Molson Hart

I'm trying to understand what kind of effect adding lanes has on the effectiveness of highway transport and I...can't. Suppose you have a single lane highway between A and B. In theory, its capacity is infinite.

In practice, cars can only go as fast as the slowest car on the highway (1). Merging causes slowdowns (2). And accidents stop traffic all together (3). When you add a second lane, you solve 1, you mitigate 2, and you sort of improve 3 (an accident increases merging which slows).

When you add a third lane, you really get diminishing returns. You slightly improve 2 again, by reducing the chance of merging (from slow roads to fast highways) interfering with another faster car.

You probably don't improve 3 because there's a 1/3rd chance that the accident happens in the middle of the road, which is very disruptive. And then, even if it happens on the sides, people slow to look at the accident, and emergency vehicles will block other lanes.