Previous articles of mine have covered 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 it’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.
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.
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, one previously suggested here as unnecessary and was recently rebuilt.
Our “X” data points are 313,300,443,341 and our “Y” data points are 529,518,719,662.
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.
Also available from the site are peak hour intersection diagrams. For warrants, a turn lane counts as an approach lane if half the traffic is turning so this is where we learn whether to count turn lanes or not (as well as giving a graphical sense as to how the intersection is being used. Bloomington Ave and 46th St only have two lanes, but this data becomes relevant at other intersections when determining which line to use.
For the second signal, lets look at Hiawatha Ave and 46th St.
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.
This one exceeds warrants so far we had to blow up the graph. As a side note since it’s not worth a separate article and we’re talking about this intersection, it’s evident it has horrific congestion as well as pedestrian unfriendliness; here’s something I drew up a while ago plopping down the tight diamond interchange at MN 7 and Woodale Ave. People in cars on Hiawatha Ave get a straight shot through the area, people walking or on bicycles get narrow ramps instead of a huge road to cross, and it’s an excuse to take out the one-story buildings for something more urban. The right turns are necessary for trucks because of the skew of the intersection, but they could be signalized.
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.
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 a minimum 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.
Part Two of this series will systematically analyze every signal on Lyndale Ave and comment some more on Lyndale Ave and 25th St. and Part Three will cover warrants and justifications for other traffic control devices.