If you make something more costly, . . . .

Here is a simple example of economics. If you increase the cost of concentrating on driving, people will drive more slowly, be less likely to change lanes, etc.. In this case, the advent of cell phones have raised the cost of people concentrating on driving.

Compared with undistracted motorists, drivers on cell phones drove an average of 2 mph slower and took 15 to 19 seconds longer to complete the 9.2 miles. That may not seem like much, but is likely to be compounded if 10 percent of all drivers are talking on wireless phones at the same time, Cooper says. . . .

In medium and high density traffic, drivers talking on cell phones were 21 percent and 19 percent, respectively, less likely to change lanes (roughly six lane changes per 9.2-mile drive versus seven or eight lane changes by drivers not on cell phones). . . .