(Mobile, Ala.) The Bankhead Tunnel, Mobile's original underwater highway, was built in 1942 with automobiles in mind.
Well, not so much.
But twice in the last two weeks, large trucks have gotten stuck in the tunnel, ripping off the roofs of their trailers.
"They just don't pay attention to the signs that are out there," says David Johnson, works for the Alabama Department of Transportation and oversees Mobile's highway tunnel system.
And he said mechanisms are in place to warn truckers that trucks taller than twelve feet won't fit into the Bankhead.
One device is very simple. Chains hanging above the roadway near the entrance of the tunnel warn truckers that their rigs are too tall.
"The chains are just another thing to grab their attention," said Johnson. "If they drive through there, it's something to hit up against the vehicle."
But something else happens well before the chains.
And it has everything to do with a pole set back from the tunnel entrance, almost to Government Plaza.
It's a height sensor, a beam of light that is broken when a tall truck approaches.
"And whenever there's a vehicle that comes through in that lane going in into the Bankhead tunnel, that over height sensor will go off," Johnson explained. "And as soon as it's tripped, we have an alarm that sounds, the message on the sign immediately changes to DO NOT ENTER, there is a double red on the traffic signal that immediately comes on."
The alarm is sounded somewhere else, too... at the tunnel monitoring center across the river.
There, an alarm sounds, and operators also gets a warning on the screen.
"There are enough warnings provided before somebody ever reaches the portal," said Johnson.
The warning signs and alarms are on both sides of the tunnel.
But is there enough time to stop once you've seen them?
Yes, said Johnson, if the driver is driving the speed limit, which is 30 miles per hour.