I-25 in Denver near 20th St. and 23rd Ave. has clear indications painted on the bridges and the signage before the bridges indicating lower clearance on the shoulders. It's not that hard to do.
NYC takes the cake for inaccurate bridge height signs, though. Some bridges have signs indicating 12'9" or lower clearance when standard 13'6" trucks can clear them easily. It's a complete mystery why they can't mark them correctly. It's been this way for decades (literally).
I got burned by this in upstate New York and it caused an unnecessarily long delay. I was following the directions provided by the customer and came across a 12'6" marked bridge towing a 13'6" trailer on a narrow two-lane highway. I stopped, obviously, and had to ring the highway patrol for help. Traffic behind me was stuck waiting and this low clearance bridge was not properly marked in the trucker atlas and (if it had truly been 12'6") not up to spec for a state highway overpass. I'd done my due diligence so this was the highway patrol or customer's problem to sort out.
About an hour and a half later, we established that the road had been recently re-paved and no one was exactly sure what the height was. Backing up to the nearest intersection was not an option because there were none close by and turning the tractor-trailer around was impossible. The cops were cool about it and suggested that I creep forward very slowly and see if I'd fit. I was able to do this and stop just as the trailer reached the bridge. I got out of the cab and climbed up behind the tractor to have a peek and had about an inch to spare. I went for it, very slowly, and made it under. No fun but I did get a cool photo as a memento: http://i.imgur.com/64uYu2z.jpg
I took a different route on the way back of course!
No, this is not practical and you risk damaging the tires. It is possible to deflate the air ride suspension for the tractor drive axles to drop a few inches (useful for backing under a new trailer when the nose is low) but I did not do this because the rear of the trailer would still be too high.
Not one of my finest but it is a view leaning out the driver's window and looking up and back at the top front left corner of the trailer, with the (very dark) bridge girders looking like they are resting on the trailer. There's about an inch of clearance and I am just coming out from under the bridge, nearly to the edge, with most of the trailer yet to pass under. Very difficult to see though, I really needed more flash but I was concentrating on other things and holding up traffic.
NYC takes the cake for inaccurate bridge height signs, though. Some bridges have signs indicating 12'9" or lower clearance when standard 13'6" trucks can clear them easily. It's a complete mystery why they can't mark them correctly. It's been this way for decades (literally).