Category Archives: traffic signals

Four on Green Line / Green Lights

Updated 9/18/2014 with MPR article

Fred Melo writes in the Pioneer Press about Always Green Traffic Control “Green Line: Inventor proposes using timing, speed to improve travel time

“Ultimately, the decision whether or not to implement a system such as this along the Green Line would need to be made by Metro Transit’s project team,” said Kari Spreeman, a spokeswoman with the St. Paul Department of Public Works.

David Levinson, an engineering professor at the University of Minnesota who specializes in transportation issues, has blogged about “Always Green” on his website,

“It’s an interesting idea,” Levinson said. “Even if the total travel time is the same in both cases, it’d be better than going fast and then stopping. You might even save some time. After you stop, you have an acceleration-deceleration loss (in travel time).”

Levinson acknowledged one drawback, however. “It’s never been tested,” he said.

Tim Harlow at the Star Tribune writes in his column The Drive: Nick Musachio and the Always Green Traffic Control

David Levinson, a transportation expert at the University of Minnesota, says the Always Green Traffic Control has potential.

“I think it would work best for isolated intersections on rural expressways, but there is no reason it couldn’t work in an urban area,” Levinson said. “Static speed signs have been used for decades on Connecticut Avenue in Washington, D.C. Something dynamic should do even better. I do believe it warrants a field test.”

Musachio faces the challenge of getting somebody to do just that. He’s been bending ears of the St. Paul Public Works Department, but so far they have not bitten.

“In theory the system could work, but it has not been tested in a real environment. Until that happens, we won’t consider it,” said city spokeswoman Kari Spreeman.

Marion Renault at MPR writes “Inventor says synchronized lights could boost Green Line travel time” [I hope by “boost” she means reduce.]

“You can either change the lights to match the vehicles,” said David Levinson, a civil engineering professor at the University of Minnesota. “Or you can change the vehicles to match the lights.”

Levinson said keeping cars moving at a steady speed is optimal for traffic flow. But he said it takes a lot of coordination and a tightly-maintained fixed traffic system to create a grid of alternating, forward-moving platoons of cars and trains.

Jessie Bekker at MnDaily writes “Signs could speed up Green Line
The signs would suggest speeds to help drivers and light rail conductors make green lights on time and increase efficiency.

Though Musachio is certain the system would work “perfectly” with the Green Line, David Levinson, associate professor [sic] in the Department of Civil Engineering [double sic], isn’t so sure.

“The Green Line is all tied up in politics,” he said.

Levinson said the system warrants a field test but believes a rural or suburban traffic signal is the place to start before it moves to an urban area.

Previously here and here and here.

Frank-Lyn – Thinking about streets and places in three dimensions. |

The development at the intersection of Franklin and Lyndale Avenues in Minneapolis has gotten a lot of attention, but primarily because of buildings proposed at the corners, to replace under-developed buildings at this highly accessible, emerging locale.

The intersection itself has gotten little consideration. It is an at-grade 4-way traffic signal. However, Franklin Avenue finds itself in a valley at Lyndale, such that a 3-dimensional option presents itself.

Urbanists are often aghast at the notion of highway overpasses in cities, and certainly most have been done poorly with no respect for urban form. But that is no reason to throw out the concept altogether.

Using my extensive computer drafting skills, I present two diagrams. The Plan view (from above) and Side view (facing west) illustrate a concept in cartoon fashion. These are, as they say, not-to-scale and obviously not engineering diagrams.

Frank-Lyn conceptual reconfiguration -  Plan View
Frank-Lyn conceptual reconfiguration – Plan View

The top diagram shows how the middle two lanes on Franklin Avenue (the left lanes Eastbound and Westbound) bridge over Lyndale Avenue (the blue bar represents the bridge). Since there are already two lanes, additional land required is only for bridge barriers, and hopefully that is minimal. Lanes can be narrowed as necessary.

This does several things. It gets cross-traffic on Franklin (going to or from Hennepin mostly) off of Lyndale. This reduces pressure on Lyndale itself, reduces traffic delay, reduces pedestrian delay, reduces bicyclist delay, reduces pollution at the intersection, reduces street crossing times for pedestrians on Lyndale going North or South (there are two fewer lanes to cross). A median boulevard could be added to Lyndale (the green bars).

The intersection of Franklin and Lyndale thus becomes an urban diamond.

There would be an option to eliminate some or all left turn movements as well, and make Lyndale more Boulevard like with no at-grade cross traffic from Franklin. The intersection could be just right-in/right-out for motor vehicles. Pedestrians could be given a Hawk signal if they wanted to cross Lyndale, with a median refuge island. The purple bar shows this region.

The downside is making it more difficult to access businesses on Lyndale (e.g. The Wedge Co-op) which are already difficult to access by car.

But even if there were left-turns allowed, traffic would be much lighter at the intersection.

Franklin Avenue at Lyndale Avenue looking Westbound (side view)
Franklin Avenue at Lyndale Avenue looking Westbound (side view)

The second diagram shows a Side view / cross-section. The idea here is not the particular architecture or building heights, but to illustrate that just because there is a 2 lane overpass, the underside of the bridge can have a pedestrian serving business (that is no more than 26 feet wide). (This need not be a cafe, but in every urban rendering I have ever seen, there are cafes, so there must be a reason).

Previous posts have discussed the underside of bridges before (1) (2). We don’t do this well here, but it doesn’t mean it can’t be done well.

Most intersections are not situated such that 3-dimensions is such a natural solution, but there are some, and we should consider the possibilities.

Full disclosure: I don’t live very near there, and only use the intersection occasionally as a motorist.

Cross-posted at

Green Line / Green Lights

Suppose you have a train moving along (parallel to) an East-West (EW) signalized arterial.

Westgate Station
Westgate Station

Case 1: If the signals are pre-timed, and the timings are known in advance, the train should never have to stop for the signals (aside from emergency signal pre-emptions and other edge cases). Instead, the train should be able to adjust its speed so that it doesn’t have to stop. It might go at an average speed of say 10, 20, 30, or 40 MPH in order to ensure it hits a green light or better a green wave from whenever it departs a station. The train driver can be apprised of the optimal time to leave the previous (upstream) station, and the speed to travel to hit “green” lights.

Typical Signal Schedule and Traffic Flow Diagram, North-South across Market (1929)

Green waves have been around since the 1920s (See Henry Barnes’s autobiography: The Man with the Red and Green Eyes. Dutton. 1965. OCLC 522406). Static signs to tell travelers the speed of the green wave has been in standard use in some places (e.g. Connecticut Avenue in Washington, DC) for almost as long. Dynamic real-time signs which tell travelers what speed to adjust to to make the green wave has been recently patented and tested in simulation for automobiles: Always Green Traffic Control. The time is ripe for some carefully controlled field experimentation.

Still, pre-timing with information certainly doesn’t guarantee the fastest speed possible for the train, but it does guarantee no stops except at stations, which is good for a variety of reasons, including both travel time (avoid acceleration/deceleration loss), traveler comfort, energy use, and train wear and tear.

Case 2: If the signals are actuated, that is, their phase and perhaps cycle timings depend on traffic levels, and traffic “actuates” the signal, usually through an in-ground loop detector, transit signal priority from a fixed upstream distance should be sufficient to ensure the train doesn’t stop at a “red” light. The traffic light controller would know that a train was coming, and either keep the lights in the direction of the train green (if they are green), or change them to green and hold them, if it is currently red and the green is coming up. The train, knowing when the green will be on, should be able to adjust its speed (faster or slower) to make the green without stopping.

The distance that trains can currently notify a downstream signal controller is when they depart the upstream station, which is up to 1/2 mile or so (the spacing between stations). 1/2 mile at 30 mph takes 1 minute. With a cycle time of 2 minutes, and at least half the green time (1 minute) for the signalized arterial, a green can be guaranteed. If the light is currently red, it will be green within a minute. If it is currently green, it can be kept green for up to a minute. The worst case is it was just about to turn red and instead the green is extended for an additional minute. Alternatively, if it is currently green, a shorter than usual red phase can be inserted to clear the crossing traffic, before the light is turned back to green.

For traffic signals less than 1/2 mile downstream (say 1/4 mile) the warning time is only 30 seconds at 30 MPH. The same logic applies, but it is potentially more problematic as there is less lead time to adjust the timings, so the phase shortenings might be more severe. On the other hand, if more than 50% of the green time goes to the EW movement (say 75%) you aren’t really any worse off.

At 1/10 of a mile the warning time is less, but train departure from the station should be able to be coordinated with the light directly.

Case 3: But let’s say your traffic engineers are incapable of making this work. Should the train and its passengers suffer? This is where traffic signal pre-emption comes in. Most widely used for emergency vehicles, this potentially changes the sequence of phases, so maybe a phase is dropped (it doesn’t occur within the cycle, or within the usual place in the cycle).

This system does ensure that the vehicle requesting the pre-emption gets a green light as quickly as possible (safely turning the conflicting movements to a red phase) and thus can drive at as high a speed as possible. While trains should not need to stop at traffic lights with priority and speed adjustments, with pre-emption, they neither need to stop nor adjust their speed.

What could go wrong?

Pedestrians. Thus far we have been talking about a system with cars and trains. Pedestrians too can actuate signals, though “beg buttons“. These may function similar to vehicle actuators, in telling the traffic signal there is someone who wants to cross. The difficulty for priority or pre-emption is that a pedestrian phase may need to be longer since pedestrians take longer to cross the street than a vehicle does, especially if the street is very wide. So a pedestrian actuator may also extend the green time, in addition to calling for green time. This makes it more difficult to quickly change lights from red to green, since for safety reasons you can’t strand a pedestrian. This makes the ability to adjust train speeds in concert with the traffic signals more important.


Firetruck on University Avenue blocked by LRT train
Firetruck on University Avenue blocked by LRT train

Emergency vehicles. Emergency vehicle on emergency vehicle crashes are a known problem, and pre-emption may make it worse as firetrucks approaching a scene from two directions may both demand a green light, but only one gets it. The driver of one vehicle, not realizing he didn’t get the green (especially if he had the green as he was approaching), fails to yield. There are solutions to these problems.

Any of this will likely lead to additional delays for conflicting vehicle movements (cars making left turns or North-South traffic crossing our East-West arterial). With priority, this may even lead to extra delay for some vehicles on the parallel arterial who have been given a short green so the conflicting traffic can also get a short green before the EW arterial returns to green.

However the train usually has more people on it than are queued up at the other directions, so total *person* delay will generally be reduced.

For a variety of reasons, delay is bad (unless your goal is punishing drivers and air-breathers), we want to minimize total person time (or weighted total person time – recognizing long weights are more onerous than short weights) in the system (because time is money), and minimize pollution outcomes as well.

In short, the Green Line not getting green lights on University Avenue is a solvable problem. It should have been solved already. It eventually will be solved.

Further reading, with math: See Fundamentals of Transportation/Traffic Signals

Don’t Walk |

Cross-posted from

Don't Walk
Don’t Walk: Washington Avenue at Union Street, just west of the East Bank station. I see a pedestrian signal telling me not to walk in the landscaped median area from east to west. There are no through roads for traffic either to the north or south of Washington here, since Union has been closed at Washington. Are they concerned I will bump into a bike or other pedestrian. Are we now signalizing pedestrian on pedestrian traffic and pedestrian on bicycle traffic? I mean, maybe I could understand this after a concert let out (though I have never seen such a thing), but on a relatively unbusy summer afternoon? Other theories?
Don't Walk, Green Light.
Don’t Walk, Green Light. Are they concerned I will move east-west parallel to a moving train or bus? That happens all the time.
Don't Walk
Don’t Walk: Washington Avenue at Church Street, just west of the East Bank station. Again there is no regular motorized cross traffic here, since Washington Avenue is a pedestrian/transit mall


Time to walk: A brief excursion into a needless incursion on the law-abiding pedestrian’s time

The traffic signal at Oak St. SE and Fulton St. SE in Minneapolis is mis-timed/mis-phased/mis-indicated. It has been so for several years.

Northbound Pedestrians on the west side are given a red indicator (Don’t Walk) even though Southbound traffic has a green light and green left turn arrow. Clearly Northbound Pedestrians on the east side of the intersection should have a red indicator in such a configuration, as they are in conflict, just not those on the west side.

We can imagine why this might have occurred, but basically the walk signals are tied together even though vehicle traffic has a split phase. Obviously the technology exists so this is not necessary. This occurs from the 0:36 to the 0:49 mark in the video below. It’s “only” 13 seconds of course, but we could say the same about vehicle delays. It’s 13 seconds every minute of every day for every pedestrian at the intersection. This is near the University of Minnesota campus so the number of pedestrians is non-zero. In late spring it’s not an unpleasant wait. Talk to me in January.

Video (looking SB on Oak Street, East on the left, West on the right):

How often does this occur? Where else do the traffic engineers not think through the implications for pedestrians?

I cannot comment on what the optimal traffic signal timings are for this intersection, but this is clearly not it.

Further reading:

Flashing red at Franklin Avenue/East River Road/27th

My favorite five-way intersection that should be a roundabout was on flashing red yesterday, and I concluded that was a time for me to monitor this natural experiment. At 4:30 on May 12 I video of the intersection for 4 minutes. I counted 100 cars and 11 pedestrians and bikes in just over 4 minutes. Simply multiplying 111*15 gives a capacity of 1665 units per hour. This is somewhat off my estimates from a signalized intersection. On the other hand, delays were rather short, and while there were short queues at some of the approaches (which helps ensure the intersection is fully utilized), they were not getting longer over time.

The video is below:

A map of the location.

Previous videos can be found here (4-way stop) and here (signalized).

Flipping Signals

In places that are, or want to be, walkable, and serve pedestrian traffic, traffic signals should have a default setting of pedestrian scramble (Barnes Dance), and only switch to a green light for motor vehicles from a particular approach (for a short time period) when it is actually actuated by a vehicle. Buses and emergency vehicles would still be able to get priority by signaling from upstream.

Shibuya, Japan (from wikimedia commons)
Shibuya, Japan (from wikimedia commons)


Today, in the United States, traffic signals are usually designed with the objective of minimizing motor vehicle delay, yet many policies and plans have a stated aim of reducing the amount of vehicle miles traveled or the automobile mode share. How does lowering the cost of driving and increasing the cost of every other mode help with that objective?

Now we place pedestrians in the supplicant position of begging for a green light. Let’s give walkers some dignity, and instead of making them “scramble” at the intersection, allow them to simply purposefully walk, or even amble. If instead of pedestrians waiting for cars, cars had to wait for pedestrians, vehicle delay would undoubtedly rise. But vehicle counts would fall, and pedestrian demand would rise. Where would the vehicles go, would they disappear or reroute?

Think about places this would work in your community. In the Twin Cities, I think this would be great for Dinkytown and Uptown.

It is a change, it would need to be tested somewhere before it could be done everywhere. There will always be resistance by the stalwarts. But we should experiment.

The measure of success would be change in pedestrian and bike counts, the reduction in vehicle counts (at this location), and maybe the change in sales at nearby businesses.