Waht's a linear accelerator? Something to make lines move faster?

Speaking of something to make lines move faster, imagine this scenario:

You arrive at the drive-up teller at your bank. There are four lanes, and each lane has exactly one car in it. Ideally, instead of pulling into a lane you'd wait a bit and see which car pulls out first and take their place, but you can't do that because there's another car behind you. So, you have to take your chances and pick a lane. Why doesn't the bank have some sort of signaling device to let you know which one of the four customers' transactions is being worked on so you can pull into that lane?