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HPMS Field Manual

Appendix N: Procedures for Estimating Highway Capacity

Simplified Procedure For Rural Signalized Intersection Capacity And Urban Sections With Missing Data

Application

In the coding of HPMS data, several data items required for applying the urban signalized capacity analysis are either "optional" or not required for rural sections. The data items of interest for capacity calculations, since they are used to identify lane groups, are:

• Data Item 88: Left Turning Lanes
  
  
• Data Item 89: Right Turning Lanes.

Procedure

Although some states do code these items for rural sections, a provision must be made to handle cases where the data are not present; this could also be true for some urban sections. In the cases where rural signalized sections have nonzero values coded for these data items, the signalized intersection capacity is used. When these data are coded as zero, the following procedure is used:

where:

The g/C ratio default values given above attempt to account for, in a general way, the presence of exclusive turn lanes and phases. These values are higher than the default values listed in Table 9-3 of NCHRP 387 (0.40-0.45) but those values consider only through movement capacity.

Stop Sign-Controlled Highway Capacity

Application

All highways that have at least one stop sign and meet the density requirement (> .5/mile) on the inventory route.

Procedure

The HCM 2000 procedure for stop sign-controlled intersections is complex and highly dependent on turning movements on all the approaches. Procedures also vary for two-way controlled intersections versus all-way controlled intersections. None of these data are present in HPMS. Therefore, a highly simplified method is recommended. It is based on the HCM 2000 procedure for two-way controlled intersections. This is the simpler and less data intensive of the two methods.

The procedure is based on first calculating the potential capacity for each turning movement. (It is assumed that the potential capacity is equal to the "movement capacity", and none of the HCM 2000 procedures for converting potential capacity to movement capacity are used.) Where shared lanes exist for turning movements, an extra calculation is required to derive the potential (movement) capacity of the shared lane. This step combines the potential capacities for two or three turning movements (depending on the lane configuration) into a single potential capacity. The final capacity of the stop sign-controlled approach is then the sum of all the potential capacities calculated for the turning movements.

Step 1: Calculate Potential Capacity of Each Turning Movement

The procedure starts with HCM Equation 17-3. It first calculates the potential capacity for each of the movements on the stop sign-controlled approach:

Where:

Table 11. Default Values for Calculating Potential Capacities (C_p,x)
of Stop Sign-Controlled Highways

Table 12. Default Values for Conflicting Flow Rates

Clearly, the major assumption in this Step is that of the conflicting volumes. Basically, it is assumed that the intersection is composed of a higher class facility in one direction and a lower class facility in the other. Many stop sign-controlled intersections, particularly those with all-way control, are comprised of similar class facilities intersecting. However, without detailed knowledge of such occurrences, the assumption is a reasonable default.

Step 2: Determine Approach Capacity

Several adjustments to potential capacity are defined in the HCM 2000 in order to derive "movement capacity". Most of the adjustments are based on knowledge of turning movements on and the geometry of all approaches of the intersection. Since none of these data are available in HPMS, major assumptions would be required. As an alternative, the movement capacity is set equal to potential capacity for most cases, except for shared lanes. The adjustment for shared-lane capacity is essential for calculating intersection approach capacity. HCM Equation 17-15 is used in these cases:

Where:

Volumes for the turning movements on the intersection approach in question must be made in order to apply Equation 30. It is assumed that 10 percent of the approach volume is composed of left-turning vehicles and 10 percent composed of right turning vehicles (recommended defaults from Chapter 10 of HCM 2000). Total approach volume is calculated as:

Where:

Values for V_x are then computed for left turns, through movements and right turns by applying the assumed turning percentages to the approach volume: 10%, 80%, and 10%, respectively.

The number of lanes handling turning movements on the approach are then determined. This is based on Data Items 88 and 89 (Left and Right Turning Lanes) for urban sections and for rural sections where these data are coded. Table 3 shows how to handle each of the possible cases. The approach capacity is based on Equation 30 for nonshared lanes and Equation 31 for shared lanes. In some cases both terms are used. Note that C_A is adjusted to account for the number of lanes handling each movement. Where HPMS indicates "multiple" turning lanes, it is assumed that two such lanes exist. For rural sections - or for any sections where Data Items 88 and 89 are coded as zero - it is assumed that no exclusive left turn or right turn lanes exist.

Table 13. Determining Stop Sign-Controlled Intersection Approach
Capacity (C_A)

Where:

Step 3: Set Peak Capacity to Approach Capacity

For HPMS, the Peak Capacity of the section is then based on the approach capacity of the stop sign-controlled intersection analyzed above. For urban sections, since the HPMS definition of urban capacity is one-way, the Peak Capacity is set equal to the approach capacity. For rural sections, it is based on the total number of through lanes (Data Item 34). This is done to account for the definition of capacity for 2- and 3-lane sections in HPMS, which is two-way capacity.

Rural 4+ through lanes

Peak Capacity = Approach Capacity

Rural 3-through lanes

Peak Capacity = Approach Capacity * 1.67 (assumes that the direction with two lanes is the inventory direction)

Rural 2-through lanes

Peak Capacity = Approach Capacity * 2