Question: What’s 80 seconds to a traffic engineer? While this may seem like the beginning of a groaner transportation joke, it is an interesting nugget about the way traffic engineers view streets and the way cars and others use them.
Answer: It’s the maximum acceptable delay for a car to get through an intersection.
I saw this recently in a presentation about street design and asked an engineer, Jack Broz of Avenue Design Partners, about it. He said that in standard engineering practice a delay of 80 seconds means something’s wrong and the intersection doesn’t work in some way.
He explained that traffic engineers score how well traffic moves on a scale called the Level of Service. If cars are not moving quickly enough, then the stretch of road gets a bad grade. And, as we all know, bad grades mean some attention needs to be paid to make things better.
So, the obvious next step is to figure out what isn’t working in the intersection and try to fix it. But, there are other things, Jack suggested, that might call the flunking grade into question.
1) The grade is based on how well traffic moves at the most congested time of day.
If traffic flows smoothly for most of the day but clogs up at rush hour, then the 20 percent of the day with bad traffic gives the road a bad grade. The approach to fixing the street is based on fixing this problem at rush hour.
Imagine if your whole day was graded based on your afternoon commute. Say you had a great lunch or a really productive meeting but had a slow trip home. That day flunked. And, this bad grade means you’d better rearrange everything else in your day to make that trip home better.
2) There is no maximum acceptable delay for people waiting to cross the street.
How long a person has to wait to get a walk signal is based on the way streets are designed for motor vehicles. The goal is to keep the flow of cars going during rush hour. How well it works for people walking or riding bicycles is not the main concern, no matter what time of day.
Which brings up another possible groaner of a transportation joke.
Q: Why did the person cross half the road?
A: Because cars started coming before she could get all the way across.
Have you ever seen someone standing on the centerline of a street? A person waiting to cross sees no traffic coming, so starts across the street. But, before there’s time to get all the way across, a light changes and cars start coming, trapping the person in the middle.
Why does this happen? As Jack explains it, people encountering delay make choices. When a driver gets to know that a particular intersection takes forever, that driver may try a different way.
People waiting to cross the street also are making choices. If they are crossing at some off-peak time, say mid-afternoon, the flow of motor vehicles may be sparse, making it seem reasonable to go across, even against the light. Even at rush hour, cars tend to move in clumps (traffic engineers call them platoons). After a platoon of cars moves through an intersection, there’s often a gap in traffic. A person wanting to cross may see a chance to go, rather than waiting for a light to change. And remember, there’s no standard for how long a pedestrian should have to wait.
There are some design elements that work well for multiple users of the road. At some intersections, roundabouts have replaced traffic signals. Why? One reason is safety. According to the Federal Highway Administration, “There are an estimated 300,000 signalized intersections in the United States. About one-third of all intersection fatalities occur at these locations, resulting in roughly 2,300 people killed each year.”
Installing a roundabout reduces severe or fatal crashes by 78 percent. This is in part because vehicles go so much slower and have to pay attention to approaching traffic to safely enter the roundabout. Roundabouts also are safer for people walking. Crossing each part of a roundabout is much shorter than crossing a full intersection. And, because cars are going slower, they can see and stop for pedestrians. Which leads to the last transportation Q&A.
Q: How long does it take to cross two legs of an intersection on foot?
A: Longer than to walk all the way around a roundabout, even with traffic.
So, the next time you’re waiting for a light to change, whether in a car or on foot or a bicycle, consider that the delay for cars at rush hour likely has determined how that intersection is designed. Unless it’s a roundabout, in which case you won’t be waiting for the light.
Hilary Reeves is communications director for Transit for Livable Communities.