Sunday, 8 January 2012

The Pathology of Tailgating

I think I might have figured out why some drivers follow too close. It's not the reason they'll likely give if you ask them, of course, which is usually to complain that the guy in front of them is just driving too slow. After all, a large percentage of the time, the car in front of them is stuck behind another car going the same speed, and having someone crowd up behind isn't going to help at all.

I suspect the problem is actually one of an unconscious error in thinking, which owes a lot to the fact that we can see space, but not time. You're trying to get closer to your destination, which is of course ahead of you. The rear of the car in front of you appears as an immediate (if temporary) limit to how much closer you can get to your destination, and so it just visually looks like you're making better progress if there's less space. And it's true, of course, that you are a few meters closer at any given instant to your destination than you would be if you were following two or three seconds back. But contrary to this intuition, it does not mean you're going to get there materially sooner.

(A related motive, and somewhat less innocent, is to deter people from merging into your lane ahead of you. Sometimes this is owing to a self-righteous anger at those jerks who try to zoom up as far as they can in a lane that's about to close and muscle in ahead of their proper place in line. Sometimes it's an instinctive competitiveness, that not being "first" in a column of vehicles is somehow a failure. Sometimes it's just an unconscious intuitive resistance to the idea of having yet another car between you and your destination, seeing these vehicles as physical barriers to where you want to go, when in fact they have negligible impact on your average speed, unless of course there is a physical impact.)

It's well-known, or at least it ought to be, that following too close is a contributing factor in many accidents. Yet this doesn't seem to influence the drivers who habitually follow too close. Perhaps they (like most drivers) think that they have above-average reflexes and don't need as much warning to stop in time. Or maybe they feel the risks are worth the gains.

So let's consider these putative gains. Does driving a fraction of a second behind the car ahead of you actually gain you an advantage? There are two scenarios where I can imagine it might appear to do so. One is trying to squeeze past a traffic light before it turns red, in which case the time potentially saved could be up to a minute for a very long traffic light. Of course, this is risky, since it assumes the driver of the car ahead won't stop. These kinds of collisions happen all the time, and it is always the fault of the driver behind. You're just not entitled, morally or legally, to assume that the driver ahead of you is going to speed up when the light turns amber. (Even so, I have actually heard the driver behind complain that the driver ahead "didn't leave me enough room to stop!" But really, who had control over the amount of room available to stop?)

The other scenario is where the car ahead of you is slowly overtaking a car in another lane, and you want to change into that lane. The closer you are to the car ahead of you, the sooner you'll be in position to change lanes. Yet this isn't a good reason, either. If you need to get into the other lane in order to exit, why can't you just drop back and change behind the slower car? You're less likely to overshoot your exit in that case, and also less likely to inconvenience the slower car when you slow down to take the exit if they are proceeding straight. Or if you want to squeeze into the other lane in order to pass the car ahead of you, bear in mind that the car ahead of you might also want to move into the slower lane to let you pass, and your impatience prevents him from doing so. Or the car in the slow lane might like to get over into your lane, leaving their lane open for you to exploit, but since you're crowding up to the car ahead of you, there's no room for them to change lanes until you pass. Either way, you're actually short-circuiting your own objective, as well as inconveniencing other drivers and exposing them and yourself to an unnecessary risk.

This is true in the big picture as well. Following too close often creates traffic jams, even where there is no actual obstacle present to slow down traffic. Traffic flows more smoothly when people have room to change lanes and exit or merge. (This website has a very interesting analysis of the phenomenon, as well as what driving strategies can prevent or even in some cases evaporate traffic jams.)

The point I'm trying to make is that the gains you think you make by following closer than two seconds behind the car ahead of you are actually illusory; most of the time you gain nothing, or even end up arriving at your destination later than you would had you just dropped back a good two seconds. It's an instance of being penny-wise, pound-foolish; by gaining two transitory seconds, you lose minutes, hours or potentially the rest of your life.


  1. Very nicely done, and congratulations to you for knowing "the two second rule."

    I found it rather entertaining some years back when some organization or other in Massachusetts [may have been the Registry, the State Police, or a consumer safety group, I don't recall] did a spot on that.

    An interviewer asked about ten "person on the street" interviewees to define the two second rule. Most of the respondents were male. The first about nine respondents had no clue what that meant.

    Finally, a respondent stated something along the lines of: "It's a way you determine safe following distance between your car and the one in front of you by counting 1001, 1002 between the time the car in front passes a stationary object and yours passes that same object."

    The one respondent they found that knew that was a woman.

    People are notorious for unsafe following distances around here. It drives me crazy.


  2. Thank you for your comment. You know, I remember reading in some driver manual many years ago (maybe even when I took my written test for my learner's permit) a table of distances for different speeds. That is, rather than just teaching drivers to stay back two seconds, they were trying to get them to memorize a series of distances, sometimes even described in terms of car lengths. There may still be a question on some of these exams: "You are travelling at 50 km/h. How many meters should you leave between your vehicle and the vehicle ahead of you?"

    It's a valid physics problem, but unnecessarily complicated, and worse, trains us to look a the distance rather than the time. Visually estimating a distance, whether in feet, meters or car-lengths, is a much more difficult mental task than selecting a marker and counting seconds.

  3. I find that most traffic problems can all be traced to selfishness (greed?). I do recall my graduate statistics adviser telling telling us that there had been studies done on traffic that showed that if people behaved in a co-operative, rather than selfish, manner traffic would flow faster and accidents would be reduced. Sadly, too many people are not conscious enough to do so. Ignorance is not always bliss.

  4. You're right that it's largely attributable to a kind of selfishness, but I think it's a little more complex than that. To be sure, there ARE situations where a driver can actually gain some kind of advantage by driving aggressively, but my point here is not that the selfishness itself is to blame, but a short-sighted and ultimately self-defeating "selfishness".

    I wouldn't mind so much if tailgating actually made logical sense as a self-interested tactic. I'd object, still, but at least I'd understand. What infuriates me is that it imposes costs on everyone else for NO actual benefit to the tailgater. The tailgater THINKS he's gaining an advantage, but is actually harming his own interests along with everyone else's. So I suppose I'm making a plea here that if we're stuck with self-interested drivers, could we at least please have some enlightened self-interest?

  5. That was actually one of the coolest side-effects of the Prius, as I'm sure you're aware. Having the efficiency and consumption logs in real time, as I'm driving actually provided an incentive to drive efficiently. It encouraged thinking well ahead, and leaving large gaps to manage my fuel incidentally also meant I was managing my time and space on the road more efficiently as well.

    It was always more effective to drive that way, but the visual feedback made it a very clear and immediate reward mechanism.

  6. Indeed. In fact, I think auto manufacturers are starting to put real-time fuel consumption displays in even non-hybrid vehicles now, because it is so successful in helping to promote fuel-efficient driving, and for drivers who value fuel economy, it's a big plus.