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Comment by antoko | Hacker News Reader

Hate to say this because I loved the movie too. But it always bugged me that the timing of the repetitive noise helped them determine the bridge. The unknown speed of the vehicle would make it pretty difficult to determine anything based off the noise pattern.

It has been a very long time since I watched the movie so there may be something that addresses this that I'm not aware of, if that's the case please let me know!

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Watch that scene again. They adjust both the pitch of the tires on the pavement and the timing of the "bumps" from the seams. While two different bridges could have the same timing at different speeds, or the same pitch, they're betting on the combination of pitch and "bump" timing to be unique for a given bridge and a given speed.

Aw crap, you're right, I had forgotten about the sound of the tires on the pavement in between the bumps.

The pitch of which is determined in large part by the tire tread pattern, so the sound could not easily be compared / transferred between the bad guys' vehicle and the good guys' unless they knew what kinds of tires the bad guys had.

If you had a rough estimate on time-of-day you could make an educated guess for speed based on traffic patterns. Assuming the bad guys wished to blend in with the traffic to avoid suspicious behavior that would potentially lead to being pulled over by a police officer. Even then, it's only one variable in the equation which is easily verifiable in a location with only a few bridges where some of them are eliminated almost immediately before even getting in the car.

But I don't recall them covering anything on that subject in the movie.

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