Hacker News Reader

Comment by cwmma | Hacker News Reader

How speed limits are enforced in America always bothers me, because there is this great disconnect between planners and everybody else.

Planners think of speeds of roads being intrinsic to the design of the road thus if a people are going to fast on a road, you need to change it by narrowing it or putting in bumps or something.

The other side of this is to think of speeds of a road as based on whats around it, so if there are a lot of houses on a road, people should go slower so you don't hit people so you put in speed limits.

But the problem with limits is that since the road feels faster then the the speed limit, people just go faster then the limit, but since changing the road is a lot more expensive then just putting up speed limits that tool is used a lot less frequently.

View on HN

It is bizarre that we are discussing making cars entirely autonomous as a way of improving safety, but refuse to consider the much more feasible option of technologically restricting speed on public roads.

It might be technically feasible, but not politically feasible.

While it is a no brainer to say every vehicle should be traveling sufficiently far behind another such that they have enough time to stop before they hit the one in front, this spacing out of vehicles will result in longer travel times as you are effectively reducing road capacity by having each vehicle take up more space on the road. Instead, it's (politically) easier to have each person take individual risks of getting into a collision.

However, with autonomous vehicles, it should be politically easier to force this constraint because the liability will be shifted from individuals to presumably the manufacturer so the manufacturer isn't going to stick their neck out so the individuals can save time on their commute.

Slowing down traffic can, in fact, increase the throughput of the road, especially when it is busy.

This is for two reasons:

1) slower traffic requires a smaller safety gap between each vehicle.

2) the dominating factor in traffic jams is often people responding to the brake-lights of the people in front of them. People over compensate and therefore a "braking bubble" (Soltion: https://en.wikipedia.org/wiki/Soliton ) forms in the traffic which causes it to get even more spaced out.

Keeping traffic flowing is more important to throughput than keeping it flowing it fast. Constant braking and speeding up on a busy road means that the flow rate is constantly disrupted and this causes the traffic to tend towards clumping and congestion.

You're not "in traffic"; you "are traffic".

It's inevitable that people will over or under compensate, and differences in acceleration speeds of different vehicles means the rubber banding is inevitable, especially if you add any elevation climbs to the equation. Add in lane changes, merges, exits, and there's no situation where I would expect a slow and steady flow of traffic.

Maximum throughput at a fast speed with minimal spacing between vehicles is more than maximum throughput at a slower speed with minimum spacing between vehicles. I see this in every urban area, people sacrifice safety for speed.

Reply on news.ycombinator.com