Sean_Marshall / Flickr

In many cases, there are benefits to replacing signals with stop signs—starting with saving taxpayers a lot of cash.

Americans are driving less than they have in the past, so it's a natural time to consider different approaches to car-related infrastructure. That means maintaining existing roads rather than building new ones that will someday need repairs themselves—a fix-it-first approach. And it might mean reevaluating another part of the street system introduced when driving was on the rise: traffic lights.

Now, obviously we need traffic lights on lots of city streets, but whether or not we need as many as we've installed is another question.

For help with an answer, we turn to new research from civil engineers Michael Schrader and Joseph Hummer of Wayne State University, who studied the traffic light situation in Detroit. A couple years back, in the throes of its budget crisis, officials determined Detroit could safely remove at least 20 traffic lights in downtown and midtown. Schrader and Hummer expanded that study to look at a sample of 100 traffic lights across the whole city.

The researchers report that 21 of these signals could be replaced with a two-way stop, and 24 could be replaced with a four-way stop, without any negative impact on traffic flow. Extrapolating those findings to all the lights deemed eligible for removal (1,021), Schrader and Hummer concluded that Detroit could remove 460 signals, or 30 percent of its total inventory. (And if anything, they write, that figure might underestimate removable signals.)

As a primary example, they point to a traffic signal at the intersection of Charlevoix and Lakeview, a crossing surrounded by vacant lots. During a weekday site visit, the researchers found the intersection "completely devoid of activity, save the operation of the signal itself." In one three-minute survey period, the signals went through three cycles and serviced a total of one vehicle. The need for careful study aside, Google Street View tells a more concise story:

Google Maps Street View

Schrader and Hummer framed their analysis around Detroit's shrinking population—down 61 percent in 2010 over 1950 numbers. There's a basic traffic rule-of-thumb that says one light is necessary for every 1,000 residents. But it seems equally valid in the era of peak driving to tweak that rule to fit vehicle miles, which are moving in the same downward direction as Detroit's population, and of course serve as a closer proxy to road use.

To that end, we turn to the latest driving trends from the Southeast Michigan Council of Governments (the same agency from which Schrader and Hummer drew data). SEMCOG data show that daily vehicle miles in Detroit fell from just over 16 million in 2000 to just over 14 million in 2012—a decline of nearly 13 percent. Driving in the entire region of Southeast Michigan, as in many other U.S. metro regions, has taken a dive over this period:

SEMCOG

Now for the benefits of removing traffic signals. Schrader and Hummer point to four:

  1. Cost. Estimates suggest that operating a single traffic light can cost a city upwards of $8,000 a year. By that measure, Detroit could save nearly $3.7 million in annual taxpayer costs by removing its lights, according to Schrader and Hummer. (Though there would be initial costs associated with tearing down signals and putting up stop signs.)
  2. Traffic flow. In high-traffic corridors, signals do a great job keeping cars moving. But in low-traffic or off-peak situations they can often slow down vehicles—take the lone car that waited at the light at empty Charlevoix and Lakeview as a case in point. Additionally, in high-density areas filled with pedestrians and cyclists as well as cars, vehicle flow should no longer be seen as the only measure of street success.
  3. Safety. Counterintuitive as it might seem, there's plenty of evidence that removing urban traffic lights can improve overall traffic safety. While that's certainly not true for every intersection, researchers have found that replacing signals with stop signs on one-way city streets can reduce collisions—perhaps because, in the absence of street governance, everyone pays closer attention.
  4. Equity. Schrader and Hummer point out that many of the signals in their study that did not seem eligible for removal were lights serving through traffic between the city and the suburbs. "In effect," they write, "a poor city is subsidizing the travel of residents of wealthier ones." That Detroit taxpayers might be maintaining signals for the convenience of people who live outside the city suggests an equity component to light management as well.

Once again, obviously the indiscriminate removal of traffic lights at city intersections is a bad idea. But at a time when vehicle miles are on the decline, the same could be said for indiscriminately keeping them.

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