On running red lights and traffic signal cycles

Somebody (Jeff Schneider, I think) commented a day or two ago that many red lights are run by the last car (or cars) trying to make the light. I've been thinking why that seems so. Not only does it seem to be the case, but those last cars through often seem to be accelerating, not holding a steady speed or slowing down.

There was a time (at least in my childhood, if not more recently) when the north/south light turned green at the same time that the east/west light turned red (and vice versa). Somewhere along the way, the traffic engineers added a momentary delay between the red light activating in one direction and the green light activating in the other. Somewhere else along the way, the onset of the green light was further delayed because the pedestrian walk light was activated.

Now we often have something like this sequence of events when a traffic light changes to red:

  1. north/south light turns red
  2. one mississippi
  3. east/west pedestrian walk light changes to "walk"
  4. two mississippi
  5. east/west light turns green

I think what's happening is that the last guy heading north or south as the light is changing is taking advantage of the fact that the east/west traffic won't really be able to go for a heartbeat or two. So, lots of red lights are run.

I don't know why the original green light pause was inserted, too many fender benders when red light bandits got there late and the stop light racers left a touch early? I suspect the pedestrian pause was inserted to give them a fighting chance to cross the street before cars turning right could cut them off. Then there's the added danger of what seems to be frequent red light bandit acceleration. Not to mention which, all those one mississippis must add up to a bit of wasted gasoline when considered in aggregate. Maybe it's time to rethink this whole mess.

Are there any traffic engineers here who can comment?

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Permalink Reply by lmh on May 1, 2014 at 10:30am

Oh god, Evanston lights drive me bats**t. Pretty sure their only purpose is to confuse and irritate everybody on the roads.

Tricolor said:

That or you could be like Evanston which makes **** up as they go.

Reboot Oxnard said:

Determining traffic signaling requires decisions trading safety for flow; finding the appropriate balancing point is intrinsically subjective. There are lots of factors that go into these decisions, including:
• the posted speed limit,
• actual traffic speed,
• vehicle types,
• vehicle mix,
• road surface conditions,
• sight distances,
• geometric considerations,
• driver age,
• turning movement

You’ll notice that pedestrians and cyclists aren’t mentioned; for the purpose of traffic signaling, they count as particularly difficult-to-handle vehicles that skew the vehicle mix and also alter the geometry of intersections.

You discussed something called ‘all-red phasing’, where all the lights will show red between cycles. It improves safety (just as you supposed) at the expense of reducing flow.

One big issue is what constitutes running a red light. Most states (including Illinois) have what is known as a ‘permissive yellow’ light, where vehicles may not enter an intersection on a red light but are not in violation if they don’t clear the intersection before the light turns red. This leads to the stacking of vehicles in the intersection, especially when traffic is congested and during left turns, and increases the tendency to accelerate into/through a yellow light. Alternatively, some states have ‘restrictive yellow’ rules, where being in the intersection when the light turns red constitutes running the red. Permissive yellow increases traffic flow (mostly) at the cost of more accidents.

Protected and passive left turn signals also have a significant impact on intersection safety and flow, especially whether or not a protected left turn signal leads or lags the cycle. Combine a restrictive yellow light with a lagging protected left turn (pretty much the opposite of what Illinois does) and you improve intersection safety but at the expense of slowing traffic.

Protected pedestrian crossing improves safety but significantly impedes the flow of other traffic. When protected pedestrian crossing is used, it’s almost imperative that right turn on red be prohibited, that the pedestrian crossing signal leads the cycle and that any protected left turn signal lags the cycle.

Finally, what might be the biggest factor in preventing red light running is the length of the yellow light. Simple things like a slope approaching an intersection make a huge difference in the amount of time required to stop, likewise an asphalt road requires much more time for a stop than concrete and accounting for all of these factors is important. Some communities, in the wake of implementing red light camera enforcement, have experimented with lengthening the yellow light phase and have found that it can reduce the violation rate so much that it makes the camera enforcement unnecessary and even non-viable.

But it all comes back to this: safety and flow are incompatible objectives and finding the appropriate balance between them requires decisions based as much upon values as data.

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