The Highway Trust Fund, which helps repair roads, bridges and mass transit, has hit a bump in the road.

The federal fuel tax isn't generating enough revenue to keep the fund solvent. Some lawmakers have a novel alternative that would switch to a fee per vehicle miles traveled, a system that's already being tested in Oregon and Utah, with Washington state close behind.

But how does a tax-by-mile system work? Watch as CQ Roll Call's transportation and infrastructure reporter Jessica Wehrman explains the approach.