Monday, April 03, 2006

Driving Solo at 78.0 mph

I merge onto I-75, gun the accelerator of the beast of a V6 engine I have on my mid-size family sedan, and quickly hit 70 mph. A trucker is barrelling down on me in the right lane, but then politely merges to the center to allow me to get in. I hit 80 mph as I merge into the right lane and speed ahead of the truck who has conveniently created an open region in the middle lane just in front of him. I never pass unless I can see the car in my rear-view mirror, which usually gives me at least three or four car-lengths of passing buffer. I set my cruising speed to 78 mph. I am cruising.

Why 78 mph? It's the ideal interstate driving speed. See, in the state of Florida, the speed limit on the interstates is usually 70 mph. However, the penalties for speeding are such that it's not really worth the officer's time to pull you over unless you're running at least 15 over the limit. It's all a matter of balancing risk with payoff.

The difference between risk (a $150 fine and 4 points on your license) and payoff (getting to your destination 7 minutes faster on a 200 mile drive) with 90 mph vs 85 mph is huge. Pass a cop in a speed trap going 90 and you're almost certain to be pulled over. Your risk factor is extremely high. Going 85 is 15 over, which isn't too bad, but will still get you in trouble with irritable cops. Go 80, on the other hand, and you'll fly through speed traps with impunity 95% of the time. For a number of reasons, it's just not worth the cop's time to pull you over. For one, it's only 3 points on your license for a moving violation under 15mph over the limit, and secondly, the fine is significantly less.

Now why not 75? At that point, the fine is only about $80 and you have practically 0% of getting pulled over for speeding. But that's the point-- the difference in risk between 75 and 80 mph is probably only about 4%. For most people, that's worth the 7 minutes they'd save. On the other hand, the difference in risk between 80 mph and 85 mph is probably about 20%. That is, if you were to pass a speed trap going 85 mph, I think you'd have a one in four chance of getting pulled over. Just a guess. Risk at 90 mph is probably 75% and risk at 95+ is probably about 98% (the cop could have been looking down to wipe off the jelly from his donut). Those extra 7 minutes look increasingly less appetizing as the speed ramps up.

So what's the solution? I think that, unless I'm in a huge hurry, the 25% risk of 85 mph is just not worth the time saved. That said, the 4% difference in risk between 75 and 80 is not enough to keep me from saving a little time. So why 78 mph? Well, the second speed "echelon" is actually 75-79 mph, not 76-80 mph. I go 78 simply to account for random error and drift in cruising speed, along with a potentially inaccurate radar gun. By extension, when I'm in a hurry, I'll cruise at 83 mph. Eh, you only lose a minute or so. Managing risks and rewards: the key to good decision-making.

*Updated 12/10/06*


Anonymous Anonymous said...

you know what's a bad decision? reading blogs while i should be writing. parents never warn their kids about intellectual crack

4/03/2006 4:05 AM  

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