Tuesday 27 May 2008

Two-way traffic

A few weeks ago we hired a driver and drove for two and a half hours (on the first day of a holiday weekend, no less) to one of the Great Wall sites. We thought we had negotiated a price that included road tolls so I was surprised to find us on small toll-free highways in so much traffic. I spent a good portion of the way there looking at sometimes-solid-sometimes-dotted expressway markings on the map that seemed to be running largely parallel to our route and telling myself that perhaps it just wasn't open yet. But the expressway isn't really the point of this story.

On several occasions, I've observed a certain traffic phenomenon here and this trip provided a good illustration. In China, you see, when someone or something on the road is in your way, you just... go around. Even if it means moving into an empty space in the oncoming lanes. Ignoring the danger of an accident, it seems reasonable enough at first glance: save yourself some time, keep the traffic flowing.

An hour or more into the trip we came along a particularly heavy patch of traffic and, as we inched along, I began to suspect what had happened: you see, this section of road had five lanes of traffic in total but there were four lanes going our way. Not a normal distribution. Sure enough as we moved along we came to a point where there were suddenly four lanes going the other way and ours all merged into one. Can you picture this? There was now one through-flowing lane in each direction and, in the middle, three lanes of traffic coming each way facing head-on into each other.

You can picture how this would happen: one direction slows up for some reason, someone darts around into a gap but doesn't make it back in because the traffic is moving slowly and there are no gaps. This blocks the traffic coming the other way and cars further back in that line, in frustration, dart around the stopped traffic ahead and find they can't get back in because the traffic has stopped. This is similar to the effect known as shockwave traffic, which produces traffic jams from something as simple as one car slowing too quickly in heavy traffic. Scientists in Japan recently managed to recreate this effect in a controlled environment. If you've never heard of this, check out the video mentioned in the above article or check out this computer simulation (click on "1: Ring-road" once it loads). Then you'll understand why that huge traffic jam you're stuck in often has no obvious cause when you get to the end of it. If you want to read more, check out here and here.

So, as the traffic keeps backing up, cars keep trying to go around and end up blocking more and more lanes. If the cars would only cooperate, they'd all get where they're going faster. Because everyone's trying to maximize their own forward progress, however, they end up in near gridlock. This kind of greedy algorithm reminds me of the Prisoner's Dilemma.

On our way home, we came upon the same patch of road and came to a complete stop for 15 minutes. Eventually, somebody opened a side street that had been blocked off and we were able to begin diverting around the stalled traffic into the countryside. As we moved our way forward, guess what I saw laid out in front of us?

Yup, 5 lines of traffic all pointing in the same direction...


Sarah said...

Can you imagine if people starting trying this technique on the Lions Gate or in the Massey Tunnel?? Haha.


Mike said...

I've experienced a similar phenomenon in the NYC subway system: Passengers on the platform are supposed to let all exiting passengers leave first, then everyone moves on.

In many cases, some self-interested passengers try to force their way through the stream of disembarking passengers, slowing the whole system down and lowering the odds of both exiting and entering passengers getting where they need to go. People getting off the trains occasionally bump these people back off the train or into a pole by the door, which I have to admit makes me smile.

Julian Fitzell said...

Yeah, subways suffer the same problems here. The Chinese government is really trying to educate people to let passengers off first: there are signs everywhere and markings on the floor. Sometimes they're not so bad but often they are.

Julia just passed her written test for a Chinese driver's license. Apparently most of the questions are about how much the fine is for breaking the (quite numerous) rules rather than the rules themselves. :)