Low clearance maps are not always accurate. Viaduct at 115th and Cottage Grove is listed as 13'6'' or better on the Low Clearance map for Chicago. The bridge is actually 12'8". Unless you drive here all the time you wouldn't know that.
Sometimes a viaduct is marked 13'6", then they repave the road and forget to remeasure and change the sign. Or an inch of compacted snow at the entrance to the viaduct can turn a 13'6" into a 13'5".
Bottom line though, the driver is responsible for every bridge he attempts to go under. G.O.A.L. Get Out And Look. No matter how many four wheelers you tick off.
But remember drivers, when asked by someone when you get stuck if you are stuck, just say, "Stuck? Heck no, I'm delivering a bridge".
Yours is the voice of experience. I drive big rigs in and around the Big Apple. Plenty of surprises, even for the experienced driver.
Heading east on 495, modern highway, all the bridges at 13-6 plus. Suddenly there is a little sign on one of them: “Clearance 12-8”. Nice!
My favorite is on the west side of Colorado Springs on a railroad trestle. “Clearance 96 FEET” IIRC