Curious Contraption

Saw this thing working on the street in front of the office this
morning. The yellow gizmo has a guy strapped into the side, sitting
about 6 inches above the road. There's a little mechanical arm he
controls, which hocks a big tar loogie on command. Then, he takes one
of those reflective lane markers and sticks it on, and the trick
drives six feet forward; repeat as necessary.

I always wondered how they did that!