We put a simple 18" pipe thru about 4 feet down from the top of the dam as our principal spillway. We got lucky and were able to create an emergency at the other end of the pond, so the dam should never be tested.
Anyway, we brought a full size concrete truck across it with about 1/2 a load, no issues. A large backhoe has passed it several times, along with a couple of 40K loads of rock in a triaxle. In our case, though, the dam in a substantial mass. The roadbed is 20' wide and the pond side is 3:1, the backside 12:1. Having the E-spillway at the opposite end was un unintended bonus when we built it.