You can still take the Traffic Quiz and get an instant score, but you’d better do it before reading any further. As promised, here are explanations and sources for the answers in the quiz, which was created by Tom Vanderbilt, the author of “Traffic.”

1. At what speed have four-lane highways been found to achieve maximum flow — that is, move the most cars past one point per hour?



60 miles per hour. (This answer comes from a number of sources, including (in pdf format) this 2005 article by Pravin Varaiya in the journal Access.)

2. What state, as ranked by fatalities per million vehicle miles driven, is the most dangerous in the United States?

3. How long are white stripes that divide lanes on a typical highway?

Ten feet is the recommend standard; in many cases they can be as long as 15 feet. A good rule of thumb is they are long as the average car itself.

4. True or False: Drivers sitting relatively high above the road tend to drive faster than drivers sitting closer to the ground.

5. By what estimated factor increase does the fatality risk in crash increase as speed increases from 25 to 50 mph?

6. What walking speed do engineers use in deciding the length of “walk” and “don’t walk” phases?

Four feet per second.

7. At what average speed do we “overdrive” our low-beam headlights — lose our ability to stop in time if coming upon a dark object?



Forty-five miles per hour. (Answer provided by Michael Sivak, head of the human factors division at the University of Michigan Transportation Research Institute.)

8. Imagine there are two road improvements a city can make. One will improve the speeds of travelers from 40 kilometers per hour to 50 k.p.h. The other, on a road of the same length, will increase speeds from 80 k.p.h. to 130 k.p.h. Which would be better in terms of time saved for travelers?

Both save about the same amount of time. (See Ola Svenson’s 2008 article in Applied Cognitive Psychology.)



9. If you drove aggressively, how much time could you typically expect to save on a typical car trip that would ordinarily take half an hour?

One minute. (Aggressive driving studies have been done in a number of different contexts, but see, for example, this report by J. Archer and colleagues at the Monash University Accident Research Center.)



10. What percentage of crashes in the U.S. were found to be related to “vehicle factors” as opposed to driver behavior?