Simplified Highway Capacity Calculation Method for the Highway Performance Monitoring System

Chapter 2. Development of Capacity Computation Methods

Review Highway Economic Requirements System Capacity Procedures for Use in Highway Performance Monitoring System

The Highway Economic Requirements System (HERS) capacity procedures, completed in 2014, adapted the Highway Capacity Manual (HCM) 2010 capacity calculation methods. (Task 6 Technical Memo: Procedures for Estimating Highway Capacity, May 2014.) It utilized Highway Performance Monitoring System (HPMS) data items to the maximum extent but also had to make many assumptions about default values. The HPMS data items that were used are shown in table 1. HCM capacity input data that had to set to default values are as follows:

Freeways—ramp density, peak-hour factor, and driver population factor.
Multilane highways—driveway density, peak-hour factor, and driver population factor.
Signals—lane group assignments (based on presence of right- and left-turning lanes), grade, number of parking maneuvers per hour, bus blockage, area type, lane utilization, left- and right-turn lanes.
Rural two-lane highways—driveway density and peak-hour factor.
Stop signs—conflicting flow rates, base critical gap, follow-up times, and assignment of turning movements to lanes.

The project team compared the HERS procedures with National Cooperative Highway Research Program (NCHRP) Project 07-22, Planning and Preliminary Engineering Applications Guide to the HCM. The report has recently been published as NCHRP Report 825. (Dowling, Richard et al., Planning and Preliminary Engineering Applications Guide to the Highway Capacity Manual, NCHRP Report 825, 2016.) Its midlevel analysis methods for capacity are very similar to the HERS procedures which preceded it. Where the two methods differ, we developed a test procedure using 2014 HPMS data to compare the capacity values obtained with each method. The results of the comparison are as follows.

Table 1. Highway performance monitoring system data items used in the highway economic requirements system capacity calculation procedures.
HPMS Data Item	Facility Type—Freeways	Facility Type—Multilane	Facility Type—Signals	Facility Type—Rural Two Lane	Facility Type—Stop Sign
Lane Width	Yes	Yes	Yes	Yes	No
Right Shoulder Width	Yes	Yes	No	Yes	No
Left Shoulder Width	No	Yes	No	No	No
Through Lanes	Yes	Yes	No	No	Yes
Peak Lanes	No	No	No	No	Yes
% SU Trucks	Yes	Yes	Yes	Yes	Yes
% Comb. Trucks	Yes	Yes	Yes	Yes	Yes
Median Type	No	No	No	No	No
No. Other Intersections	No	Yes	No	Yes	No
Right-Turn Lanes	No	No	Yes	No	Yes
Left-Turn Lanes	No	No	Yes	No	Yes
K-factor	No	No	Yes	Yes	Yes
D-factor	No	No	Yes	Yes	Yes
Percent Green Lime	No	No	Yes	No	No
Terrain	Yes	Yes	No	Yes	No
Functional Class	No	No	No	No	Yes
Speed Limit (Speed limit is generally used in the computation of service measures but except for rural two-lane highways is not used in capacity calculation)	No	No	No	Yes	No

Freeways

The HERS capacity procedure is very similar to that of NCHRP 825. The HERS procedure follows the HCM procedure verbatim, with adjustments for lane width, lateral clearance, interchange density, heavy trucks, and the peak-hour factor. The NCHRP method excludes the peak-hour factor. However, their formulations for the final capacity value are slightly different. When applied to 2014 Sample Panel data (21,940 freeway sections), HERS-developed capacity values are five percent lower than those of NCHRP 825. We consider this difference negligible, so we recommend the NCHRP 825 method for capacity and (Level of Service) LOS calculations.

Figure 2. Equation. Capacity calculation for freeway capacity.

Figure 2 is an equation that shows that the capacity calculation for freeways is a function of free flow speed, percent of heavy vehicles in the traffic stream and number of lanes.

Figure 3. Calculation or free flow speed for freeways.

Figure 3 is an equation that shows that free flow speed is calculated as 75.4 minus adjustment factors for lane width and right side lateral clearance.

The HERS method follows the HCM faithfully. NCHRP 825 recommends using the HCM method, so the two methods are the same. Therefore, the HCM method (which also is the HERS method) is recommended for capacity and LOS calculations.

Multilane Highways

The HERS method follows the HCM faithfully. NCHRP 825 recommends using the HCM method, so the two methods are the same. Therefore, the HCM method (which is also the HERS method) is recommended for capacity and LOS calculations.

Figure 4. Calculation of capacity for multilane highways.

Signalized Highways

Different philosophies are taken between the HERS method and NCHRP 825 method. In the HERS method, the capacity of the entire approach is calculated. This done because in HERS’ speed estimation, turning movement volumes are not available in HPMS and the volume on the entire segment is used. In the NCHRP 825 method, only the through movement is used. (It offers a solution if only total volume is available: if exclusive left-turn and right lanes exist, deduct 10 percent for each.) To be comparable, we used the HERS method to analyze just the through movement. In the HERS method, saturation flow rate is adjusted downÂward for lane width, heavy vehicles, parking, and the peak-hour factor. In the NCHRP 825 method, saturation flow rate is unadjusted. As a result, when applied to the Sample Panel data (26,504 signalized sections), the HERS method produces capacities that are 14 percent lower than NCHRP 825. The main problem with the HERS method is that the turning lane and their volume assignments are based on many assumptions. Because of its simplicity in dealing with turning movements, we recommend the NCHRP 825 method for capacity and LOS calculations.

Figure 5. Calculation of capacity for signalized highways.

Figure 5 is an equation that shows the capacity of a signalized highway is equal to the amount of green time divided by the cycle length times the number of lanes times 1,900.

Two-Lane Highways with No Traffic Control

Completely different approaches are taken by the two methods. The HERS method assumes that no traffic control is present while the NCHRP 825 method includes delay at intersections if they are present. The HERS procedure calculates two-way capacity while the NCHRP method uses one-way capacity and assigns a fixed value, either 1,450 or 1,490 vehicles per hour, but capacity is not used in the calculation of LOSâ€”it is only used to create a v/c ratio for screening. Because of this difference, no tests were made. The HERS method is quite complex, accounting for grades, heavy vehicles, peak-hour factor, and percent no passing zones. The NCHRP 825 approach is simpler for capacity, but the calculation of speed and associated LOS basically follows the same method as for the capacity calculation in HERS. This is because capacity is “backcalculated” in the HERS method so that the HERS speed equations, which are based on the AADT-to-capacity ratio, can be used. In the NCHRP 825 method, capacity is largely irrelevant for two-lane highway LOS as it is based on average travel speed or percent of time following. Because it is more closely tied to HCM methods for the calculation of average travel speed (the basis for LOS), we recommend the NCHRP 825 method for capacity (one-way) and LOS calculations.Â That is, the one-way capacity is set at 1,490 vehicles per hour.

Stop-Controlled Highways

Very different capacity calculations are used in the HCM 2010 depending on whether an intersection has two-way stop control or all-way stop control. Unfortunately, it is impossible to distinguish the two conditions with HPMS data. Both HERS and NCHRP 825 procedures are complex and highly dependent on turning movements on all the approaches. The HERS procedure is based on the HCM 2000 procedure for two-way stop-controlled intersections. It is data intensive and makes many assumptions about turning movements based on the presence of turning lanes. In contrast, the NCHRP 825 procedure uses the highly simplified adaptation of the HCM procedure for all-way stop control and a more complex procedure for two-way stop control which is based on the update to the HCM 2010. For this reason, we did not run any comparisons between the two methods. Because, the two-way stop control procedures in NCHRP 825 is based on the recently updated version of the HCM 2010, we recommend that the NCHRP 825 procedure be used for capacity and LOS calculations. In doing so, it is assumed that all stop control is two-way.

Figure 6. Calculation of capacity for stop-controlled highways.

Figure 6 is an equation that shows that the capacity of a stop-controlled highway is equal to 1,200 vehicles per hour where the number of lanes is equal to 1 or 1,500 vehicles per hour otherwise.

Update Capacity Procedures Based on New Highway Capacity Manual Edition

The Major Update to the HCM 2010 has been released. We reviewed the new procedures against the HERS capacity procedures and found no changes in the way capacity is calculated except for two-way stop-controlled intersections. The NCHRP 825 method is consistent with the new HCM as the researchers had access to the draft chapters. Therefore, the recommendations made above are not changed.

Page last modified on August 28, 2020