HAVERHILL — It was a truck driver's nightmare.

A Mayflower moving van was headed east on River Street yesterday afternoon when it had to come to a sudden stop. The driver was confronted by the downtown railroad bridge, which has a clearance of 131âÑ2 feet.

Fearing his large rig would not be able to get under the bridge, he stopped and within minutes he had to contend with a traffic jam.

Police were called to the scene around 2:45 p.m. and they stopped cars long enough so the driver could back his truck up Washington Street about 100 feet or so. He then headed across the Merrimack River via the Comeau Bridge.

"It has happened a few times," said Sgt. John Arahovites, police spokesman, when asked how often large trucks have to back away from the railroad bridge, which was built long before tractor-trailers started hauling much of the nation's goods. Typically, the police have to go to the area, stop traffic and help the embarrassed trucker back up and take a different route.

Arahovites guessed this particular traffic challenge happens less than once a year. Fortunately for everyone concerned, the new Comeau Bridge, opened in 2007, was available so the trucker could drive out of the bind.

The new bridge was under construction for several years and there was no way to get across the river at that point.