Warrants and Justifications for Traffic Signals

October 4, 2016 at 10:26 pm | Posted in Traffic Signals | Leave a comment

Other articles of mine cover just about every facet of traffic signals, including how they work, how timing and phasing are set, and even how to identify who made a particular signal. But what about how it’s decided whether to add or remove a signal in the first place? That is done through “warrants” and “justification.”

The MUTCD (Manual of Uniform Traffic Control Devices) is the engineer’s bible for traffic control, so of course it is the basis for whether a signal is needed or not. Minnesota has warrants-01it’s own local version of the MUTCD, but it’s mostly copied from the national MUTCD, and the section on signal warrants is identical.

The following is a primer on the language used in the MUTCD:

Shall means something is an absolute requirement. For example, the colors of traffic signals cannot vary, evenif a city wants to honor Prince and put purple lenses in their traffic signals (although purple was a very early pedestrian indication).

Should is a very strong suggestion for items less acutely dangerous than things covered by shall, but supported by years of engineering experience and studies.

May is an option. You don’t have to put up a sign saying there’s a McDonald’s at the next freeway exits, but standards exist if you want to.

The MUTCD Speaks:

So lets see what possible warrants there are for traffic signals (in all of these, emphasis mine).

An engineering study of traffic conditions, pedestrian characteristics, and physical characteristics of the location shall be performed to determine whether installation of a traffic control signal is justified at a particular location. The investigation of the need for a traffic control signal shall include an analysis of factors related to the existing operation and safety at the study location and the potential to improve these conditions, and the applicable factors contained in the following traffic signal warrants:

Warrant 1, Eight-Hour Vehicular Volume.

Warrant 2, Four-Hour Vehicular Volume.

Warrant 3, Peak Hour.

Warrant 4, Pedestrian Volume.

Warrant 5, School Crossing.

Warrant 6, Coordinated Signal System.

Warrant 7, Crash Experience.

Warrant 8, Roadway Network.

Warrant 9, Intersection Near a Grade Crossing

The satisfaction of a traffic signal warrant or warrants shall not in itself require the installation of a traffic control signal

The last sentence is extremely important. Just because the warrants are met does not mean a traffic signal will be installed, or even that it’s necessarily a good idea. If the intersection meets objective warrants, a subjective, common sense call based on engineering judgement is made to see if it is “justified.” Every justified signal is warranted, but not all warranted signals are justified.

Although these aren’t too hard to understand in English as opposed to “engineer-speak,” I’ll explain a few nonetheless. The first three are vehicle volumes at given times of day. To meet the warrant it has to meet the threshold for at least eight, four and one hours in a 24 hour period. It doesn’t matter which ones, or even if they are consecutive. Warrant 1 is the normal one used to warrant a signal, and the MUTCD is clear Warrant 3 is only to be used for unusual situations:

This signal warrant shall be applied only in unusual cases, such as office complexes, manufacturing plants, industrial complexes, or high-occupancy vehicle facilities that attract or discharge large numbers of vehicles over a short time.

Warrant 8 is if you want to encourage traffic to use certain streets to try and establish a hierarchy where none exists (say encourage traffic to use a collector as opposed to a local street to exit a neighborhood), and the signal doesn’t already meet other warrants, and Warrant 9 is if a stop or yield sign on an intersection near railroad tracks is causing queued traffic to back up over the tracks.

Analyzing Some Signals

It’s been suggested that Minneapolis has too many signals, some of them perhaps unwarranted. So I thought it would be illustrative to do my own “warrant analyses”, starting with vehicle volume, the normal one that is used. In many cases only six hours of data is available from the city Traffic Management Center web site, which means Warrant 2.

Here is the chart for Warrant 2. If the plot is at or above the red line for any four hours of the day, the warrant is met.

warrants-02

Here are the charts for Warrant 4, pedestrian volume. The first chart is any four hours in a day; the second is any one hour, if either threshold is met the signal is warranted.

warrants-03warrants-04

Bicycles get counted as vehicles or pedestrians, depending on if they’re on the road or on the sidewalk.

For the first signal, let’s look at Bloomington Ave and 46th St in Minneapolis

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46th Street and Bloomington Ave, Minneapolis

Our “X” data points are 313,300,443,341 and our “Y” data points are 529,518,719,662.

warrants-07

Since all the data points fall above the red line, this signal is warranted. But is it justified? Maybe not, since it doesn’t exceed it by much (it’s still on the chart), and since the original article reported it functioned fine with a temporary stop sign. Maybe this was a conversation that should have happened before money was spent rebuilding it.

For the second signal, lets look at Hiawatha Ave and 46th St.

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Hiawatha Ave and 46th Street

Commons sense would tell us that this one is warranted, but lets be sure. Our “X” data points are 2843, 2836, 3187, 2910; our “Y” data points are 629, 663, 720, 741.

warrants-10

This one exceeds warrants so far we had to blow up the graph.

For our third signal, let’s look at Grand Ave S, and W 34th St.

I saw this one browsing around on the TMC web site, and thought “how in the world could this possibly be warranted? Our “X” Values are 70, 150, 152, 176, and our “Y” values are 23, 36, 29, 22.

warrants-14

This one is so far from meeting warrants it’s off the plot area to the bottom left, even if it still fits on the image. But shouldn’t we have a signal here because of the school? Well, let’s check Warrant 4 to see if it’s warranted for pedestrians. Our X values are the same and our Y values are 8, 11, 8, and 5. This is so low I’m not going to even try graphing this one.

But what about Warrant 5, because it’s near a school? Well, it still has to meet a certain, albeit lower, pedestrian count:

01 The School Crossing signal warrant is intended for application where the fact that schoolchildren cross the major street is the principal reason to consider installing a traffic control signal. For the purposes of this warrant, the word “schoolchildren” includes elementary through high school students.

02 The need for a traffic control signal shall be considered when an engineering study of the frequency and adequacy of gaps in the vehicular traffic stream as related to the number and size of groups of schoolchildren at an established school crossing across the major street shows that the number of adequate gaps in the traffic stream during the period when the schoolchildren are using the crossing is less than the number of minutes in the same period…and there are aminimum of 20 schoolchildren during the highest crossing hour.

03 Before a decision is made to install a traffic control signal, consideration shall be given to the implementation of other remedial measures, such as warning signs and flashers, school speed zones, school crossing guards, or a grade-separated crossing.

 So it doesn’t appear that this signal meets Warrant 5 either, and there’s no reason this signal shouldn’t be removed.  I’d even make the city an all cash offer for the old Eaglelux signal heads like they have on this intersection, the kind  desirable on the collectors market that they currently throw into the scrap bin.

Why so few kids is an interesting question; the data was collected on from what I could tell was a warm, dry fall school day. (Wed Oct 12, 2011). Are kids living a half-block away put on a bus to go to some far-flung school to achieve racial balance or attend a magnet school? Are parents giving kids a ride in their car? Are they riding bicycles on the street and thus being included in the vehicle count? Is the data somehow misleading? I don’t know, but the point is that the official data does not warrant a signal here.

I’ll also comment on Lyndale Ave and 25th St.

After a car vs pedestrian crash that was caught on a security camera and made the local news, there were calls that a signal was needed. As it turns out, data for this intersection exists. The city response to calls for a signal were “warrants aren’t met,” and I confirmed that to be true. Traffic on 25th St would need to double to meet vehicular warrants, and with five pedestrians crossing Lyndale Ave in an eight hour period, it’s even farther from meeting pedestrian warrants. If there is a particular, unusual crash problem at an intersection that could be corrected by a signal, that’s another story (Warrant 7), but skimming the crash data in the neighborhood suggests this doesn’t seem to be the case.

warrants-15

Now, to get a sense, let’s analyze all the signals in a corridor. I picked South Lyndale Ave (with the exception of the signal at the ramp to MN 62, which is Mn/DOT operated. I’m going to cheat and just say that one is warranted rather than try to get data). This corridor has many busy intersections, some not so busy. Some signals have been there forever, and some have been recently reconstructed.

warrants-16

So the  Four Hour Vehicle Warrant, the signals at the I-94 Ramp, Franklin Ave, 22nd St, 24th St, 26th St, 28th St, Lake St, 31st St, 35th St, 36th St, 38th St, 40th St, 46th St, 50th St, 54th St, and the west ramp of MN 62 meet vehicle warrants.  33rd St, 34th St, 43rd St, 48th St, Minnehaha Pkwy, 53rd St, 56th St, 58th St, and 61st St do not.

A few random notes:

1) I didn’t note what times I took data for because it’s not important beyond being the highest time available, it was generally 7:00-9:00 AM and 4:30-6:30 PM, provided data existed for those hours. If the PM rush hour wasn’t available, the noon “mini-rush hour” was. Asterisks are where Lyndale Ave is the side street.

2) The volume required to meet warrants I got from eyeballing the graph. Real engineers use complicated spreadsheets with macros. Undoubtedly if you’re a programmer (which I most certainly am not) you’d be able to scrape data from the TMC web site and plug it in to the warrant spreadsheets.

3) 22nd was the only intersection that also met pedestrian warrants; I didn’t include the data for that here but I looked at pedestrian warrants if it looked like there was a chance of meeting it.

4) I was a bit surprised by how few pedestrians were crossing Lyndale Ave at Minnehaha Parkway, nowhere close to meeting pedestrian warrants,  only 34 in the four hour-long periods. But when I’m there it’s always on my bicycle on a nice summer weekend, not a typical rush hour when traffic is counted.

5) The 4-3 conversion south of Lake St. magically made some signals warranted that were not before.

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Here’s a map where I’ve indicate warranted signals in green, and those that do not meet the warrant  in  red.

Why no Warrants?

So why are there warrant-less signals? Warrants have existed since the beginning of the MUTCD in 1935. So “warrants didn’t exist then” isn’t an excuse. Sometimes a signal was warranted at one time, but traffic patterns have changed. This is particularly startling in Detroit, the archetype for urban evisceration, where a lot of traffic signals aren’t needed simply because no one is around anymore. But patterns change in other cities too. Usually, whenever local residents hear a traffic signal may be removed, local residents are strongly opposed, predicting all kinds of death and mayhem.  So cities are reluctant to be too motivated to remove them; normally it comes up when continuing to operate the signal would involve spending money because the signal needs replacing due to deterioration or road reconstruction.

warrants-16

Eaglelux “Tall Fin”, Minnehaha Ave and 46th

Sometimes too, cities just do whatever they feel like. Just about every signal on Michigan Ave. in Chicago has a configuration that is a direct violation of the national MUTCD, prohibiting left turns with a sign instead of a red arrow. And Washington state for years used a flashing yellow ball to indicate permitted left turns.

A Notorious Intersection

Going back to 25th St and Lyndale Ave, where a widely publicized car vs pedestrian crash was caught on security cam, it’s hard to make a judgement about who was immediately at fault for the crash. State statute 169.21 says basically that motorists must stop for pedestrians in unmarked crosswalks, but pedestrians must not cross if a motorist cannot reasonably stop. That the pedestrian said it was her fault is not relevant, and all the public has is grainy, zoomed in CCTV footage.  So rather than point fingers, lets look at potential ways to fix the underlying problem. Engineers are very pragmatic; if there’s a reasonable engineering way to fix a problem that won’t create other equal or worse problems, they’ll try to fix it.

Lyndale Ave is above the threshold where a 4-3 road diet is workable so we’re stuck with the 4-Lane Death Road. Generally speaking, anything over 15,000 to 20,000 vehicles a day isn’t workable because congestion overwhelms the road, and if there’s a lack of a hierarchical road network, motorists start using local streets instead (this part of Lyndale is at 22,000). Motorists become angry and impatient, thus making poor and even reckless decisions zipping down local streets that weren’t designed for them and where residents may not expect. This seems to be a recipe for disaster.

If you look only at the arterial, the costs of congestion might be worth the benefits of safety on the arterial. But dealing with motorists diverting onto local streets adds a lot of costs and complexity. As we’ll find out later, stop signs (to say nothing about “Slow, Children” signs and similar) are completely inappropriate, ineffective, and even dangerous. Thus they’re very strongly discouraged in the MUTCD for traffic calming or speed control. And when you start stationing police there on a semi-permanent basis or making physical changes like traffic circles, diverters, and chicanes to each and every local street, soon you’re talking about a lot more money than some new paint on the arterial.

I’d also suggest that despite the tendency toward the reaction to blame suburban commuters for all the cars on the streets, Lyndale Ave is probably almost entirely Minneapolis residents. Despite the horrific congestion on our freeways, it’s simply not attractive for suburbanites heading home from downtown to try to get off and navigate city streets, especially with the MnPass lanes available for people that are unlucky enough to have to drive in rush hour regularly. Traffic counts would seem to back me up. There’s over 30,000 vehicles a day north of Franklin. Half the traffic is gone by the time you get past Lake St, and on the MN 121 entrance to I-35W and MN 62, there’s only 7000, and only a subset of that would have traveled the entire length.

warrants-17

Lyndale Ave at 25th St.

So what else can be done?  Legally it’s a crosswalk (despite the media calling it jaywalking) and there’s not a ‘No Pedestrians’ sign. But the lack of a painted crosswalk, and the lack of even curb cuts, sends a message to both motorists and pedestrians that this isn’t a place for pedestrians to cross. Adding marked crosswalks and curb cuts would be a good place to start. Maybe there’d even be space to build a pedestrian refuge island. But what if we could do even better and make motorized traffic just disappear for a short time to allow pedestrians to cross? We’ve talked about a lot of the warrants, but here is the final one: Warrant 6: Coordinated Signal System:

Progressive movement in a coordinated signal system sometimes necessitates installing traffic control signals at intersections where they would not otherwise be needed in order to maintain proper platooning of vehicles.

Standard:
The need for a traffic control signal shall be considered if an engineering study finds that one of the following criteria is met:

  1. [not relevant]
  2. On a two-way street, adjacent traffic control signals do not provide the necessary degree of platooning and the proposed and adjacent traffic control signals will collectively provide a progressive operation.

The Platoon

When you have a number of signals, you can time them to create a “Green Wave” with platooning. The idea is that you want to keep the cars together and try to give them green lights as they arrive at each intersection. Spaces between platoons give pedestrians a chance to cross and motorists a chance to enter from side streets and driveways. Doing this in a stream of traffic would at best slow the traffic down, and at worst, it results in crashes when pedestrians and motorists get impatient and start to take chances.

In the name of research, I did something I normally try to avoid at all costs: drive surface streets at rush hour. Driving southbound at 6:00 PM, I tried to drive a steady 30 mph. North of Lake St I was unable to do that with the streets being swamped with traffic and turning cars and stopping buses. South of Lake St I was able to do it for the most part. I was stopped for seven out of the 21 signals between Franklin and 58th, in some cases arriving a few seconds too late or soon.

There’s obviously been some attempt at progression but it needs tweaking.  Minneapolis is way, way behind in terms of traffic signal technology and ideology; in the suburbs every one of the signals would have sensors for the main street (as well as flashing yellow arrows) and with advances in traffic control, the signals could figure out the queue length of the next signal and traffic speeds and adjust themselves accordingly. There was obviously no attempt to ensure platooning in both directions, leading to excessive delays when people want to cross at unsignalized intersections. There’s plenty of time when traffic has ceased in one direction, but it’s almost always flowing in the other. (It should be noted that this is where refuge islands would help immensely.)

So how do we set up a situation where motorists on Lyndale Ave can travel without hitting too many lights and motorists and pedestrians at non-signalized intersections get a chance to enter the roadway and cross it? An engineer I talked to, in typical engineer fashion, didn’t want to give an answer without having done a traffic study, but when pressed, the engineer commented that generally on a 30 mph urban street, a 1/2 mile spacing is enough; a source online suggested 1/4 to 3/8th mile. In order to enable a bi-directional green wave, they they should be as evenly spaced as possible, or failing that, in even multiples. (In no case can signals be less than 1000 feet; that’s not a problem on Lyndale Ave, but in downtown it’s best to just forget about it and turn all the signals green at once.)

Obviously some unwarranted signals would have to be removed, some warranted only by Warrant 6 kept, moved, or built. Maybe even some warranted signals might have to be removed–remember, just because you have a warrant does not obligate you to build or maintain a signal. But in the end, you make things better for all people, whether in a car driving down Lyndale Ave, in  a car trying to enter or cross, or on foot trying to cross. Even in an urban environment, transportation doesn’t have to be a zero sum game.

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