Friday, November 09, 2018

Why traffic jams happen and how to stop them happening

An article about the right way for cars to merge lanes on a busy highway has reminded me of another (video) article I saw recently which tried to make sense of some of the apparently inexplicable phenomena we regularly experience in traffic.
The article in today's Globe focuses specifically on the thorny issue of merging lanes, for example when joining at an intersection or when encountering construction blockages. Whatever you may feel about it, the science most definitely and definitively concludes that "late merging" or "zipper merging" is the way to go. If done properly, it can be up to 40% more efficient to go the the end of the lane and seamlessly merge by alternating one by one, with as little braking as possible on both sides, rather than trying to merge early and holding up people behind you. The sting, though, is in that phrase "if done properly": it relies on vehicles in the continuing lane letting people in, on the joining vehicles not peremptorily pushing their way in and causing everyone else to brake, and on people not doing crazy things like haring off down the hard shoulder or moving out of a continuing lane to a merging lane in order to get a few vehicles length ahead (and yes, this kind of thing is increasingly common, at least around Toronto).
The YouTube piece I was referring to is an episode of Hank Green's SciShow called "Why is it so hard to fix traffic?" I confess that I find Mr. Green's style intensely annoying (my 23-year old daughter loves him, so maybe it's a millennial thing?), but his shows are often interesting and always well-researched and explained.
In addition to confirming the wisdom of zipper merging as examined above, the video explains (using fluid dynamics and the simple analogy a bag of rocks) that it is possible for a traffic jam to occur purely due to the sheer number of cars on a road, without the need for any bad behaviour or poor driving skills. It also shows why building new roads or adding lanes does not necessarily help traffic flow, and can even make it worse (a phenomenon known as Braess' Paradox). Conversely, completely removing some busy roads has been shown to actually improve traffic flow, as the examples of Seoul and San Francisco have demonstrated.
The video also explains the theory of "traffic waves", which is so often the reason for those inexplicable slowdowns which appear to have absolutely no visible cause. When aggressive drivers weave in and out of traffic, or force themselves into inadequate gaps, the cars behind them have to brake for safety, leading the cars still further behind to brake even more. Ultimately, on a busy road, some cars way further back in the line will be forced to come to a complete stop, all due to some idiot a long way ahead who just couldn't rein in his patience (and yes, I use the masculine pronoun advisedly). Then, as people move out of the jammed lane to avoid the hold-up, they cause the same thing go happen in the adjoining lane too, until there is a full-blown traffic jam in all lanes for no apparent reason (the actual reason being that thoughtless yahoo in the Beemer a few minutes ago).
Of course, the obvious solution to this problem is to leave more space between cars so that there is time and space to brake if needed without affecting others behind. This solution does not seem to have occurred to Canadian drivers, and whenever I leave plenty of space in front of me (which I always try to do) it usually gets filled in repeatedly by others, who seem to abhor a vacuum... Interestingly, studies also show that leaving more space in city diving can speed up your
journey because, although you may be a little further away from that traffic light.that is about to change, you can actually start accelerating again earlier than a car that is jammed up against the vehicle in front, and you certainly use much less fuel that way (up to 20%).
So, come on guys, give everyone a break and leave more space - tailgating helps no-one not even the tailgaters. And stop it already with that aggressive ducking and weaving - you may get home twenty seconds earlier, but everone else is still stuck in that traffic jam you created.

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