Yeah, you tell the speed by how fast the sound gets louder. Where that's helpful is that you can pick out the vehicle that is moving faster (or slower) than all the rest of the traffic. When all the traffic is doing 60 mph, that's OK ... everyone is doing something predictable. But if you hear one vehicle moving faster than everyone else, that's when you'll want to take a glance in your mirror to see what's going on. Chances are he's passing, and if he's passing, then perhaps the car he is passing is moving over onto the shoulder where you're cycling ... which could pose a problem to you. The sound of a slower vehicle could be a problem too, because that vehicle might be driving on the shoulder to keep out of the way of other traffic ... however, it might still be travelling faster than you are ... which could pose a problem to you.

And if you're really listening, you can place almost exactly where the vehicle behind you is. If you're on the shoulder of a 4 lane highway, for example, you can tell which lane the vehicle coming up behind you is in ... or if it is on the shoulder. On narrower roads, you can tell if the vehicle coming up behind you is going to whistle past your elbow or if it has pulled out and is going to give you some room.

By listening you can also hear the cars pulling up to a hidden intersection you might be approaching, or about to pull out of driveways, and so on.