I caught myself drifting onto the shoulder at two in the morning

My eyes closed for a second and the car drifted. I don't remember deciding to open them. — Nurse after a night shift, 2008

By 1994 the Pennsylvania Turnpike had a new rumble strip on its shoulders, the Sonic Nap Alert Pattern. SNAP. The Turnpike knew exactly who it was for. The Turnpike's own study found run-off-road crashes fell by about two-thirds. Most new cars now carry the same idea on board: lane departure warning beeps or buzzes when you cross a line. Crash studies say it cuts those crashes some, not all, and plenty of drivers switch it off because it nags. A car that does the driving would be the real answer, and driverless taxis already carry paying riders in several cities. No one has put a date on the one in your driveway. Until then, lane warning wakes you up. It doesn't keep you up.

For now: if your car has lane departure warning, leave it switched on, and if you're drowsy, pull off somewhere safe and nap for 15 to 20 minutes.

The full file

The Turnpike's engineers tuned SNAP so a truck driver would feel it in the cab without it being too harsh for cars or motorcycles. That's a narrow target. California had run its own test back in 1985, on stretches of two interstates in San Bernardino County, and found 49% fewer run-off-road crashes.

Later, wider studies were less excited. A national research report puts the cut at 15 to 22% of single-vehicle run-off-road crashes, depending on the road. The early numbers ran as high as 80%. The strips are loud for the neighbors, and awkward for bicyclists. Still, Kentucky calls them among the most cost-effective ways to prevent roadway departure crashes.

Drowsy-driving crashes peak between midnight and 6 a.m., Kentucky says, and again in the late afternoon. Two in the morning is when the strip earns its keep. Naps are cheaper.