Rethinking Traffic Signal Warrants

Martin Bretherton and I had an email exchange a while ago discussing the traffic signal warrants in the Manual on Uniform Traffic Control Devices (MUTCD).  We sell a spreadsheet to run the calculations and in researching the curves for some of the warrants we confirmed Martin’s criticism.  Mainly, the warrants are a 50+ year old historical remnant that are not based on research (there is no mathematical equation for the four hour and peak hour warrants that we could find – we had to make them up to match the line on the graph).

It’s time to rethink the traffic control warrants in the MUTCD.  Martin proposes going to more of an economics based model and I agree with him.

We could develop a mathematical, economics based model that incorporates the following criteria:

  • Cost of the past three years’ worth of crashes at the intersection. We have costs for different types of crashes from fender benders all of the way up to fatal crashes at a national level and each state could even fine tune their values each year.  The factor related to crashes should be a research based value that society would save by changing the traffic control vs. the arbitrary criteria of five correctible crashes over the past three years.
  • Cost of the peak hour delay with a traffic signal vs. roundabout vs. all way stop sign vs. side street stop sign control. This could be based on calculations run per the Highway Capacity Manual.  I think we’d run both the a.m. and p.m. peak hours for a typical weekday.
  • An analysis of the gaps available for the minor crossing street. MnDOT has a procedure for factoring in a gap study and I think this is a nice piece of data to add to the decision.
  • A factor based on the Daily Traffic Volumes of the crossing corridors. This would likely take some additional research, but I think we could quickly come up with a more rational criteria than the eight hour warrants to account for traffic control being reasonable for most of the day.  The existing eight hour warrant thresholds appear to be made up and not based on any research.
  • Like the Highway Capacity Manual level of service calculations, we could add in adjustment factors such as being in a Central Business District, near a school, near a railroad crossing, pedestrian activity, etc.
  • Factor in a construction estimate for the intersection treatment.

With the above criteria, we could determine whether or not the proposed traffic control will cost or save society xx dollars over xx years (the life cycle of the treatment?).  Then that gets factored into the justification process instead of the old warrants.

The warrants in the MUTCD were a great 1950s solution.  Thankfully we’re not stuck using slide rules anymore.  The methods in the Highway Capacity Manual have evolved to take advantage of research and computing power. It’s time to modernize the warrants in the MUTCD, including adding warrants for those newfangled roundabouts.

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4 thoughts on “Rethinking Traffic Signal Warrants

  1. You are right about the warrants being outdated, the first 6 were develop after WWII when there were few actuated signals and the roadway user costs, signal construction costs and maintenance costs were high.

    However, instead of the using the costs for just delay (which is typically only measured at the stop or yield line) I would suggest using total lost time caused by the traffic control selected and attendant intersection design. This is particularly important when one is looking at a junction on a 55 mph highway with over 25,000 ADT and heavy commercial traffic approaching 20% with a low speed intersecting roadway. Considering only delay at such a location may make a roundabout look overly attractive. The lost time for a roundabout even without a stop (no delay), for a mainline truck would be nearly a minute and for a car would be over 20 seconds. A properly designed and timed signal would allow much of that high speed traffic to not encounter any added travel time. However, in this type of location an RCUT may be the best answer.

    • Thanks Denny – good thoughts as usual! Adding in the lost time to the criteria is very rational. Mike

  2. I like that these warrants are being reviewed. I’d like to add two points to consider.

    First, the warrants should have some flexibility such that, at least in some or maybe many cases, more than one intersection control may be reasonable (think yield versus stop signs, or roundabout versus traffic signals). This allows engineers and our customers, the public, a bit of flexibility to deal with specific situations. If we do not have this flexibility, we set ourselves up for a fight with the public.

    Second, when using crash stats in warrants, please include flexibility to take action in absence of crash stats, or because of a risk analysis. If we do not, sometimes we put ourselves in the position of waiting for a serious crash before we take action.

    For example, the Protected-Only Left Turn warrant is a good tool, but sometimes because of speed, volume, and number of opposing lanes, the risk of a collision and its consequences is enough reason to install the Protected-Only Left Turn phasing rather than waiting for the inevitable serious crash.