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Tuesday, June 22, 2010

What Sir Isaac Newton Would Say to Washington's Drivers, Lawmakers, and Traffic Engineers

Funny thing about the laws of physics: You can study them, you can conduct experiments about them, and you can expand your understanding of them. But you can never, ever, break them.

This in contrast to, say, the laws of the Washington State Legislature.

This is a distinction that many drivers do not seem to have grasped. How many times have you seen a half-dozen cars tailgating each other up the fast lane at 80 miles an hour? If the State Patrol's not around, nobody gets a ticket. But if the guy in front has to hit his brakes suddenly, there's gonna be a crash, police presence or no. Sir Isaac could have told them that.

Drivers aren't the only ones who could use a physics lesson. Lawmakers and traffic engineers who set speed limits could use a refresher course, too.

Let's begin with vehicle stopping distance. There are two components to consider: driver reaction time, and braking distance once the driver does react.

Logic dictates that the higher a vehicle's speed, the farther it will travel before the driver reacts to, say, a pedestrian who has stepped off the curb. According to an excellent paper by the Australian Academy of Science, small increases in speed can make a big difference. For example, given typical driver reaction time, a car traveling at 65 km/h (40.4 mph) will travel almost 89 feet between the time the driver sees the pedestrian and the time he or she slams on the brakes. That's about seven feet farther than a car traveling at 60 km/h (37.3 mph). And reaction time can be significantly longer for distracted drivers.

Add to this the fact that braking distance is proportional to the square of velocity, and you end up with strikingly longer total stopping distances given relatively modest increases in speed. For instance, in the example above, the car traveling at 65 km/h will take almost 15 feet longer to stop than the car traveling at 60 km/h--a 12 percent increase in stopping distance, even though the speed difference is a little more than 3 miles an hour.

But there's more: if a car does hit a pedestrian, the chances that the pedestrian will die increase drastically with speed. This is because force of impact increases as the square of speed; in other words, a car traveling at 40 miles an hour will strike the pedestrian with four times the force of a car traveling at 20 miles an hour. Here's what that means for pedestrian death and injury rates, according to statistics from the UK as cited on SF Streets Blog; a wealth of information from other sources supports these numbers:

In Europe, a growing number of countries have moved to lower residential speed limits to 30 km/h (about 20 mph). Look at the chart above and you'll see why. 30 km/h speed limits are widespread in the Netherlands, which has the lowest vehicle accident death rate (and safest cycling) in the world.

And the BBC reported that the introduction of 20 mph zones in London cut road injuries by more than 40 percent and reduced by half traffic deaths and serious injuries among children. In all, researchers estimated that 200 lives a year had been saved. A British safety official called 20 mph zones "one of the most effective ways of protecting vulnerable road users, especially children."

What can we take away from this little physics lesson? For drivers it's obvious: slow down, don't push the speed limit, and don't tailgate. A few miles an hour, and a few feet, can mean the difference between life and death (Note to Seattle drivers: No, it's really not OK to average 40 mph in 30 zones).

For traffic engineers: expand traffic calming and consider lowering speed limits, sometimes dramatically. Just as important, police should emphasize strict enforcement.

And for our state's lawmakers: consider lowering the default speed limits set under state law, and give local officials the clear legal authority to establish widespread 20 mph zones. Current state law appears to prohibit this except under special circumstances, such as in school zones.

I'm sure that everyone agrees that saving lives is more important than saving a few minutes. Now it's time to act on that belief.

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