In praise of roundabouts

The first time I ever noticed roundabouts, I was on vacation in the UK. This was many years ago, but they seemed to be everywhere. At first I thought it was just one more peculiar feature of local transportation, along with narrow roads, streets that changed their name with each new block, and driving on the left. In fact, I didn't realize how much they improved the flow of traffic until I found myself immobilized in a column of cars for what seemed a very long time, only to realize we were all stopped for a traffic light.

That afternoon I saw why roundabouts were so common in the UK, and I began to wonder why they aren't just as common everywhere else. Of course rebuilding an existing intersection is time-consuming and expensive. But the statistics comparing roundabouts to traditional intersections are remarkable.

• The Federal Highway Administration estimates that replacing a traditional intersection with a roundabout reduces the number of fatal and injury crashes by about 80%. (That's 82% for a two-way stop-controlled intersection, and 78% for a signal-controlled intersection.)*
• When collisions do occur, they are generally side-to-side collisions from merging or changing lanes, rather than head-on or right-angle collisions. On the whole, side-to-side collisions are less damaging and less often lethal than the other kind.
• Not only are they safer for cars, but roundabouts have 10-40% fewer pedestrian or bicycle crashes than traditional intersections.
• Since cars typically don't have to wait as long at roundabouts as at traditional intersections, they contribute to reducing air pollution from idling engines.    

And all these benefits stem from one root cause: Roundabouts are far simpler than traditional intersections. In this context, simplicity is measured by counting "conflict points." These are points in the intersection where a collision can be foreseen. A traditional intersection of two roads, each supporting two-way traffic and crossing at right angles, has 32 conflict points. If those same two roads meet in a roundabout, the number of conflict points drops to eight. No wonder the roundabout is safer!

Why am I writing about traffic this week? There's no special reason, except that it illustrates in a dramatic way another important Quality principle: when other things are equal, simplicity is better than complexity. In other words, if you have two solutions to a problem and they both solve it equally well, choose the simpler one. The complex tool has more parts; the complex procedure has more steps; the complex intersection has more conflict points. In all events, this means that the complex solution has more ways to go wrong: more parts that can break, more steps that can be mis-executed, more collisions that can happen. From the perspective of risk management, the complex solution is always—again, other things being equal—more fragile and more at risk of failure.

Years ago, a colleague at work was telling me about the cup holders in his cars. He owned two cars, and the cup holders were very different. One car had a sleek and elegant design. The cup holder was tucked discreetly out of sight until you pushed a button; then a little motor gently unfolded it for you. But at the time we talked, a tiny part had broken in the motor, so it had stopped working. He had contacted the dealership, but it would be several weeks before they could get the piece in stock.

The other car wasn't nearly so elegant. But it had a piece of plastic in arm's reach, molded to hold a cup. The overall look wasn't nearly as sleek and beautiful as the first car, but there was nothing that could break. From the perspective of customer satisfaction, the simpler solution clearly had the higher Quality. And it's often like that.

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* You can find more detailed numbers, for example, here.