The Newport Arch in Lincoln, a gateway in the Roman city walls - this 3rd century arch is believed to be the only Roman arch in the world that still has traffic passing through it - though as it has been hit twice by lorries in the last 40 years this seems a bit risky! The gate carried traffic to the north of the Roman city of Lindum (Lincon) along Ermine Street - the modern A15 road still follows the line of this road, travelling over 20 miles in a pretty much straight line.
For more information see:
http://en.wikipedia.org/wiki/Newport_Arch