Sunday, September 29, 2013

Signal Timing Manual, 2nd Edition editing - Benefits of short cycle lengths

Nearly every traffic engineer that has a shred of experience with research knows about Webster and the concept of an "optimal" cycle length at signalized intersections. The research completed by Webster is almost 50 years old yet the traffic signal timing tools that we use continue to use these days are based on these traditional methods. As I am reviewing the 2nd Edition of the Signal Timing Manual as the Chair of the National Cooperative Highway Research Program (NCHRP) project, I was very pleased to see this thoughtful text by the authors (lead by colleagues at Kittelson & Associates, Inc.) 

critical intersection methods use Webster’s model to determine optimal cycle length, which has a de facto outcome focused on vehicle flow, not specifically considering the needs of other users, such as pedestrians, bicycles, and transit.

Bicycle traffic in downtown Portland (after a bridge lift)
There has been some work by my colleagues at the Texas Transportation Institute focused on the newer versions of the Highway Capacity Manual that focus on updating the 50 year old research and while that's good, the point of this post is to focus on the importance of policy. Policies should be carefully consulted in a cycle length selection process. The Signal Timing Manual, First Edition, highlights that in Chapter 2. Originally, that Chapter was met with some dismissive comments from the Panel of experts reviewing it. One professional cited too much use of the term "priority" meaning they didn't want transit to get special treatment. 

A community with a emphasis on multimodal transportation should select cycle lengths that respond to the needs specific to the intersection. Lower cycle lengths are better for multimodal transportation in general. Lower cycle lengths are also best with shorter block lengths because of the potential for longer periods of red to stack traffic up between signals. Downtown Portland has both the policy and the aforementioned physical attributes (I originally wrote constraint but corrected myself because many consider this an asset) , that necessitate a very short range of cycle lengths: 48- (midday), 56- (a.m. peak), or 60-seconds (p.m. peak). These short cycle lengths provide quick changes for pedestrians.
Pedestrians in a downtown environment benefit from short cycle lengths In Portland, there's progression for pedestrians too, but that's another post. 
 and limited progression for buses that stop every two, three, or four blocks. 

The progression speeds that follow from the short cycle length are slow to promote a safe environment  for people on bicycles to take the lane and feel comfortable doing so.   

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