Category Archives: traffic signals

Westgate Station

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


Intersection of Oak and Fulton Streets, SE Minneapolis

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).

Shibuya, Japan (from wikimedia commons)

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.

Sioux City Safety

Sioux City has automatic red-light running detection cameras. These are doing their job. In the Sioux City Journal by Molly Montag article on the topic, the facts are all clear, unfortunately the headline takes the small negative instead of the large positive as the lede: “Sioux City data: Rear-end crashes increased at 5 red-light intersections”

"A truck passes a traffic safety camera at Lewis Boulevard and Outer Drive in Sioux City on April 8. State data show rear-end crashes increased at five of eight Sioux City intersections where red-light cameras were installed. ... photo by Tim Hynds, Sioux City Journal file"
“A truck passes a traffic safety camera at Lewis Boulevard and Outer Drive in Sioux City on April 8. State data show rear-end crashes increased at five of eight Sioux City intersections where red-light cameras were installed. … ” photo by Tim Hynds, Sioux City Journal file”

In Sioux City’s case, new police data obtained by the Journal show a 40 percent decrease in crashes from motorists running red lights at intersections with cameras and a 15 percent reduction in all crashes. Iowa Department of Transportation data also show a decline in accidents involving red-light running. “In general, this is a positive result, as rear-end crashes (though not desirable) are not as severe (or dangerous) as right-angle crashes that might otherwise occur,” David Levinson, professor of transportation engineering at the University of Minnesota, said in an email.

University of Missouri-St. Louis transportation studies professor Ray Mundy said rear-end crashes typically increase after systems are installed and drivers slam on the brakes when they see the camera flash. Such accidents usually decrease over time as people get used to driving through camera-controlled intersections. He echoed Levinson in saying more slower-speed rear-end crashes are a tradeoff for reducing higher-speed, T-bone crashes that happen when drivers run red lights.

The University of Minnesota’s Levinson agreed the decrease in red-light crashes showed the systems improved safety.

“That is the important takeaway,” he wrote.

I looked at the data. While in my analysis the total number of crashes did not change, it is clear that automated traffic enforcement reduced the number of “ran traffic signal-involved” crashes and increased the number of “rear-end-involved” crashes. The differences before and after installation by crash-type are statistically significant and meaningful in both cases. This is consistent with general results nationally about the effects of automated traffic enforcement. (And what you would expect if there were more sharp braking at intersections as people strive to avoid fines).

Cost to users is a transfer to the city (and should otherwise reduce some other taxes the city is collecting), and though there is some cost to administering the system, that is outweighed in general by the safety benefit.


Always Green Traffic Control

Nick Musachio, local inventor in Minnesota, has just been issued a patent (No. 8,711,005) for his Always Green Traffic Control System. (Since this is transportation, we will abbreviate this AGTCS)

Imagine you have an isolated signalized intersection, operating near but below capacity. If vehicles were able to travel at the correct speed when approaching the intersection for a significant distance, they should be able to  travel through the intersection without hitting a red light or being delayed by standing queues. If at 45 MPH they would hit a red light, but at 35 MPH would get a green, they should be informed to reduce speed to 35 MPH. This not only reduces driver delay, but should decrease crashes and decrease emissions, both of which are exacerbated by intersection control and braking and acceleration.

How would drivers know which speed to travel? An upstream Variable Message Sign with Dynamic Speed Limits (tied into the traffic signal controller cabinet, or with the pre-programmed traffic signal timings) would tell them the best speed to avoid stopping. If only the first car in a platoon does this (on a 1 lane road), all following cars are controlled by default.

Audi has a similar in-vehicle system. That is only useful if the traffic agencies produce live feeds of traffic signal timings. Comment: it is appalling that such a traffic signal timing live feed doesn’t generally exist (even transit agencies, not historically known for their cutting edge research) have GTFS.

AGTCS is infrastructure based, and works for all vehicles anywhere an agency wants to set it up.

Some videos and simulations below.

Begging for Simplicity

Cross-posted from Begging for Simplicity … in which I complain about beg buttons, and general second class treatment for pedestrians.

Begging for Simplicity


Pedestrian actuators call for a pedestrian signal at an intersection which is semi-actuated (where the green-time goes to the mainline except when a vehicle is on the side street, subject to a maximum cycle length and a minimum green time for the side street) or fully-actuated (where the green time is allocated to approaches which are actuated subject to a lot of constraints). When they do this, they also tell the controller to extend the green time (and parallel walk signal) given to a phase to be sufficiently long to allow pedestrians to safely cross.

Replacement Walk Button (Franklin and Seymour)

Decommissioned Walk Button (Franklin and Seymour)

But this has problems. Imagine you are on a side street about to cross a main street and the light turns green for the cars, but the Don’t Walk sign remains (since no Pedestrian actuation was recorded). You did not get to the actuator quickly enough. Should you cross on the Green but against the Don’t Walk, or should you wait almost an entire cycle for this to come around. You may or may not have enough time to make it across.

Second imagine the actuator is broken. It may never give a walk signal. (There aresolutions for that, where the default state of broken is “on” instead of “off”, but that doesn’t seem to be widely deployed). For instance, recently I reported toSeeClickFix a broken pedestrian actuator at Franklin and Seymour Avenue in Southeast Minneapolis, which was corrected within 18 days (i.e. the case was closed within 18 days). I am fairly confident a broken traffic light serving cars would have been corrected sooner.

The next  two “beg buttons” (pedestrian actuators for traffic signals) were recently photographed. The one on the University of Minnesota campus (at Beacon and Harvard) recently had a time exemption added, implying that the actuator need not be pressed between 8 am and 6 pm weekdays. (This time exemption seems to have been removed since the photo was taken). This is an improvement over the previous situation (pushing the button in the middle of the day). Still one expects this will, like so many others, become a placebo button, or just break and make it so there is no pedestrian phase.

Beg button at Harvard and Beacon, University of Minnesota campus

However, the complexity is still needless. Traffic signals on streets with sidewalks (which implies pedestrian traffic either exists or is desired) should ALWAYS have an automatic walk phase, just as every cycle gives green time to cars from every approach. (This is especially true in pedestrian areas like a college campus which has a plan that aims to prioritize walking.) Actuators are fine if they make the walk signal come sooner, but being unpushed should not be used as an excuse not to have a walk phase at all. Car drivers don’t have to go out of their way to press actuators, why should pedestrians?

(If traffic is so low you are concerned the time devoted to a pedestrian phase (~12 seconds (36 ft at 3 fps)?) is too long (will cause too much vehicle delay) for this two lane roadway, maybe it shouldn’t be a signal but instead a stop sign (which requires no pedestrian signal) or a yield sign. This can be implemented with flashing red lights if you must you electrical gear.)

Beg button at Franklin Avenue, East River Road, and 27th Avenue SE

Fortunately it is not as complex as the last, at Minneapolis’s favorite five-way intersection (Franklin Avenue, East River Road and 27th Ave SE), which gives instructions for something that should be tacit. That it is not tacit indicates it is a flawed design. If I can read the instructions, I already know how to cross a street. It is not like pedestrian actuators are a new technology. While I want more information at bus stops, crossing a street should be straight-forward, and not require an 11 line instruction set with five graphics. Sadly there is more information here than at the nearest bus stop.

Update. I found the tweet below, which seems appropriate.

Erik Griswold (@erik_griswold) 11/16/13, 18:13 "Making pedestrians press. buttons to cross streets is the death of the city" -Eric Fischer (@enf) at #transpowest

You have got to love renderings


I found this rendering of 222 Hennepin earlier today, after reading Bill’s post on architecture. The part that I liked best is the traffic light. First it is sideways, a design that is used in some places in the world, but not Minneapolis, for a lot of good reasons.

I grew up in Columbia, Maryland, which for many years had sideways traffic lights, mainly as an architectural distinction, but which were abandoned because of the confusion created.

A major problem of such uncommon lights is that color-blind person might not know if the green is on the left or on the right. In Wisconsin (as everywhere else in the US where it is done, and standardized in the MUTCD, it is red on the left, green on the right.

In the rendering, it is the opposite.

It makes you wonder what other tricks and graphical shortcuts are going on to make the rendering desirable to approve and move into, but won’t really turn out as implied.



Auctioning Green Time

Eddie, a traveler in a hurry arrives at a traffic light from the East. He would pay up to $18 to save an hour. Sue, a less-hurried casual traveler arrives from the South, she would only pay $6 to save an hour of travel right now. Who gets the green light, who gets the red? Presently this is decided without consideration of how much Eddie or Sue would be willing to pay to save a few seconds or a few minutes. No one has the ability or the authority to make a transaction occur where Eddie can pay a few cents to Sue and get the green light while Sue waits for the light to change. Until recently, this was because it was technologically infeasible, but in recent years, advances in transportation signal technology and real-time wireless vehicle-infrastructure communications have made this once impossible transaction possible. Now it is institutional constraints that prevent this from happening. Traffic signals are in almost all towns, cities, counties, and states publicly owned and managed. Imagine instead that this was a service that private firms would bid to supply.

A new organization, LightSpeed Traffic, has paid your city $100,000 a year for the privilege of managing traffic signals. Instead of this being a cost center for the city, it is now a revenue generator. Why do they do this? A private operator is able to use traffic signals more efficiently from an economic perspective than a public agency, they can obtain revenues from sources such as:

  • Acting as the facilitator of transactions between travelers as described above to minimize weighted travel delay, Eddie pays $18/hour (30 cents a minute) and saves a minute, Sue is compensated at $6/hour (10 cents a minute), and LightSpeed Traffic keeps the difference, 20 cents a minute, to cover the costs of operating traffic signals, paying the city for the franchise, and earning some profit for shareholders. This could scale up, by summing all of the traffic from each approach, and multiplying by their respective values of time. (See for instance Varirani and Ossowski (2012), or Dresner and Stone (2008) for illustrations.)
  • Providing real-time traffic data to a new generation of GPS companies that aim to provide routing information to travelers. By investing in the signals and sensing technologies around the intersection (and at nearby intersections they also manage), LightSpeed has accurate estimates of arterial travel time, and can make predictions about future travel times, data that is extremely valuable to those providing real-time advanced traveler information.
  • Administering red-light running cameras.
  • Advertising at the traffic signal when it is red (as suggested by the linked patent). Like transit companies who sell advertising on the interior and exterior of buses and bus stops, traffic signals have laid out before them a captive audience that might be interested in real-time information, especially information that was customized by place and traveler. LightSpeed has the authority to coordinate advertising with traffic signal timings. Not making the light extra long to force drivers to wait, but simply to use variable message signs to display ads when the light is red anyway, and to benefit travelers by displaying real-time travel information when the light is green.

Presently a few companies operate traffic signals under contract to municipalities, notably in Sandy Springs, Georgia. None yet use signals innovatively as described above.

Obviously this can get quite complex: there may be more than one approaching driver, how do you decide the baseline to estimate vehicle time-saved or time-list by adjusting signal timings, how does this work in networks instead of just isolated intersections.

But scarce resources (like two vehicles seeking to use the same space at the same time) can be allocated in many ways other than arbitrarily or first-come first-serve, to the benefit of all. Sometimes the best solution is a yield sign, sometimes a stop sign, sometimes a roundabout, sometimes a traffic signal, sometimes a grade separation. To be clear, it is not always a traffic signal. When it is a traffic signal, there might be some merits to thinking creatively about the organization and operation of the market that is created by the rationing of time for the benefit of all.