What are clearance lights?

What are clearance lights?
Clearance lights are common on Semi-trailers and taller vehicles. What they are are lights at the top of a vehicle that marks the outline so you can make sure you can clear an obstacle.

For example: Look at a big truck going down the freeway at night, see those lights at the top of his trailer? those are clearance lights. These mark out the outline of a vehicle so you can see them and so the driver can make sure if he is going under something he will clear it.