Core Concepts for the Civil PE Exam:
Transportation Depth
Civil Morning Breadth and Transportation Depth Practice Problems and Quick Reference Manual
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PE Exam Transportation Depth

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Traffic Engineering
Uninterrupted Flow
The capacity of a roadway, for a given stretch of road with defined characteristics, is a measure of the amount of traffic it can handle to maintain design speeds. The Highway Capacity Manual (HCM) is used for guidelines on the analysis of roadway capacity. Roadways must first be classified into one of two categories: Uninterrupted or interrupted flow. As the name suggests, uninterrupted flow includes roads where there is no disruption of the traffic from intersections or traffic control measures. These are typically highways or freeways. Interrupted flow is the opposite in which there are locations in which the traffic is controlled. Interrupted flow will be discussed in the next section below.
To classify roads by how they perform, the HCM has established a metric called Level of Service. A roadway can be rated from A being the best to F being the worst. The level of service is determined from charts in the HCM and is a function of the calculated density of the roadway. For freeways and multilane highway segments use HCM exhibit 1215.
The heavy vehicle factor converts flow of trucks and buses into passenger car equivalents. The calculation of the variables is based on the characteristics of the roadway. They can be classified into either general terrain segments or individual segments. The general terrain is applicable for grades up to 3% for lengths of 0.25 to 1.0 miles. The terrain is then classified as level or rolling. ET is then determined in exhibit 1225.
PT is the proportions of trucks and buses.
Free flow speed is the speed of the traffic flow when the volume is low enough to not impede the speed of the vehicles. This can be calculated off of a Base Free Flow Speed. This is the speed of a roadway under perfect geometric conditions:
Interrupted Flow
Interrupted flow conversely to uninterrupted flow contains some restriction for the analysis of the capacity of a segment of roadway. This includes intersections both signalized and unsignalized, roundabouts, urban street flow, and pedestrians.
Because of these restrictions the traffic can often not reach the free flow speed and instead can only reach a running speed. This is the speed at which a vehicle is able to travel when accounting for the factors created by the interruption of flow. This speed can be calculated from the following equation HCM 1848:
Intersection Capacity
Traffic Analysis
Volume studies as the name indicates is an analysis of a roadway or intersection by field measurements. The study most often consists of observers on site counting traffic volumes and recording the numbers. The parameters of the study need to be determined by engineering judgement based on the intention of the study. The range and duration of the study can vary to achieve these intentions. The results of the study can be used to calculate parameters used in analyses such as average daily traffic, intersection volumes, and observed speeds.
It is important to set the limits of the segment in which the speed is to be recorded. This segment needs to be determined by judgement based on the intention of the study. The average speed over a given segment can be calculated by the following equation:
Savg = Average speed for the given segment
L = Length of segment
Nt = Number of cars observed
t = Observed time of each vehicle
Modal split is the measure of the percentages of different modes of transportation for an observed stretch. The modes often include cars, buses, trucks, bikes and pedestrians and any other uncommon mode. It is a good representation of the distribution of traffic for a given location or stretch of roadway.
Trip Generation
Trips are the act of a type of modal transportation leaving an origin and arriving at a destination. It is important to characterize the amount and type of trips which occur in a given area. This is a trip generation analysis. This is often used to characterize trips and observations are classified as data points for a given type of trip. These data points are then charted and fit to an equation to help approximate anticipated trips. The best fit equation can be linear or nonlinear:
T=y+bx linear
lnT=y+blnX (nonlinear)
T = Number of trips
y = yintercept
b = Slope of best fit line
X = Trip generation parameter
Accident Analysis
When traffic movements create a potential for a crash, these can be reviewed as a part of a conflict analysis. This identifies all of the potential movements for an intersection or roadway and determines where there is the possibility for a crash. Conflict diagrams show all movements and the types of conflicts associated with other movements. This can be used to see where there are troublesome areas and the potential for improvements to avoid undesirable conflicts.
Accident analysis is, as the name suggests, an evaluation of the number of crashes for a given intersection or segment. This information can be used to evaluate if improvements are required. The accident rate is a ratio of the number of crashes to the exposure, which is the number of vehicles for a defined time or length of roadway:
Nonmotorized Facilities
The analysis of pedestrians is important to the flow of vehicle traffic, to ensure the area can handle the number of pedestrians, and to ensure safety. Just as with vehicles we can calculate the pedestrian flow rate at a given location:
SP = Walking speed
DP = Pedestrian density
It is important to note the speed of pedestrians will decrease as the density is increased. This is because people have more trouble maneuvering and walking at a normal pace if they are obstructed by other people. The Highway Capacity Manual has a number of graphs which show the relationship between density, speed, space, and flow in exhibits 414/15/16/17.
A walkway or sidewalk has a certain width. However, there are often objects in the walkway which will reduce the effective width. The reduction in the original walkway width is the sum of the shy distances. Typical values for reductions can be found in HCM exhibit 249.
Just as in the analysis of vehicular traffic, the performance for pedestrian flow at a particular walkway or intersection can be classified by a Level of Service. The flow of pedestrians can be uninterrupted or interrupted. The unit flow rate is the determining variable for the LOS but is most often taken at 15 min intervals. The 15 min pedestrian flow rate is (HCM Eq. 243):
The LOS for average walkways can be determined from HCM exhibit 241.
There is also platoon level of service. This accounts for the fact that pedestrians will often travel in groups. Platoon LOS can be determined from HCM exhibit 242.
Traffic Forecast
Predicting traffic is important for allocating funds and prioritizing projects for the future. Often traffic can be estimated using historical data to obtain a growth rate. Future traffic can be predicted using the following equation:
P = Growth rate (decimal)
n = Number of years
Highway Safety
The AASHTO Highway Safety Manual (HSM) provides guidelines for the prediction of crashes for a given segment or location. The frequency of crashes can be predicted by using equations called Safety Performance Functions (SPF) based on the characteristics of the roadway and the desired time period. The equations must be determined through statistical modeling and are most often based on annual traffic volume and segment length but may also include other roadway characteristics. These SPF’s are used to determine a predicted crash frequency which can then be adjusted to determine the actual predicted frequency from the following equation:
C = Calibration factor
CMF = Product of all Crash Modification Factors
The Crash Modification Factors (CMF) are based on proposed modifications to a site. It is the ratio of the expected crash frequency of the changed site to the crash frequency of the original condition:
CMF= Modified Crash Frequency/Original Crash Frequency
Horizontal Design
Intersection Sight Distance
When a vehicle is approaching or is stopped at an intersection, they must have an adequate line of sight along the perpendicular roadway to be able to safely stop or maneuver if necessary. This sight distance can be approximated by sight triangles where the hypotenuse is the required sight distance and the base is the required stopping distance. The diagram below exhibits this where X is the stopping distance of the vehicle on the major road and H is the sight distance:
Interchanges
An interchange is a gradeseparated crossing of 2 or more roadways in which ramps are used in such a manner so that the flow of traffic is not interrupted. On ramps and off ramps need to be designed such that there is enough length for acceleration and sight distance for the seamless merging of traffic. Mostly the design lengths can be determined from the appropriate tables in the GDHS.
There are a number of types of interchanges which have advantages and disadvantages based on the site constraints. Some examples include trumpet, diamond, partial and full cloverleaf, or fully directional
As with traffic signals, GDHS provides warrants for the consideration of the use of interchanges. These include:
1. Design Designation
2. Bottleneck or Spot Congestion Relief
3. Safety Improvements
4. Topography
5. User Benefits
6. Traffic Volume
At Grade Intersection Layout
Intersections must be detailed to minimize disruption of traffic and to ensure a safe driving condition. To achieve this, the layout must facilitate both proper sight distances and maneuverability. Acute angles at intersections provide difficulties for both of these aspects and should be avoided as much as possible. The AASHTO Policy on the Geometric Design of Highways and Streets (GDHS) provides a wide range of tables and figures. Chapter 2 focuses on vehicle dimensions and the ability to make turns. Chapter 9 provides guidance on the geometry of the traveled way and intersections to account for minimum turning requirements.
Vertical Design
Intersection Sight Distance
When a vehicle is approaching or is stopped at an intersection, they must have an adequate line of sight along the perpendicular roadway to be able to safely stop or maneuver if necessary. This sight distance can be approximated by sight triangles where the hypotenuse is the required sight distance and the base is the required stopping distance. The diagram below exhibits this where X is the stopping distance of the vehicle on the major road and H is the sight distance:
Interchanges
An interchange is a gradeseparated crossing of 2 or more roadways in which ramps are used in such a manner so that the flow of traffic is not interrupted. On ramps and off ramps need to be designed such that there is enough length for acceleration and sight distance for the seamless merging of traffic. Mostly the design lengths can be determined from the appropriate tables in the GDHS.
There are a number of types of interchanges which have advantages and disadvantages based on the site constraints. Some examples include trumpet, diamond, partial and full cloverleaf, or fully directional
As with traffic signals, GDHS provides warrants for the consideration of the use of interchanges. These include:
1. Design Designation
2. Bottleneck or Spot Congestion Relief
3. Safety Improvements
4. Topography
5. User Benefits
6. Traffic Volume
At Grade Intersection Layout
Intersections must be detailed to minimize disruption of traffic and to ensure a safe driving condition. To achieve this, the layout must facilitate both proper sight distances and maneuverability. Acute angles at intersections provide difficulties for both of these aspects and should be avoided as much as possible. The AASHTO Policy on the Geometric Design of Highways and Streets (GDHS) provides a wide range of tables and figures. Chapter 2 focuses on vehicle dimensions and the ability to make turns. Chapter 9 provides guidance on the geometry of the traveled way and intersections to account for minimum turning requirements.
Intersection Geometry
Intersection Sight Distance
When a vehicle is approaching or is stopped at an intersection, they must have an adequate line of sight along the perpendicular roadway to be able to safely stop or maneuver if necessary. This sight distance can be approximated by sight triangles where the hypotenuse is the required sight distance and the base is the required stopping distance. The diagram below exhibits this where X is the stopping distance of the vehicle on the major road and H is the sight distance:
Interchanges
An interchange is a gradeseparated crossing of 2 or more roadways in which ramps are used in such a manner so that the flow of traffic is not interrupted. On ramps and off ramps need to be designed such that there is enough length for acceleration and sight distance for the seamless merging of traffic. Mostly the design lengths can be determined from the appropriate tables in the GDHS.
There are a number of types of interchanges which have advantages and disadvantages based on the site constraints. Some examples include trumpet, diamond, partial and full cloverleaf, or fully directional
As with traffic signals, GDHS provides warrants for the consideration of the use of interchanges. These include:
1. Design Designation
2. Bottleneck or Spot Congestion Relief
3. Safety Improvements
4. Topography
5. User Benefits
6. Traffic Volume
At Grade Intersection Layout
Intersections must be detailed to minimize disruption of traffic and to ensure a safe driving condition. To achieve this, the layout must facilitate both proper sight distances and maneuverability. Acute angles at intersections provide difficulties for both of these aspects and should be avoided as much as possible. The AASHTO Policy on the Geometric Design of Highways and Streets (GDHS) provides a wide range of tables and figures. Chapter 2 focuses on vehicle dimensions and the ability to make turns. Chapter 9 provides guidance on the geometry of the traveled way and intersections to account for minimum turning requirements.
Roadside and Cross Section Design
Forgiving Roadside Concepts
Drivers, for a number of reasons, may veer off the road whether it be distraction, fatigue, or to avoid collision. For proper roadway design, there needs to be a minimum horizontal distance so that the driver can safely return to the roadway unharmed. This horizontal distance which begins at the edge of the roadway is called the clear distance. The AASHTO Roadside Design Guide (RSDG) provides guidelines on the safety of cars which have traveled off of the roadway.
The land just outside of the roadway may not always be flat. The slope of the clear distance has an effect on the cars ability to safely recover. Slopes less than 1 Vertical to 4 Horizontal are considered recoverable slopes since the car’s ability to stop or maneuver will not be greatly affected by the slope. A nonrecoverable slope is one which is steeper than 1:4. If a nonrecoverable slope is present, the bottom of the slope must have a vehicle runout area which will allow the vehicle to stop. Table 31 of the AASHTO RSDG can be used to determine minimum clear distances based on slopes and design speeds.
When traveling on a horizontal curve, the cars traveling along the outside of the curve will struggle to recover moreso than a straight roadway due to the centrifugal force. Therefore, an adjustment factor needs to be applied to the clear zone on the outside of the curve only. The adjustment factor is found in table 32 of the AASHTO RSDG.
Barrier Design
Often objects outside of the roadway must fall within the clear zone. A barrier must be provided to both protect the object and prevent the vehicle from a collision. An appropriate barrier will minimize the damage to the vehicle and safely redirect it onto traffic. The runout length, LR, is the minimum distance away from an object that a vehicle may leave the roadway and strike the object. This will define the length of barrier needed. AASHTO RSDG Table 510b provides minimum values based on volume and design speeds. Barriers which are too close to the roadway may be troublesome to drivers and cause them to slow down. To prevent this, a minimum shy distance is provided in RSDG Table 57. The geometry of a barrier must be determined for a safe condition by the following equations:
LA = Distance from edge of road to back edge of object
b = Rise of taper slope
a = Run of taper slope
L1 = Length from object to beginning of flare
L2 = Distance from edge of road to face of barrier
LR = Runout Length
Crash attenuators can be used to prevent vehicles from crashing directly into an object or from entering an area which would be unsafe for the driver or pedestrians. When the vehicle strikes the attenuator, it begins to decelerate at a rate of the following equation:
F = Stopping force (lbs)
w = weight of vehicle (lbs)
d = Deceleration rate
g = Force due to gravity (32.2 ft/s2)
SF = Safety factor
Cross Section Elements
While a roadway often has to fit the area and purpose of its proposed location, the geometric features must meet certain minimum and maximum values. The Policy on Geometric Design of Highways and Streets provides a large number of requirements for the design of a roadway or walkway cross section. For the PE exam it is best to become familiar with the location of these requirements and most importantly be able to find them quickly since it is unreasonable to be expected to memorize all values.
ADA Design Considerations
The American Disabilities Act of 1990 outlines the requirements for structures to ensure proper treatment of individuals with disabilities. The guidelines outline many topics including parking, ramps, egress and others and the requirements which must be met to ensure the proper accessibility and safety. For the PE exam you will likely be asked a question or two requiring you to lookup certain aspects of the code. You should not spend excessive amounts of time reading the code but be familiar with the sections and be able to navigate and find information quickly.
Signal Design
Uninterrupted Flow
The capacity of a roadway, for a given stretch of road with defined characteristics, is a measure of the amount of traffic it can handle to maintain design speeds. The Highway Capacity Manual (HCM) is used for guidelines on the analysis of roadway capacity. Roadways must first be classified into one of two categories: Uninterrupted or interrupted flow. As the name suggests, uninterrupted flow includes roads where there is no disruption of the traffic from intersections or traffic control measures. These are typically highways or freeways. Interrupted flow is the opposite in which there are locations in which the traffic is controlled. Interrupted flow will be discussed in the next section below.
To classify roads by how they perform, the HCM has established a metric called Level of Service. A roadway can be rated from A being the best to F being the worst. The level of service is determined from charts in the HCM and is a function of the calculated density of the roadway. For freeways and multilane highway segments use HCM exhibit 1215.
The heavy vehicle factor converts flow of trucks and buses into passenger car equivalents. The calculation of the variables is based on the characteristics of the roadway. They can be classified into either general terrain segments or individual segments. The general terrain is applicable for grades up to 3% for lengths of 0.25 to 1.0 miles. The terrain is then classified as level or rolling. ET is then determined in exhibit 1225.
PT is the proportions of trucks and buses.
Free flow speed is the speed of the traffic flow when the volume is low enough to not impede the speed of the vehicles. This can be calculated off of a Base Free Flow Speed. This is the speed of a roadway under perfect geometric conditions:
Interrupted Flow
Interrupted flow conversely to uninterrupted flow contains some restriction for the analysis of the capacity of a segment of roadway. This includes intersections both signalized and unsignalized, roundabouts, urban street flow, and pedestrians.
Because of these restrictions the traffic can often not reach the free flow speed and instead can only reach a running speed. This is the speed at which a vehicle is able to travel when accounting for the factors created by the interruption of flow. This speed can be calculated from the following equation HCM 1848:
Intersection Capacity
Traffic Analysis
Volume studies as the name indicates is an analysis of a roadway or intersection by field measurements. The study most often consists of observers on site counting traffic volumes and recording the numbers. The parameters of the study need to be determined by engineering judgement based on the intention of the study. The range and duration of the study can vary to achieve these intentions. The results of the study can be used to calculate parameters used in analyses such as average daily traffic, intersection volumes, and observed speeds.
It is important to set the limits of the segment in which the speed is to be recorded. This segment needs to be determined by judgement based on the intention of the study. The average speed over a given segment can be calculated by the following equation:
Savg = Average speed for the given segment
L = Length of segment
Nt = Number of cars observed
t = Observed time of each vehicle
Modal split is the measure of the percentages of different modes of transportation for an observed stretch. The modes often include cars, buses, trucks, bikes and pedestrians and any other uncommon mode. It is a good representation of the distribution of traffic for a given location or stretch of roadway.
Trip Generation
Trips are the act of a type of modal transportation leaving an origin and arriving at a destination. It is important to characterize the amount and type of trips which occur in a given area. This is a trip generation analysis. This is often used to characterize trips and observations are classified as data points for a given type of trip. These data points are then charted and fit to an equation to help approximate anticipated trips. The best fit equation can be linear or nonlinear:
T=y+bx linear
lnT=y+blnX (nonlinear)
T = Number of trips
y = yintercept
b = Slope of best fit line
X = Trip generation parameter
Accident Analysis
When traffic movements create a potential for a crash, these can be reviewed as a part of a conflict analysis. This identifies all of the potential movements for an intersection or roadway and determines where there is the possibility for a crash. Conflict diagrams show all movements and the types of conflicts associated with other movements. This can be used to see where there are troublesome areas and the potential for improvements to avoid undesirable conflicts.
Accident analysis is, as the name suggests, an evaluation of the number of crashes for a given intersection or segment. This information can be used to evaluate if improvements are required. The accident rate is a ratio of the number of crashes to the exposure, which is the number of vehicles for a defined time or length of roadway:
Nonmotorized Facilities
The analysis of pedestrians is important to the flow of vehicle traffic, to ensure the area can handle the number of pedestrians, and to ensure safety. Just as with vehicles we can calculate the pedestrian flow rate at a given location:
SP = Walking speed
DP = Pedestrian density
It is important to note the speed of pedestrians will decrease as the density is increased. This is because people have more trouble maneuvering and walking at a normal pace if they are obstructed by other people. The Highway Capacity Manual has a number of graphs which show the relationship between density, speed, space, and flow in exhibits 414/15/16/17.
A walkway or sidewalk has a certain width. However, there are often objects in the walkway which will reduce the effective width. The reduction in the original walkway width is the sum of the shy distances. Typical values for reductions can be found in HCM exhibit 249.
Just as in the analysis of vehicular traffic, the performance for pedestrian flow at a particular walkway or intersection can be classified by a Level of Service. The flow of pedestrians can be uninterrupted or interrupted. The unit flow rate is the determining variable for the LOS but is most often taken at 15 min intervals. The 15 min pedestrian flow rate is (HCM Eq. 243):
The LOS for average walkways can be determined from HCM exhibit 241.
There is also platoon level of service. This accounts for the fact that pedestrians will often travel in groups. Platoon LOS can be determined from HCM exhibit 242.
Traffic Forecast
Predicting traffic is important for allocating funds and prioritizing projects for the future. Often traffic can be estimated using historical data to obtain a growth rate. Future traffic can be predicted using the following equation:
P = Growth rate (decimal)
n = Number of years
Highway Safety
The AASHTO Highway Safety Manual (HSM) provides guidelines for the prediction of crashes for a given segment or location. The frequency of crashes can be predicted by using equations called Safety Performance Functions (SPF) based on the characteristics of the roadway and the desired time period. The equations must be determined through statistical modeling and are most often based on annual traffic volume and segment length but may also include other roadway characteristics. These SPF’s are used to determine a predicted crash frequency which can then be adjusted to determine the actual predicted frequency from the following equation:
C = Calibration factor
CMF = Product of all Crash Modification Factors
The Crash Modification Factors (CMF) are based on proposed modifications to a site. It is the ratio of the expected crash frequency of the changed site to the crash frequency of the original condition:
CMF= Modified Crash Frequency/Original Crash Frequency
Traffic Control Design
Uninterrupted Flow
The capacity of a roadway, for a given stretch of road with defined characteristics, is a measure of the amount of traffic it can handle to maintain design speeds. The Highway Capacity Manual (HCM) is used for guidelines on the analysis of roadway capacity. Roadways must first be classified into one of two categories: Uninterrupted or interrupted flow. As the name suggests, uninterrupted flow includes roads where there is no disruption of the traffic from intersections or traffic control measures. These are typically highways or freeways. Interrupted flow is the opposite in which there are locations in which the traffic is controlled. Interrupted flow will be discussed in the next section below.
To classify roads by how they perform, the HCM has established a metric called Level of Service. A roadway can be rated from A being the best to F being the worst. The level of service is determined from charts in the HCM and is a function of the calculated density of the roadway. For freeways and multilane highway segments use HCM exhibit 1215.
The heavy vehicle factor converts flow of trucks and buses into passenger car equivalents. The calculation of the variables is based on the characteristics of the roadway. They can be classified into either general terrain segments or individual segments. The general terrain is applicable for grades up to 3% for lengths of 0.25 to 1.0 miles. The terrain is then classified as level or rolling. ET is then determined in exhibit 1225.
PT is the proportions of trucks and buses.
Free flow speed is the speed of the traffic flow when the volume is low enough to not impede the speed of the vehicles. This can be calculated off of a Base Free Flow Speed. This is the speed of a roadway under perfect geometric conditions:
Interrupted Flow
Interrupted flow conversely to uninterrupted flow contains some restriction for the analysis of the capacity of a segment of roadway. This includes intersections both signalized and unsignalized, roundabouts, urban street flow, and pedestrians.
Because of these restrictions the traffic can often not reach the free flow speed and instead can only reach a running speed. This is the speed at which a vehicle is able to travel when accounting for the factors created by the interruption of flow. This speed can be calculated from the following equation HCM 1848:
Intersection Capacity
Traffic Analysis
Volume studies as the name indicates is an analysis of a roadway or intersection by field measurements. The study most often consists of observers on site counting traffic volumes and recording the numbers. The parameters of the study need to be determined by engineering judgement based on the intention of the study. The range and duration of the study can vary to achieve these intentions. The results of the study can be used to calculate parameters used in analyses such as average daily traffic, intersection volumes, and observed speeds.
It is important to set the limits of the segment in which the speed is to be recorded. This segment needs to be determined by judgement based on the intention of the study. The average speed over a given segment can be calculated by the following equation:
Savg = Average speed for the given segment
L = Length of segment
Nt = Number of cars observed
t = Observed time of each vehicle
Modal split is the measure of the percentages of different modes of transportation for an observed stretch. The modes often include cars, buses, trucks, bikes and pedestrians and any other uncommon mode. It is a good representation of the distribution of traffic for a given location or stretch of roadway.
Trip Generation
Trips are the act of a type of modal transportation leaving an origin and arriving at a destination. It is important to characterize the amount and type of trips which occur in a given area. This is a trip generation analysis. This is often used to characterize trips and observations are classified as data points for a given type of trip. These data points are then charted and fit to an equation to help approximate anticipated trips. The best fit equation can be linear or nonlinear:
T=y+bx linear
lnT=y+blnX (nonlinear)
T = Number of trips
y = yintercept
b = Slope of best fit line
X = Trip generation parameter
Accident Analysis
When traffic movements create a potential for a crash, these can be reviewed as a part of a conflict analysis. This identifies all of the potential movements for an intersection or roadway and determines where there is the possibility for a crash. Conflict diagrams show all movements and the types of conflicts associated with other movements. This can be used to see where there are troublesome areas and the potential for improvements to avoid undesirable conflicts.
Accident analysis is, as the name suggests, an evaluation of the number of crashes for a given intersection or segment. This information can be used to evaluate if improvements are required. The accident rate is a ratio of the number of crashes to the exposure, which is the number of vehicles for a defined time or length of roadway:
Nonmotorized Facilities
The analysis of pedestrians is important to the flow of vehicle traffic, to ensure the area can handle the number of pedestrians, and to ensure safety. Just as with vehicles we can calculate the pedestrian flow rate at a given location:
SP = Walking speed
DP = Pedestrian density
It is important to note the speed of pedestrians will decrease as the density is increased. This is because people have more trouble maneuvering and walking at a normal pace if they are obstructed by other people. The Highway Capacity Manual has a number of graphs which show the relationship between density, speed, space, and flow in exhibits 414/15/16/17.
A walkway or sidewalk has a certain width. However, there are often objects in the walkway which will reduce the effective width. The reduction in the original walkway width is the sum of the shy distances. Typical values for reductions can be found in HCM exhibit 249.
Just as in the analysis of vehicular traffic, the performance for pedestrian flow at a particular walkway or intersection can be classified by a Level of Service. The flow of pedestrians can be uninterrupted or interrupted. The unit flow rate is the determining variable for the LOS but is most often taken at 15 min intervals. The 15 min pedestrian flow rate is (HCM Eq. 243):
The LOS for average walkways can be determined from HCM exhibit 241.
There is also platoon level of service. This accounts for the fact that pedestrians will often travel in groups. Platoon LOS can be determined from HCM exhibit 242.
Traffic Forecast
Predicting traffic is important for allocating funds and prioritizing projects for the future. Often traffic can be estimated using historical data to obtain a growth rate. Future traffic can be predicted using the following equation:
P = Growth rate (decimal)
n = Number of years
Highway Safety
The AASHTO Highway Safety Manual (HSM) provides guidelines for the prediction of crashes for a given segment or location. The frequency of crashes can be predicted by using equations called Safety Performance Functions (SPF) based on the characteristics of the roadway and the desired time period. The equations must be determined through statistical modeling and are most often based on annual traffic volume and segment length but may also include other roadway characteristics. These SPF’s are used to determine a predicted crash frequency which can then be adjusted to determine the actual predicted frequency from the following equation:
C = Calibration factor
CMF = Product of all Crash Modification Factors
The Crash Modification Factors (CMF) are based on proposed modifications to a site. It is the ratio of the expected crash frequency of the changed site to the crash frequency of the original condition:
CMF= Modified Crash Frequency/Original Crash Frequency
Geotechnical and Pavement
Uninterrupted Flow
The capacity of a roadway, for a given stretch of road with defined characteristics, is a measure of the amount of traffic it can handle to maintain design speeds. The Highway Capacity Manual (HCM) is used for guidelines on the analysis of roadway capacity. Roadways must first be classified into one of two categories: Uninterrupted or interrupted flow. As the name suggests, uninterrupted flow includes roads where there is no disruption of the traffic from intersections or traffic control measures. These are typically highways or freeways. Interrupted flow is the opposite in which there are locations in which the traffic is controlled. Interrupted flow will be discussed in the next section below.
To classify roads by how they perform, the HCM has established a metric called Level of Service. A roadway can be rated from A being the best to F being the worst. The level of service is determined from charts in the HCM and is a function of the calculated density of the roadway. For freeways and multilane highway segments use HCM exhibit 1215.
The heavy vehicle factor converts flow of trucks and buses into passenger car equivalents. The calculation of the variables is based on the characteristics of the roadway. They can be classified into either general terrain segments or individual segments. The general terrain is applicable for grades up to 3% for lengths of 0.25 to 1.0 miles. The terrain is then classified as level or rolling. ET is then determined in exhibit 1225.
PT is the proportions of trucks and buses.
Free flow speed is the speed of the traffic flow when the volume is low enough to not impede the speed of the vehicles. This can be calculated off of a Base Free Flow Speed. This is the speed of a roadway under perfect geometric conditions:
Interrupted Flow
Interrupted flow conversely to uninterrupted flow contains some restriction for the analysis of the capacity of a segment of roadway. This includes intersections both signalized and unsignalized, roundabouts, urban street flow, and pedestrians.
Because of these restrictions the traffic can often not reach the free flow speed and instead can only reach a running speed. This is the speed at which a vehicle is able to travel when accounting for the factors created by the interruption of flow. This speed can be calculated from the following equation HCM 1848:
Intersection Capacity
Traffic Analysis
Volume studies as the name indicates is an analysis of a roadway or intersection by field measurements. The study most often consists of observers on site counting traffic volumes and recording the numbers. The parameters of the study need to be determined by engineering judgement based on the intention of the study. The range and duration of the study can vary to achieve these intentions. The results of the study can be used to calculate parameters used in analyses such as average daily traffic, intersection volumes, and observed speeds.
It is important to set the limits of the segment in which the speed is to be recorded. This segment needs to be determined by judgement based on the intention of the study. The average speed over a given segment can be calculated by the following equation:
Savg = Average speed for the given segment
L = Length of segment
Nt = Number of cars observed
t = Observed time of each vehicle
Modal split is the measure of the percentages of different modes of transportation for an observed stretch. The modes often include cars, buses, trucks, bikes and pedestrians and any other uncommon mode. It is a good representation of the distribution of traffic for a given location or stretch of roadway.
Trip Generation
Trips are the act of a type of modal transportation leaving an origin and arriving at a destination. It is important to characterize the amount and type of trips which occur in a given area. This is a trip generation analysis. This is often used to characterize trips and observations are classified as data points for a given type of trip. These data points are then charted and fit to an equation to help approximate anticipated trips. The best fit equation can be linear or nonlinear:
T=y+bx linear
lnT=y+blnX (nonlinear)
T = Number of trips
y = yintercept
b = Slope of best fit line
X = Trip generation parameter
Accident Analysis
When traffic movements create a potential for a crash, these can be reviewed as a part of a conflict analysis. This identifies all of the potential movements for an intersection or roadway and determines where there is the possibility for a crash. Conflict diagrams show all movements and the types of conflicts associated with other movements. This can be used to see where there are troublesome areas and the potential for improvements to avoid undesirable conflicts.
Accident analysis is, as the name suggests, an evaluation of the number of crashes for a given intersection or segment. This information can be used to evaluate if improvements are required. The accident rate is a ratio of the number of crashes to the exposure, which is the number of vehicles for a defined time or length of roadway:
Nonmotorized Facilities
The analysis of pedestrians is important to the flow of vehicle traffic, to ensure the area can handle the number of pedestrians, and to ensure safety. Just as with vehicles we can calculate the pedestrian flow rate at a given location:
SP = Walking speed
DP = Pedestrian density
It is important to note the speed of pedestrians will decrease as the density is increased. This is because people have more trouble maneuvering and walking at a normal pace if they are obstructed by other people. The Highway Capacity Manual has a number of graphs which show the relationship between density, speed, space, and flow in exhibits 414/15/16/17.
A walkway or sidewalk has a certain width. However, there are often objects in the walkway which will reduce the effective width. The reduction in the original walkway width is the sum of the shy distances. Typical values for reductions can be found in HCM exhibit 249.
Just as in the analysis of vehicular traffic, the performance for pedestrian flow at a particular walkway or intersection can be classified by a Level of Service. The flow of pedestrians can be uninterrupted or interrupted. The unit flow rate is the determining variable for the LOS but is most often taken at 15 min intervals. The 15 min pedestrian flow rate is (HCM Eq. 243):
The LOS for average walkways can be determined from HCM exhibit 241.
There is also platoon level of service. This accounts for the fact that pedestrians will often travel in groups. Platoon LOS can be determined from HCM exhibit 242.
Traffic Forecast
Predicting traffic is important for allocating funds and prioritizing projects for the future. Often traffic can be estimated using historical data to obtain a growth rate. Future traffic can be predicted using the following equation:
P = Growth rate (decimal)
n = Number of years
Highway Safety
The AASHTO Highway Safety Manual (HSM) provides guidelines for the prediction of crashes for a given segment or location. The frequency of crashes can be predicted by using equations called Safety Performance Functions (SPF) based on the characteristics of the roadway and the desired time period. The equations must be determined through statistical modeling and are most often based on annual traffic volume and segment length but may also include other roadway characteristics. These SPF’s are used to determine a predicted crash frequency which can then be adjusted to determine the actual predicted frequency from the following equation:
C = Calibration factor
CMF = Product of all Crash Modification Factors
The Crash Modification Factors (CMF) are based on proposed modifications to a site. It is the ratio of the expected crash frequency of the changed site to the crash frequency of the original condition:
CMF= Modified Crash Frequency/Original Crash Frequency
Drainage
Forgiving Roadside Concepts
Drivers, for a number of reasons, may veer off the road whether it be distraction, fatigue, or to avoid collision. For proper roadway design, there needs to be a minimum horizontal distance so that the driver can safely return to the roadway unharmed. This horizontal distance which begins at the edge of the roadway is called the clear distance. The AASHTO Roadside Design Guide (RSDG) provides guidelines on the safety of cars which have traveled off of the roadway.
The land just outside of the roadway may not always be flat. The slope of the clear distance has an effect on the cars ability to safely recover. Slopes less than 1 Vertical to 4 Horizontal are considered recoverable slopes since the car’s ability to stop or maneuver will not be greatly affected by the slope. A nonrecoverable slope is one which is steeper than 1:4. If a nonrecoverable slope is present, the bottom of the slope must have a vehicle runout area which will allow the vehicle to stop. Table 31 of the AASHTO RSDG can be used to determine minimum clear distances based on slopes and design speeds.
When traveling on a horizontal curve, the cars traveling along the outside of the curve will struggle to recover moreso than a straight roadway due to the centrifugal force. Therefore, an adjustment factor needs to be applied to the clear zone on the outside of the curve only. The adjustment factor is found in table 32 of the AASHTO RSDG.
Barrier Design
Often objects outside of the roadway must fall within the clear zone. A barrier must be provided to both protect the object and prevent the vehicle from a collision. An appropriate barrier will minimize the damage to the vehicle and safely redirect it onto traffic. The runout length, LR, is the minimum distance away from an object that a vehicle may leave the roadway and strike the object. This will define the length of barrier needed. AASHTO RSDG Table 510b provides minimum values based on volume and design speeds. Barriers which are too close to the roadway may be troublesome to drivers and cause them to slow down. To prevent this, a minimum shy distance is provided in RSDG Table 57. The geometry of a barrier must be determined for a safe condition by the following equations:
LA = Distance from edge of road to back edge of object
b = Rise of taper slope
a = Run of taper slope
L1 = Length from object to beginning of flare
L2 = Distance from edge of road to face of barrier
LR = Runout Length
Crash attenuators can be used to prevent vehicles from crashing directly into an object or from entering an area which would be unsafe for the driver or pedestrians. When the vehicle strikes the attenuator, it begins to decelerate at a rate of the following equation:
F = Stopping force (lbs)
w = weight of vehicle (lbs)
d = Deceleration rate
g = Force due to gravity (32.2 ft/s2)
SF = Safety factor
Cross Section Elements
While a roadway often has to fit the area and purpose of its proposed location, the geometric features must meet certain minimum and maximum values. The Policy on Geometric Design of Highways and Streets provides a large number of requirements for the design of a roadway or walkway cross section. For the PE exam it is best to become familiar with the location of these requirements and most importantly be able to find them quickly since it is unreasonable to be expected to memorize all values.
ADA Design Considerations
The American Disabilities Act of 1990 outlines the requirements for structures to ensure proper treatment of individuals with disabilities. The guidelines outline many topics including parking, ramps, egress and others and the requirements which must be met to ensure the proper accessibility and safety. For the PE exam you will likely be asked a question or two requiring you to lookup certain aspects of the code. You should not spend excessive amounts of time reading the code but be familiar with the sections and be able to navigate and find information quickly.
Engineering Economics
Forgiving Roadside Concepts
Drivers, for a number of reasons, may veer off the road whether it be distraction, fatigue, or to avoid collision. For proper roadway design, there needs to be a minimum horizontal distance so that the driver can safely return to the roadway unharmed. This horizontal distance which begins at the edge of the roadway is called the clear distance. The AASHTO Roadside Design Guide (RSDG) provides guidelines on the safety of cars which have traveled off of the roadway.
The land just outside of the roadway may not always be flat. The slope of the clear distance has an effect on the cars ability to safely recover. Slopes less than 1 Vertical to 4 Horizontal are considered recoverable slopes since the car’s ability to stop or maneuver will not be greatly affected by the slope. A nonrecoverable slope is one which is steeper than 1:4. If a nonrecoverable slope is present, the bottom of the slope must have a vehicle runout area which will allow the vehicle to stop. Table 31 of the AASHTO RSDG can be used to determine minimum clear distances based on slopes and design speeds.
When traveling on a horizontal curve, the cars traveling along the outside of the curve will struggle to recover moreso than a straight roadway due to the centrifugal force. Therefore, an adjustment factor needs to be applied to the clear zone on the outside of the curve only. The adjustment factor is found in table 32 of the AASHTO RSDG.
Barrier Design
Often objects outside of the roadway must fall within the clear zone. A barrier must be provided to both protect the object and prevent the vehicle from a collision. An appropriate barrier will minimize the damage to the vehicle and safely redirect it onto traffic. The runout length, LR, is the minimum distance away from an object that a vehicle may leave the roadway and strike the object. This will define the length of barrier needed. AASHTO RSDG Table 510b provides minimum values based on volume and design speeds. Barriers which are too close to the roadway may be troublesome to drivers and cause them to slow down. To prevent this, a minimum shy distance is provided in RSDG Table 57. The geometry of a barrier must be determined for a safe condition by the following equations:
LA = Distance from edge of road to back edge of object
b = Rise of taper slope
a = Run of taper slope
L1 = Length from object to beginning of flare
L2 = Distance from edge of road to face of barrier
LR = Runout Length
Crash attenuators can be used to prevent vehicles from crashing directly into an object or from entering an area which would be unsafe for the driver or pedestrians. When the vehicle strikes the attenuator, it begins to decelerate at a rate of the following equation:
F = Stopping force (lbs)
w = weight of vehicle (lbs)
d = Deceleration rate
g = Force due to gravity (32.2 ft/s2)
SF = Safety factor
Cross Section Elements
While a roadway often has to fit the area and purpose of its proposed location, the geometric features must meet certain minimum and maximum values. The Policy on Geometric Design of Highways and Streets provides a large number of requirements for the design of a roadway or walkway cross section. For the PE exam it is best to become familiar with the location of these requirements and most importantly be able to find them quickly since it is unreasonable to be expected to memorize all values.
ADA Design Considerations
The American Disabilities Act of 1990 outlines the requirements for structures to ensure proper treatment of individuals with disabilities. The guidelines outline many topics including parking, ramps, egress and others and the requirements which must be met to ensure the proper accessibility and safety. For the PE exam you will likely be asked a question or two requiring you to lookup certain aspects of the code. You should not spend excessive amounts of time reading the code but be familiar with the sections and be able to navigate and find information quickly.