Traffic Signals and Pedestrians

A recent discussion about a New York City request to Google proved somewhat interesting. The request itself was for Google to add an option to its navigation type products to avoid left turns. Their reasoning was that left turns are an inherently dangerous operation and reducing the number of them would be overall beneficial. One commenter brought up another question that is somewhat interesting, though.First, some background which most of my readers probably already know. In North America, at most traffic signals, pedestrians and vehicular traffic move during the same phase of the signal. To clarify, let’s consider a typical four way interesction with one road running north-south and the other east-west. In a basic signal configuration, there would be two signal phases. One for the east-west road and one for the north-south road. When the signal is green for the north-south road, all traffic proceeding north or south or turning from the north-south road proceeds. Any pedestrian traffic that is crossing the east-west road also proceeds.

It’s obvious from the preceding description that there are some conflicts. Left turns must cross the oncoming traffic. All turns must cross the pedestrian traffic. That sounds like a problem. However, a close examination of the situation reveals that this is exactly the same situation we have if the east-west road simply faces a stop sign as far as the north-south road is concerned. The same traffic conflicts exist.

It turns out the conflicts can be managed quite well with a couple of simple rules. Left turning traffic must wait until there is a sufficient gap in the oncoming traffic that the turner can clear the oncoming traffic lanes. In other words, the oncoming traffic has priority, including oncoming right turns. Opposing left turns may proceed at the same time assuming both have a clear path since they do not cross each other’s path. The other rule means that all turning traffic must yield to pedestrians in the crosswalk (marked or implied).

Of course, these rules require adequate visibility through the whole intersection. That means substantial barriers and other obstacles that block a driver’s view must be avoided. Also, for this situation to work with maximum saftey, pedestrians must take some responsibility for their own safety by not crossing against the signal and also making sure the drivers can see them before they enter the intersection. Running up to an intersection and entering the street without stopping or looking is a sure way to get hit because a driver was already moving through his turn before you were visible. Motor vehicles take a longer time to stop than a pedestrian without considering the reaction time of the driver. Whether you had the right of way or not, you really do not want to argue with a tonne or two of motor vehicle.

In more complex signal configurations, there are often phases specifically for left turns during which pedestrians face a “don’t walk” signal. In others, pedestrians have their own phase during which they can cross in any direction (a so-called “scramble” instersection). The former improves safety for all and throughput for turning traffic. The latter optimizes for pedestrian heavy intersections but also improves safety overall.

The interesting comment touched on the above. The commenter is not from North America and queried why pedestrians and vehicles are moving at the same time. Why doesn’t all traffic stop while pedestrians are moving? How can it possible work?

Well, as described above, there are specific rules that drivers are required to know and obey. In cases where there is a protected turn phase on a signal, the pedestrians whose path the turn would cross are also stopped, just like the opposing vehicular traffic.

Why do we do it that way? Efficiency. In most cases there is either relatively little pedestrian traffic, relatively little vehicular traffic, or relatively little conflict between the two (small amount of turning traffic or low enough pedestrain volumes that turning traffic rarely has to wait long after the opposing traffic clears for the pedestrians to also clear). It’s simply not resource efficient to add protected turn phases to all signals. It is also inefficient for traffic when turn volumes are low. Additionally, such phasing also delays pedestrians which is not desirable if one is to encourage walking.

Now, the comment itself ignores the fact that NYC’s request to Google wasn’t to reduce left turns at signals but to reduce left turns everywhere. That includes at intersections where there is no signal at all. In such cases it is even less practical to install a fully channelized (signal phases for every movement through the intersection).

However the setup in North America came about (historical accident, deliberate planning), it does improve traffic flow overall for the most part as long as all participants behave, which is something traffic requires anyway. Indeed, even with the apparently dangerous rules, basic traffic lights often improve overall safety at an intersection once traffic volumes (pedestrian and vehicular) reach a certain point, even before it makes sense to have any protected phases. They are often installed at imbalanced intersections so that a busy main road doesn’t completely starve the cross traffic of the ability to cross the main road or make a left turn. In those cases, it would be massive overkill to fully channelize the signal. In fact, I would argue that most basic traffic lights in North America fall into that category.

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