Analyzing a Corridor
Previously we went into what the warrants for signals were and went through some examples. So now let’s look at all the signals on 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. Once again here is the chart for vehicle warrants:
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.
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.
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.
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.
The need for a traffic control signal shall be considered if an engineering study finds that one of the following criteria is met:
- [not relevant]
- 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.
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.
The next article will look at warrants and justifications for traffic control devices other than signals.