Category Archives: Traffic Engineering

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

Open Access Article: Spatial modeling of bicycle activity at signalized intersections | Institute of Transportation Studies Library

Open Access Article: Spatial modeling of bicycle activity at signalized intersections

Biking at Grand/Halsted/Milwaukee (3 of 4)

This week is Open Access Week. What’s Open Access? Here is a not very brief overview by Peter Suber. UC Berkeley also has an Open Access Initiative to help open up your research and data. 

In the spirit of Open Access Week, here’s an interesting article from an open access journal – The Journal of Transport and Land Use. Go check it out and peruse the articles. No need to depend on your institution’s sibscription because it’s free to the public! (Thanks open access!)

In “Spatial modeling of bicycle activity at signalized intersections“, Jillian Strauss and Luis F Miranda-Moreno look at the built-environment and cycling. 

This paper presents a methodology to investigate the link between bicycle activity and built environment, road and transit network characteristics, and bicycle facilities while also accounting for spatial autocorrelation between intersections. The methodology includes the normalization of manual cyclist counts to average seasonal daily volumes (ASDV), taking into account temporal variations and using hourly, daily, and monthly expansion factors obtained from automatic bicycle count data. To correct for weather conditions, two approaches were used. In the first approach, a relative weather ridership model was generated using the automatic bicycle count and weather data. In the second approach, weather variables were introduced directly into the model. For each approach, the effects of built environment, road and transit characteristics, and bicycle facilities on cyclist volumes were determined. It was found that employment, schools, metro stations, bus stops, parks, land mix, mean income, bicycle facility type (bicycle lanes and cycle tracks), length of bicycle facilities, average street length, and presence of parking entrances were associated with bicycle activity. From these, it was found that the main factors associated with bicycle activity were land-use mix, cycle track presence, and employment density. For instance, intersections with cycle tracks have on average 61 percent more cyclists than intersections without. An increase of 10 percent in land-use mix or employment density would cause an increase of 8 percent or 5.3 percent, respectively, in bicycle flows. The methods and results proposed in this research are helpful for planning bicycle facilities and analyzing cyclist safety. Limitations and future work are discussed at the end of this paper.

The full article can be found here

The price of anarchy – on the roads and in football

Colleagues at the University of Minnesota write:

Skinner, B and Carlin, B (2013) The price of anarchy – on the roads and in football Significance 10(3) pp. 25-30:

“City traffic can sometimes move faster when a road is closed. A football team can sometimes play better without its best player. The two are linked. Anarchy, say Brian Skinner and Brad Carlin, is freedom to be counterproductive.”

[I am not sure I would interpret I-35W as a Braess's paradox example, strictly speaking it fails the test, but its advantages were a lot less than might have been thought, and certainly many people were worse off after it opened, if not overall. The general analogy to sports is interesting though.]

Seven hours in New York City

I was briefly in New York yesterday. By briefly I mean I left Minneapolis when it was daylight, and returned and it was still daylight. This is of course much easier to accomplish when you are near the summer solstice, but still it suggests the technical feasibility, though definitely not the desirability, of cross-continental commutes.

On the Minneapolis side, things went very smoothly. I left my house at 5:45 AM, caught the bus at 5:53, was at the LRT by 6:00, caught the ~6:03 LRT to the airport and was there by 6:20. Security was quick, the weather was good, the 8:05 flight was on-time.

Some comments on transportation in America’s largest city.

For a city with so many airline passengers, and presumably airline profits, some of the airport terminals (JFK Terminal 2) are still quite dumpy (Yes there is a plan to fix this). One would think that if there were competitive owners of each different airport (and each terminal), they would have to compete for customers (both passengers and airlines) by differentiating quality (presumably upwards). Though there has been some terminal modernization, New York is far behind the rest of western (and eastern) civilization in this arena.

Second, there is not good transit access from the airports to the City. New York, with the US’s largest subway system has had more than 50 years since the dawn of the jet age to connect its airports to its transit system successfully, and seems to have failed to avail itself. (I am aware of JFK’s AirTrain, it seems to require a separate charge from the transit system and a transfer, surely someone could figure out how to bundle that. It also required taking the subway with 33 stops to my Midtown destination). This is not an unknown problem, and solutions are proposed for LaGuardia (via Bus, apparently the train proposal was shelved) and JFK (at about $10B, which seems excessive, but this is NYC).

At any rate, someone else was paying for my surface transportation, so I was in a car. (Which I realize makes me part of the problem, not the solution, but also gives me the perspective of enlightened commentators such as Dorothy Rabinowitz. Yet I did not notice any problems with CitiBikes on my brief stay. There were some bicycles darting in and out of traffic, but that was because cars were not moving and bikes could). On the way in I also got to hear the political philosophy of my driver (a well-educated Russian immigrant from over 30 years ago), who is probably best described as a Peter King Republican, which probably would not have happened on a subway train. The driver seemed to be of the belief that bus lanes were a bad idea because they delayed cars, and in general was opposed to the Bloomberg administration. He also thought most of the works were badly managed and timed poorly (this I agree with) so the Unions could flex their power, and that trucks should only enter the city at night. Of course where you stand depend on where you sit.

Third, New York has far more street traffic congestion than it should. Of course it is crowded, and it probably shouldn’t build more highways, but it doesn’t manage scarce roadspace the way a well-managed city would.

  • On the way in to the city, one of the lanes on the Queens Midtown Expressway was blocked so someone (1 person) could sweep the shoulder, with a broom, in the middle of the day. To be charitable, maybe there was recent broken glass that required cleaning, but this seemed far more substantial cleaning than the debris from a fender-bender. The queues formed by the lane closure were several miles in length.
  • Why is on-street parking permitted in the middle of the day on both sides of the street on major congested streets (37th Street)? This seems to be more than loading/unloading and more than temporary construction crews.
  • And why is don’t block the box not enforced. This would seem a perfect opportunity to use red light running cameras to ticket people who block cross-traffic on the red light.

This is even before considering what economists normally think about when they say pricing, some form of congestion charge, which has been proposed and not implemented because the winners could not bring themselves to pay off the losers.

Fourth, why is there congestion at the airport on a clear day with as perfect weather as one could ask for? Leaving LaGuardia, we boarded the plane on-time and the plane was 17th for take-off with about 40 minutes of ground wait. We landed “on time”, meaning the airline (Delta) built in 45 minutes of ground delay into the schedule to ensure “on time” arrivals. If the schedule is such that the same flights are repeated daily (an approximation), then our plane would take off at the same time every day regardless (unless it was worse due to weather). Which means, we could have been scheduled a half-hour later and not waited in the plane on the ground. This is a simple coordination problem that could be solved with reservation pricing. I suspect this is a problem because there are competing airlines which want to offer the same departure time (~6:45 pm), but a monopoly airport. In a different airport a dominant (hub) airline might internalize the delay costs. See Daniel (1995) on Congestion Pricing and Capacity of Large Hub Airports: A Bottleneck Model with Stochastic Queues.

County presents scenarios for Franklin/East River Parkway remake | Bridgeland News

From the Bridgeland News: County presents scenarios for Franklin/East River Parkway remake
From the article:

“Two suggestions bordered on the Swiftian: One was a modest proposal to remove all traffic control from the existing intersection. “When those signals are out, that intersection functions fairly well,” stated one man.”

I was “one man”.
The official alternatives are available here:
Project website
My letter (sent to the team and local public officials) clarifying what I am thinking about, which I sent to the project team is below:

Jim,
Thank you for hosting the public hearing on the Franklin Ave/27th Street/East River Road intersection. I mentioned the meeting you should consider a shared-space concept (including perhaps a simple roundabout, but without all of the complex signage, separation, etc.) , the ideas I have in mind are illustrated here:

http://www.shared-space.org/

The advantage is that it could cost much less, and could be easily tested (put some covers on the signals, take down the signs, and put up some warning signs telling people upstream they are approaching a new environment, without requiring full reconstruction.
A video showing some of the ideas is here:

AND

(especially at 5:00 into the second video)
I recognize the idea may appear radical to traditional engineering practice, but I think it is worth giving full consideration to, especially on a site like this with no obvious inexpensive solution, with a mix of commuter and parkway traffic, bicycles, and pedestrians, a desire to minimize land taking, and a desire to calm traffic.
Please let me know if you have any questions.
— David

Finally, in addition to having a personal interest in the intersection since I use it daily, I also supervised a Master’s Degree paper: Evaluation of a Roundabout at a Five-Way Intersection: An Alternatives Analysis Using Microsimulation on the intersection by Reuben Collins, which recommended a roundabout.
Unfortunately, judging by their response to comments, the study team clearly has not yet grokked the possibilities of alternatives to conventional (i.e. US standards-based) design, and intends to overbuild and oversign the location.