6.  Glossary

85th Percentile Speed (V85)

The 85th percentile of a sample of observed speeds is the general statistic used to describe operating speeds on a geometric feature. It is the speed at or below which 85 percent of the drivers are operating.

Design Speed

The design speed is defined by the American Association of State Highway and Transportation Officials (AASHTO) in its 2001 A Policy on Geometric Design of Highways and Streets as "a selected speed used to determine the various geometric design features of the highway." Design speed can be edited through the "Highway Editor" under the General category.

Desired Speed

In the speed-profile model, desired speed is the 85th percentile speed that drivers select when not constrained by the vertical or horizontal alignment. Empirically, desired speed is estimated by measuring speeds on portions on long tangents where speed is not constrained by either vertical gradient or horizontal and vertical curvature. In IHSDM, the default value for desired speed is 100 km/h (62 mi/h). This default value is based upon the average of 85th percentile speeds on long tangents in the research used to calibrate the model. The average was based upon speeds measured in six States at 64 long tangents on highways with 88.5 km/h (55 mi/h) posted speed limits. Among the six States, the averages ranged from 93 km/h (58 mi/h) in New York and Pennsylvania to 103 km/h (64 mi/h) in Texas. Users may modify the desired speed through the "Highway Editor" under the General category..

Speed at Evaluation Start Station

The 85th percentile initial speed when traveling in the direction of increasing stations or the end speed when traveling in the direction of decreasing stations. This speed cannot be greater than the desired speed. It is set to the desired speed by default and may be edited by the user from the Run Evaluation Wizard.

Speed at Evaluation End Station

The 85th percentile end speed when traveling in the direction of increasing stations or the initial speed when traveling in the direction of decreasing stations. This speed cannot be greater than the desired speed. It is set to the desired speed by default and may be edited by the user from the Run Evaluation Wizard.

Speed Profile

One of the ways in which operating speeds are used in ensuring design consistency is through the use of speed profiles. Speed-profile models are used to detect speed inconsistencies along road alignments. A speed profile is a plot of operating speeds on the vertical axis versus distance along the highway on the horizontal axis.

TWOPAS

Description:
TWOPAS is a microscopic model that simulates traffic operations on two-lane highways by reviewing the position, speed, and acceleration of each individual vehicle on a simulated highway at 1-second intervals and advancing those vehicles along the highway in a realistic manner. The model takes into account the effects on traffic operations of road geometric, traffic control, driver preferences, vehicle size and performance characteristics, and the oncoming and same direction vehicles that are in sight at any given time. The model incorporates realistic passing and pass abort decisions by drivers in two-lane highway passing zones. Spot data, space data, vehicle interaction data, and overall travel data are accumulated and processed, and various statistical summaries are printed.

Use of TWOPAS in the Design Consistency Module:
Steep upgrades reduce passenger car speeds. The TWOPAS traffic simulation model contains equations that can be used to represent the effect of grades on the speed of passenger cars.

Upgrades have the effect of limiting the accelerations that vehicles can achieve, thus making it difficult for drivers to maintain their desired speed. If the grade a vehicle is ascending is steep enough, the vehicle will be forced to decelerate. If the grade is also long enough, the vehicle will eventually decelerate to a crawl speed. A vehicle at its crawl speed can continue up the grade at a constant speed without decelerating further, but cannot accelerate. A vehicle's crawl speed on a specific grade is a function of the steepness of the grade and the performance characteristics of the vehicle. The length of grade required for a vehicle to reach its crawl speed is a function of the steepness of the grade and the performance characteristics of the vehicle, as well as the vehicle's initial speed as it enters the grade.

In the DCM, TWOPAS equations are used to check the performance-limited speed along the highway. If, at any point, the grade-limited speed is less than the tangent or curve speed predicted using the speed prediction equations or the assumed desired speed, then the grade-limited speed will govern.

The procedure for predicting the grade-limited speed using the TWOPAS equations can be summarized as follows:

1. Calculate vehicle performance speeds along the alignment at one-second intervals.
2. Calculate speeds based on the driver's preferred acceleration rate at one-second intervals.
3. At each time interval, compare the speeds predicted in Steps 1 and 2, and select the lowest speed. The speed selected is used as the initial speed for the next one-second interval in Steps 1 and 2. This process is continued for the entire evaluation section.

The result is an estimated operating speed profile for the selected design vehicle, based on the effects of the vertical alignment.

Vehicle Type

There are five passenger car vehicle types included in the DCM. Passenger Car- Type 5 is set as the default vehicle, and has the highest acceleration and top speed of all the loaded vehicles; it is also the most common vehicle, making up 30% of the passenger car population. Conversely, Passenger Car- Type 1 has the lowest acceleration and top speed and represents the smallest portion of the passenger car population at 10%. Changing the vehicle type for a given evaluation with all other variables remaining constant, may change the 85th percentile speed profile. This depends upon whether or not the speeds predicted by the TWOPAS equations in Step 4 of the speed profile algorithm are lower than the speeds predicted by the speed prediction equations in Steps 2 and 3. If the speeds predicted by the TWOPAS equations are lower, then the final 85th percentile speed profile will reflect a decrease. The grade-limited speeds predicted by the TWOPAS equations are most likely to control on long, steep grades. Vehicle type may be edited via the "Set design consistency attributes" screen of the Evaluation Wizard.