Because they look like they're going faster than they really are.
4 seconds sound clip from the The High and the Mighty (1954) movie soundboard.
You can hear this line at 00:37:41 in the DVD version of the movie.
Quote context
[...]
- She'll show it to all the fellas and girls who drop in to keep her company when I'm away.
- But it's there. It's like a heavy bundle in your belly. You can almost touch it.
- She spent most of her time riding around in it so she wouldn't get too lonesome.
- Guess she drove a little too fast...
- Because they look like they're going faster than they really are.
- All the old-timers say it's only temporary, like growing pains.
[...]