On some (modern and ww2) vehicles (mostly jeeps and trucks) you can find yellow circles with a black number.

Can someone tell me what the purpose of this is?

These are weight ratings... the number in the circle is the bridge rating required for the tank or vehicle to cross. It also indicates the normal weight in tons...

Lance is right, it's the bridge classification marking and it is still used today. The new type is green with black numbering. The sign has adjustable numbers that allow you to have numbers "00" through "99". Tanks and other heavy combat vehicles just use a black painted circle with the weight number in the center. Wheeled vehicles have the adjustable numbers since their weight will change depending on the load (if any).

Here's an example on a Canadian Sherman:



Nic

How does the crew of the vehicle know what the bridge rating is ? I would guess that for bridges erected by Combat engineers some type of sign is posted. How about existing structures?