Three weeks ago I walked out of my front door at 6:30 A.M., headed for the office. I made three turns in short succession, bringing me to Old Bridge Road, the local thoroughfare that leads, four miles away, to an entrance to I-95, the main north-south artery into Washington. After 20 minutes on Old Bridge, I had gone a mile and a half. I turned around and went home. At 9:15 I made another try for the office, arriving at my desk just before 11:30. I live 27 miles from work, as the tortoise crawls.
There was no specific cause for the delay that morning. There were no catastrophic accidents or road closings along the route. It was simply a bad case of traffic.
In a nontrivial sense, my life is governed by traffic. I used to live in an inner-ring suburb seven miles south of the city. On a good day, my commute took about 12 minutes. A couple months ago I moved south, to the new place. It’s just 20 miles further out, but the geographic and temporal distances are only tangentially related. I can leave my house and be sitting at my desk in 35 minutes. The hitch is, I have to be on the road by 5:30 A.M. If I depart even 30 minutes later, traffic stretches the commute to an hour. If I leave at a mildly civilized hour–say, 7:00 A.M.–it can take two hours. Or more.
As a consequence, I treat traffic the way sailors treat weather. It is always the first consideration, a condition that defines the boundaries of prudence.
I’m reasonably philosophical about it. I view traffic as the great unsolved problem of our time, a Fermat’s Last Theorem of modern life. People tend to think of traffic as either an untamable beast or an act of God. But it’s mostly science.
For instance, we’ve all had the experience of sitting in a bad jam on the highway. You come to a halt, then the traffic eases a bit, then halts again. You experience this stutter-stop over and over until, like magic, you hit a release point. The traffic disappears and you continue on your way without ever seeing an accident or incident that caused the snarl.
Yet your jam was not a random occurrence. At some point, perhaps hours earlier, something happened. Maybe it was an accident. Maybe there was debris in the road. Whatever the case, a bottleneck arose and traffic slowed. The rate of traffic approaching the bottleneck greatly exceeded the rate at which traffic was released from it. And as the traffic built, two waves–of acceleration and deceleration–rippled up and down the lengthening stream of cars, causing you to speed up, then slow down, over and over. The traffic jam is actually a math problem. And this math problem keeps jams going until the waves peter out, often long after the incident itself is resolved.
Traffic is all about math. A few years back Los Angeles Magazine ran an excellent story on traffic which explained the equation used to evaluate the efficiency of signaled on-ramps. That’s it right there in the middle of this page.
But you don’t need to know differential equations to understand the basics. The standard metric for traffic is capacity, measured in cars-per-lane-per-hour. An average highway moves at roughly 2,000 cars-per-lane-per-hour under ideal conditions. The riddle of traffic is solving for “ideal.”
There are two components to traffic: drivers and roads. To achieve ideal efficiency, you first have to get drivers to behave optimally. Which is actually harder than it sounds. Once upon a time, 45 mph was considered the ideal target for highway driving–the speed at which the most cars could move through a lane in a given period. But over the years the behavior of American drivers changed. At higher speeds, we started driving in “platoon” formations–tightly packed, very efficient groups. So the ideal highway speed became 60 mph. Thus, ideal driver behavior is not constant.
The second half of the equation is the roads, where the goal is to eliminate bottlenecks. Highways often carry obvious, built-in bottlenecks–lane merges and divides, intersections–which reduce their capacity. But bottlenecks can also result from seemingly insignificant factors. For instance, on east-west roads bottlenecks can form when the sun sits close to the horizon, blinding east-bound drivers in the morning and west-bound drivers at night, causing them to suddenly slow down. Good engineering means designing roads to smooth away as many hidden bottlenecks as possible.
As you can imagine, there are many other variables in this complex equation. But like all mathematical problems, traffic is ultimately solvable. At least that’s what I tell myself whenever I’m stuck.
JONATHAN V. LAST
