May 5


Rethinking Traffic Signal Warrants

By Mike Spack

May 5, 2016

Highway Capacity Manual, Manual on Uniform Traffic Control Devices, traffic signals

By Bryant Ficek, PE, PTOE

Martin Bretherton, principal engineer at WMB Engineering, 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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Bryant Ficke Bio

  • 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

  • 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.

  • I think the “Cost of the past three years’ worth of crashes at the intersection.” misses a BIG issue. Traffic signals result in more crashes after installation. A signal is not a safety device, it is a traffic control device. To improve side street access it shifts a HUGE crash risk to main road drivers. I do like the idea of revisiting Warrants, but certainly a more comprehensive consideration than those noted on your proposal. I also suggest a life-cycle costs as municipalities need to know long term budget commitments as well as initial installation.

  • I would also love to see an update to Warrant 4 in Chapter 4C along with corresponding modifications to Chapter 4F- Pedestrian Hybrid Beacons and I’ll throw in a discussion of Chapter 4E – Pedestrian Control for good measure.

    First, I think lower thresholds in Warrant 4 would align with the rest of the world’s commitment to provide a transportation system for people, not just an efficient system for moving cars. The typical separation requirements for our traffic signals are based on optimum vehicle progression goals (i.e. drive at a constantly high speed without stopping) and do not create a safe environment for walking or biking along or across our roadways. Lowering the thresholds to promote pedestrian safety would allow for introduction of regulatory devices like pedestrian signals that pay more than lip service to our commitment to yield to pedestrians in marked crosswalks. Corresponding reductions to the threshold in Chapter 4F for pedestrian hybrid beacons would allow for installation of these devices in more locations. At least in Florida, these are very uncommon and because of that our roadway owners are reluctant to use them. Lower thresholds would allow for more installations, which would promote familiarity and trend to more frequent usage and an overall safer system for vulnerable users.

    Second, it would be great to see America move to the more intuitive conventions used in Europe where a Walk signal with a countdown timer in white indicates remaining time available to cross and don’t walk signal with a countdown timer in red indicates remaining time until you will be given opportunity to cross. I speculate that Section 4E.02 is only necessary because without definitions no one would understand our current system.

  • NC DOT has almost exactly this in its Intersection Life Cycle Cost Comparison Tool. Developed by the Institute for Transportation Research and Education (ITRE) at NC State, the construction, maintenance, fuel, travel time, volume growth, safety, and crash costs are all monetized. It will analyze for the entire 24 hour period, not just peak period, using inputs of AADT, peak hour counts, or 13 hour counts and extrapolating as necessary.

    As an aside, another challenge with the MUTCD warrants is that because the origin is unknown, it is difficult to understand how to apply them at intersections with fewer than 4 legs. Researched just wrapped up in North Carolina developed guidance for signal needs for intersections with two or three legs (commonly found at RCUTs & one-way street intersections) based on anticipated 95% queue lengths.

  • Bryant:

    I am a member of the TRB Highway Capacity and Quality of Service Committee and chair the unsignalized intersections subcommittee. We have been discussing the relationship between the HCM level of service and the traffic signal warrants in our meetings over the past few years. Many HCM users find the gap between acceptable level of service and the warrant for installation of a traffic signal troubling. I like your ideas concerning an economic evaluation to help set the warrant threshold. Do you have a white paper or anything which could be presented at our summer meeting in Austin Texas June 20-23? Our subcommittee meetings are usually on Thursday or Friday of our summer meeting. We are always looking for presentations or material to discuss our issues.

    The subcommittee is also looking at measuring gap acceptance and appropriate gap acceptance at intersections and roundabouts.

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    Mike Spack

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