What are Clearance Lights?

Clearance lights are common on Articulated Vehicle (Semi-trailers) and taller vehicles. They are the lights at the top of a vehicle that mark out the outline so you can make sure you can clear an obstacle.

i.e.: Take a look at a large truck traveling at night, see the 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.