HERS-ST Discussion

By HPMS definition, a continuous left turn lane is not considered a median (Item 33). Generally in the real world, there are no median options available for segments that contain a continuous left turn lane, meaning that a roadway with a couple travel lanes and a continuous left turn lane usually doesn’t have a median. For the purpose of capacity analysis, the continuous left turn lane is probably nothing more than an unprotected (painted) median, though it occasional serves as a mid block left turn lane or a temporary acceleration lane.

It seems that when the continuous left turn lane is ignored, than the system has no median and no traffic separation. I would think that this would have an impact on the capacity calculations. The benefit of coding a continuous left turn lane as a median would be that it would provide some traffic separation, even though unprotected, and subsequently be instrumental in the capacity calculations. There is a minor problem generated due to the increased conflict points associated with either a left turn lane or an acceleration lane. But these might be minor. Any thoughts?????

On sections where signals exist, the type of median (raised/barrier/continuous Two-Way Left Turn Lanes {TWLTLs}/none) is assumed not to affect capacity -- the signal itself is the controlling feature and will have a much lower capacity than any midblock section.

However, there are many cases where a TWLTL exists in rural settings and/or on sections without signals. In these cases, the presence of a TWLTL is treated the same as a divided highway (i.e., raised median or positive barrier). So the separation of head-on traffic provided by a TWLTL is accounted for in the new capacity procedures (revised Appendix N of the HPMS Field Manual; see "rural mulitlane highways").

--Rich

On sections where a signal exists, the presence or absence of a median or two-way left turn lane (TWLTL) is assumed not to affect capacity. This is because the signal controls the capacity of the section, i.e., the capacity of the signal is lower than any midblock section.

However, there are many cases where a TWLTL exists in rural settings or on a section without signals. In these cases, the "Multilane Highways" procedure is used (applies to both rural and urban conditions). In this procedure, a TWLTL is treated the same as a raised median or other positive barrier. So the positive effects that a TWLTL has on separating head-on traffic is accounted for in the new procedures.

--Rich