Officials in Greensboro, NC, want to make sure truckers get the message than a railroad overpass there may be too short for rigs to drive under.

While there are already warning signs on the street pointing out the 11-foot, 10-inch overpass, the city has come up with a more alarming way to keep truckers from steering themselves wrong.

The city plans to install a photoelectric beam before the underpass, according to Greensboro’s News-Record. When trucks pass through the beam, an alarm will trigger a sign near the underpass to flash the words “Over Height.”

So far this year, six trucks have gotten stuck at the underpass at Martin Luther King Jr. Drive and Davie Street, the News-Record reported. More than 20 have done it since 2001.

City engineers think the road’s design could be a part of the problem. The road slopes downhill going under the bridge heading into downtown Greensboro, then begins to rise as it heads out the other side. So for drivers heading north on the road, the entrance to the underpass has more clearance than the exit.