Two-inch buffer notification

Beginning Monday, May 4, 2020, MDOT is implementing a two-inch buffer for structure underclearance as part of the review for all transportation permits. This two-inch buffer is taken from the measured point of minimum vertical clearance of the bridge and is not based on the posted height of the bridge.

Permit applications that exceed the two-inch buffer will not be issued.

This practice provides a buffer to account for several potential items that would cause the allowed clearance to deviate from the measured clearance, and thereby reduce the risk of over height impacts.

The reasons for adding the buffer include:

• Live load deflection of the bridge over the roadway travelled by the permit truck
• Snow and/or ice buildup on roadway below the bridge
• Debris in road
• Truck suspension (bouncing)
• Pavement movement due to frost/temperature heaving
• Overlays below the bridge due to maintenance or scheduled projects
• Electronic measuring device not picking true lowest points (especially bolt heads/etc.)
• Existing, unrepaired high load impact damage resulting in flange bending and then subsequent lower underclearance

This buffer provides a benefit to the trucking industry, MDOT and the travelling public. High load hits are an ongoing risk to our infrastructure, and the public that may be travelling under or over a bridge at the time of impact.

This effort is also intended to reduce damage to trucking industry assets, which typically result in significant insurance claims. Having a buffer for underclearance is a permitting best practice followed by many other states to reduce damage to the highway infrastructure.