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Climbing lane on a freeway section LoS procedure?

Climbing lane on a freeway section LoS procedure? was posted on August 6th, 2021 at 7:29 PM UTC
regarding Chapter 15: Two-Lane Highways

Greetings, I asked this question already on in hopes of increasing my chances for a reply, hopefully no one minds and I hope I'm in the right place to ask this question. If not, kindly accept my sincere apologies.

I'm fairly acquainted with the procedure from HCM 2000 that determines level of service for a two lane highway with a passing/climbing lane. However, recently at work I was assigned a task to determine LoS of a freeway section with a climbing lane (climbing, not a passing lane because the terrain is mountainous). The problem is that the two lane highway procedure relies heavily on parameters of the traffic flow from the opposite direction, whereas on the freeway the opposite directions are physically separated and have no immediate effects on each other, rendering half the procedure (or completely?) useless for freeway sections...?

I looked through HCM 2000 and 2010 but from what I could see there isn't a chapter or a procedure that deals with passing/climbing lanes on freeway sections.

So my question is, how do I determine the LoS of a freeway section with a passing/climbing lane? Can I modify the procedure for a two lane highway with a passing/climbing lane for my situation? Say do everything as in the procedure for two lane highways while ignoring the parameters regarding the traffic from the opposite direction? Or calculate everything as in the two lane highway procedure but only for the analyzed direction?

Hopefully I was clear and didn't make it confusing. Thank you for your inputs in advance.

Scott Washburn
University of Florida
RE: Climbing lane on a freeway section LoS procedure? was posted on August 11th, 2021 at 5:36 PM UTC

For your information, there is a new analysis methodology for two-lane highways in the HCM (version 6.1). In this methodology, the opposing flow rate is not considered for a passing/climbing lane segment. There are likely some similarities in how traffic would operate on a climbing lane segment of both a two-lane highway and a freeway, especially if the freeway also consists of just two lanes in the subject direction. In this case, the two-lane analysis methodology may give you a reasonable ballpark estimate of how the freeway climbing lane section would operate. While the two-lane analysis methodology is focused on the service measure of follower density, you can still just extract the density (flow rate/avg speed) part of that calculation. Keep in mind that the speed-flow relationship for the passing lane segment is different from that used for freeways, but is somewhat similar to that for a multilane highway.

However, if the freeway climbing lane segment consists of 3 or more lanes in the subject direction, it is probably not reasonable to try to adapt the two-lane analysis methodology to this situation. The lane segregation behavior of the non-climbing lane vehicles may result in significantly different parameter relationships (e.g., speed-flow) than for the 2-lane situation. Unfortunately, our committee has not previously had access to data from such freeway segments. While the current freeway analysis methodology does allow you to consider upgrades, it is not based on the assumption that trucks are expected to restrict themselves to a single lane on the upgrade. But this approach would probably still provide a better estimate than from adapting the two-lane highway approach.

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