Star Sign up now to gain access to all the features of this website. NEW — access is now open to ALL USERS! — Already registered? Please log in.

Limitations on Chapter 13 methodology

Andrew S. Brewer
Arkansas Highway and Transportation Department
Limitations on Chapter 13 methodology was posted on May 2nd, 2011 at 10:56 AM EDT
regarding Chapter 13: Freeway Merge and Diverge Segments

I have several comments/questions regarding Chapter 13.

-Regarding the Step 2 computational step, Estimating the Approaching Flow Rate in Lanes 1 and 2 of the Freeway Immediately Upstream of the Ramp Influence Area: it appears that this method is intended for six-lane freeways. What about a diverge on a four-lane freeway with a merge immediately upstream (or vice-versa)? Wouldn't this be a limitation of the methodology and if so, should it be explicitely stated?

-A stated limitation of Chapter 11 (p. 11-9) is the influence of downstream queuing on a segment. Similar statements are made in Chapter 12 (p. 12-9) for the weaving methodology. Shouldn't this limitation also be explicitely stated on p. 13-7?

-Another statement on p. 12-9 is the limitation of the methodology on arterials or other urban streets (including one-way frontage roads). Does this limitation apply to Chapter 13? I know it says that "guidance is also provided to allow approximate use" on such facilities, but I think there needs to be further detailed clarification. There are instances where the user needs to determine the LOS of a merge or diverge on a frontage road or an arterial (outside the realm of the Chapter 22 methodology). Many times, at least in Arkansas, those instances will have driveways and/or signalized intersections within it's influence area, which is the intent behind the stated limitation in Chapter 12 I believe (i.e., not within a true uninterrupted flow area). At least, that was my intent whenever I made that comment during the HCM2010 draft comment phase and it was accepted.

-It doesn't appear that this methodology is intended for one-lane ramps (i.e., a merge on a ramp before merging onto the freeway mainline) and/or one-lane C-D roads (i.e., a merge onto a C-D road at a cloverleaf interchange)? If that's the case, can't it be explicitely stated somewhere? In such cases, I assume that the user should refer to Exhibit 13-10 for a capacity check, rather than trying to use this methodology to calculate density and LOS.

I didn't get a chance to review this chapter before it made final draft but I figure better late than never. I know this is small potatoes, but having clarification on the intended use of any HCM methodology is helpful for the practitioner. Also, I'm not clear as to whether it was intended for questions like these to appear in the discussion forum, or as an email through the "Interpretations and Errata" page, so I did both. Thanks.

Andrew S. Brewer, P.E. Senior Planning and Research Engineer Statewide Planning Section Planning and Research Division Arkansas State Highway and Transportation Department (501) 569-2063

Paul Ryus
Kittelson & Associates, Inc.
RE: Limitations on Chapter 13 methodology was posted on May 24th, 2011 at 3:41 PM EDT

Hi Andrew,

It's appropriate to post interpretations questions here (and your questions were forwarded to the Highway Capacity Committee's User Liaison subcommittee shortly after they were posted), but be aware it takes time (up to six months) before an official interpretation is approved by the Committee, if one is required. Since no one else has posted a response to your questions, here are my unofficial thoughts on your questions:

  1. The ramps method is applicable to freeways with 2-5 lanes per direction (page 13-5). The method provides three equations (Exhibit 13-6) for estimating the proportion of traffic in the rightmost two lanes of the freeway for a six-lane freeway (3 lanes in each direction), based on the presence or absence of upstream and downstream ramps. For four-lane freeways, no calculation is needed, as 100% of the traffic is by definition in the rightmost two (i.e., only) lanes of the freeway. For eight-lane freeways, the original research had insufficient data to determine whether adjacent ramps influenced the distribution of traffic in the ramp influence area; therefore, the equations for eight-lane freeways in Exhibit 13-6 don't take adjacent ramps into account.

  2. Oversaturated conditions are discussed briefly on page 13-1 (next to last paragraph) and in the second paragraph on page 13-7, as well as in Chapter 10, but I agree that downstream queuing and queuing from ramp terminals should be expressly stated as limitations.

3 & 4. "The procedure may be applied in an approximate manner to completely uncontrolled ramp terminals on other types of facilities, such as multilane highways, two‐lane highways, and freeway C‐D roadways that are part of interchanges" (Page 13-7, first paragraph). It can be applied approximately to ramp-ramp merges and diverges within an interchange, including merges on C-D roadways within an interchange. A ramp terminal on a multilane highway or urban street that was influenced by traffic signals or driveways would not be "completely uncontrolled" and therefore could not be analyzed using the Chapter 13 procedure. (Arterial weaving has been a longstanding HCM user request; perhaps the Arkansas State Highway and Transportation Department would like to submit the Committee's research problem statement on the topic to AASHTO, perhaps also including arterial ramps?)

Sign in to add a reply. Don't have an account? No problem, sign up for free.