MDOT now applies a two‑inch buffer for structure underclearance when reviewing all transportation permits. This buffer is based on the bridge’s measured minimum vertical underclearance, not the posted height.
Permit applications that exceed the two-inch buffer will not be issued.
These factors include:
- Live load deflection of the bridge over the roadway travelled by the permit truck
- Snow or ice buildup on road below the bridge
- Debris on the road
- Truck suspension movement
- Pavement heaving
- Added pavement overlays due to maintenance or projects
- Limitations in electronic measuring equipment
- Prior high‑load impact damage that may lower the true underclearance
The buffer benefits the trucking industry, MDOT and the traveling public by lowering the risk of infrastructure damage and improving safety. It also helps reduce costly damage to trucking equipment and aligns with best practices followed by many other states.
This practice adds a safety buffer to account for factors that can cause actual clearance to differ from measured clearance, reducing the risk of over‑height impacts.