system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What causes the "singing" when you drive over bridges or certain types of asphalt at high speeds?
gpt
Regularly- and tightly-spaced bumps, basically. Same principle at work when you run your fingers down the teeth of a comb, strum a washboard, or something like that. This is most noticeable on bridges with road surfaces made out of some kind of metal grid (to let water through and reduce weight, mostly). With regular roads, it'd be "rumble strips," either across travel lanes to warn traffic to slow down, or on shoulders to warn drivers to get back on the bloody road.

Every time the vehicle's tires hit one of these bumps (whether raised or depressed), there's an impact noise. Just like running over a stick. But do a bunch of those in tight succession, and the ear/brain starts to interpret the resulting sound as a musical "pitch". Musical pitch is directly related to frequency of vibration. That's all sound is, really: shock waves propagated through a medium (usually air), that are detected by organs in the ear, transmitted to the brain, and interpreted as sound. So the faster the car is going, or the closer the bumps are together, the more of these impacts there will be in a second, producing a higher frequency of vibration and thus a higher pitch.