Show HN: A Simple Highway Simulator

If you launch new cars too quickly, they start blowing up. :)

Here's something fun and counter-intuitive:

From default settings, set speed variance to 50%, launch frequency to 120 cars/minute, and reaction time to 500ms. Unsurprisingly, this is chaos.

Then, keeping those settings, clear the cars and set the target speed to 150km/h. The traffic mostly clears up! Since the cars are going faster, they leave the road sooner, meaning fewer are on the road at a time, fixing most problems.

(Of course, in real life, this also makes the accidents that do occur worse.)

A bug in this simulator is that new settings apply only to newly-launched cars, incrasing the chaos as you explore a range of options. Increased speed shows an exaggerated immediate increase in crashes, decreased speed causes an exaggerated decrease in crashes, and so on.

The sensible conclusion here is that human drivers could be banned in order to safely minimize travel times (much faster reaction times, lower variance in speed).

Something this sim doesn't model that plays a large role in real accidents is the speed differentials during merges. If an offramp moves too slowly and gets fully congested, it starts spreading onto the highway, compounding the problem.

Playing Cities: Skylines I've noticed a similar problem when I try to use roundabouts. They're idealized as a way to keep traffic flowing in busy intersections by imposing no stops, but if they surpass a critical mass of congestion, the whole thing stalls, creating a problem worse than stop lights.

(The Cities traffic model is pretty close to how a robocar might behave, as they are polite drivers able to execute apparently dangerous merges with 0% chance of collision, but also have the problem of not detouring when the ideal route becomes congested. As a result, you have to design your city to be decent at diffusing traffic early and not rely on wide roads to do that job.)

Interesting project! I had fun playing with it. I wanted to test the theory (which I saw in an article posted here on HN a few weeks ago) that a single car slowing right down on a busy highway (e.g. to avoid hitting an animal on the road or similar) can actually cause a traffic jam that goes back miles and can last for hours.

I couldn't prove that hypothesis at this scale. I am assuming that the AI drivers have just too good a reaction time and they can co-ordinate their speeds to each other to keep things moving along fairly nicely. A thumbs up for robot driverless cars, I guess ;-)

EDIT: Actually, playing some more - it does prove it to some extent. If you increase reaction time to ~800ms and let the traffic blitz along at a steady clip, then slow down two cars side by side to almost a stop, then after the initial cascade of accidents, it take ages for the traffic to get back to the same steady pace. Interesting.