| HPMS Item 87 (No. of Peak Lanes) causes some states some trouble. For most sections, the item is intended to identify the number of peak-period lanes in the peak direction. The exception is for rural 2 and 3-lane roads. For these roads, the item should give the total number of lanes in the peak period. A problem arises on 2 and 3-lane roads that run in and out of rural and urban areas. Some states currently code this item with the total number of lanes on both rural and urban sections of these roads. This coding of urban sections presents a problem to both HPMS and HERS. Consider a 2-way 2-lane urban section with peak lanes coded as 2. HPMS calculates the one-way capacity of this section on the basis of two lanes, and doubles this value to get total capacity on the section. This is the only capacity value used by HPMS. HERS develops three values of capacity: one for the off-peak period (which does not use Item 87); and two for the peak period. For peak-period peak-direction capacity, HERS performs the same calculation as HPMS. For the opposite ("counterpeak" or "CP") direction, HERS makes its own estimate of the number of lanes in this direction, and then uses this value to calculate CP capacity. If the peak-period directional factor (DFAC) is less than 1.0, HERS will usually assume that there is one lane in this direction, but, in some case (e.g., no peak-period parking allowed on either side of the road) it may assume that there are two lanes in this direction. In either event, HERS significantly overestimates total peak-period capacity on the road. (In the case of two-way four-lane urban sections that have been erroneously coded as having four peak lanes, the effect may be somewhat worse. HERS will usually assume only one lane in the CP direction. This is likely to produce a significant underestimate of capacity in this direction and may result in HERS deciding to add lanes to the section well before they are needed.) There are several possible fixes for this problem: 1. State HPMS staff can take greater care in the coding of Item 87. 2. The State can create a simple data-checking program that identifies urban sections that are is coded as having DFAC < 1.0 and the same number of peak lanes as total lanes. This is a likely indication of an error, though, it is also possible that some of these sections actually have two lanes in the off-peak period and three in the peak period (in which case the section is properly coded). If the state determines that an error exists, it can then recode Item 87. 3. The HERS-ST GUI can be revised to detect such conditions and then inform the user of a possible error and allow the user to fix the error. 4. The GUI can be revised to detect these conditions and automatically change the value of Item 87 for sections with four or more lanes. For sections with only two through lanes, a totally automatic change is inappropriate, since sections with the above condition may include two-lane sections that have three lanes in the peak-period (and thus are properly coded as having two peak lanes in the peak direction). However, it is also possible to provide a switch to allow the user to request such an automatic change for two-lane sections as well, assuming that the user is aware of the possible miscodes that would result for sections with three lanes in the peak period. The first two of the above options would have the advantage of producing better data to HPMS. |
