Occassionally there's time during the day to review the Institute of Transportation Engineers' Listserv. This is one of the greatest information exchanges that happens in the industry, which is a bit concerning because anyone can participate. That being said, I took an excerpt of a question by Sam Schwartz who is former Commissioner of NYCDOT.
Speed limits are one of the most challenging topics in the transportation industry and is closely linked to the enforcement by police. It's really an important part of any strategy to improve safety in a community.
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NYC politicians and bike/ped advocates have been calling for a law lowering the city-wide speed limit to 20 mph from the current 30 mph. I have been warning that it may have little effect if all that is done is pass a law and post signs. I also have warned that it could create other issues including new crashes and more people breaking the law. I have been vilified by many for taking this position. I'd like opinion of peers.
I cite a quote from one of the studies "Expert System for Recommending Speed Limits in Speed Zones" published in November 2006 by researchers at the University of North Carolina Highway Safety Research Center for the NCHRP, TRB, and the NRC.
"Artificially low speed limits can lead to poor compliance as well as large variations in speed within the traffic stream. Increased speed variance creates more conflicts and passing maneuvers, which can lead to more crashes."
Also from "Effects of Raising and Lowering Speed Limits on Selected Roadway Sections" published in January 1997 by the Federal Highway Administration:
"Lowering or raising the posted speed limits at the experimental sites had little effect on driver behavior as reflected by the 85th percentile speeds. Lowering the speed limit by 5, 10, 15, or 20 mi/h (8, 16, 24, or 32 km/h) at the study sites did not result in major reductions such as 5 mi/h (8 km/h) or more in the 85th percentile speeds. "
Thoughts?
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Samuel Schwartz P.E.
President & CEO
Sam Schwartz Engineering,PLLC
New York NY
sschwartz@samschwartz.com
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Sam-
This is similar to the change & clearance interval debate. A good number of research efforts have been completed (mostly by graduate students in 12 month programs with good intentions) that show if you increase the yellow time, there is a resulting improvement in reduced crashes. The problem with these studies in my opinion is that it doesn't necessarily track the human behavior and people learning over time. The Minnesota DOT has the largest study of the effect of clearance intervals I could find when my team was writing the FHWA Signal Timing Manual. My point in this is that there are situations where research is flawed because of the relatively short duration of the study. That wasn't what you asked about, but it is a frame that I use to consider these big picture questions. This is also the sort of thinking that I believe makes policymakers think that we are irrelevant at times (kudos for your article on this, excellent by the way).
In Portland, we have some data (that we're going to write up) that shows that the change of speed limits does have an effect over time. I am not suggesting that we're going to dispute the old adage that just changing the speed limit results in a change of speeds, but our experience on some corridors is that by changing the speed limit, it begins a process of changing striping requirements and the multitude of other factors, which may change the nature of the street and the physical elements along the corridor over time. These changes don't occur over a one year period, but over a longer horizon (depending on the pace of development, level of activity, enforcement, etc).
So while I agree that a unilateral change in the speed limit may have little effect over the scope of a 18 month research project, our data suggests that there are cases where setting appropriate speed limits may be cause / reason to support a policy decision to reduce the speed limit that would affect enforcement, engineering, etc in an attempt to meet the goals of the community.
Now, as for 30 mph to 20 mph, I think when you're reducing down to 20 MPH that's quite a low speed that is appropriate on very low volume streets, but not sure how well it would work in an urban context on busy streets. Playing devils advocate now with that, if you have a dense traffic signal spacing pattern then setting progression speeds for 20 MPH is very possible and is something that Portland has had for the past 20 years. Here's an example of that in the form of a video.
Best,
Peter
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Peter Koonce
Principal Engineer
City of Portland
Portland OR
peter.koonce@portlandoregon.gov
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