eli5: would adding more lanes to a freeway/busy street really ease congestion or would you still get bottlenecks?

Question

I mean theoretically adding a lane or two should allow more cars to flow through, or would bad drivers still cause bottlenecks/gridlock despite the added capacity?

In: 28

Answer 1

First, highways cost a LOT. But let’s get to the real answer.

Short answer is every time you switch lanes, you increase the chances of an accident. Because of this, at some point, you reach the rule of diminishing returns.

Say you are in on a 4 lane highway (2 lanes each way), and want to exit soon. You lane-switch to the right lane (in America anyway) so you can exit. That’s a lane change. Even if you charge up the exit ramp at highway speed, that’s still a lane change.

Now, you’re on a 6 lane highway. That’s 2 lane changes. Each increases the odds of an accident… a little. But there are a LOT more cars on that 6 lane highway, so the odds actually do increase quite a bit.

Now go to a 12 lane highway, 6 lanes each way.

Some cars lane-change toward the faster near-the-median lanes because they’re not exiting. Some cars go the other way because they want to get off the highway. **People hit brakes. A lot.** Eventually all the switching, and slowdowns caused by other people switching, clogs up everything FAR FAR MORE than the slower but consistent lane-changing of a narrower highway with less lanes.

At some point it get really super messy. And that’s why they don’t do it.