Photos tweeted by WDIV Channel 4 in Detroit show what appears to be a camper on fire.
The fire started around noon Tuesday. It was out before 1 p.m. Smoke could be seen in from Windsor, billowing up from the American side.
Detroit's fire and police departments responded to the emergency call. Windsor Fire and Rescue was not involved.
Traffic was stopped in both directions.
Officials originally anticipated traffic would begin moving soon after 1 p.m. ET, but at 1:30 p.m., the Windsor Police Service told CBC it would not likely open again until sometime after 3:30 p.m.
At the same time Windsor police made a statement, the Michigan Department of Transportation tweeted on ramps leading from Interstate-75 to the bridge would be closed.
Transport trucks stretched kilometres down Huron Church Road in Windsor to Highway 401 in Lakeshore.
Truckers got out of their cabs and passed the time chatting on the boulevard.
The cause of the fire is unknown.
The Ambassador Bridge is North America's busiest border and a commercial lifeline for many manufacturers in southwestern Ontario and the U.S. Midwest.
The bridge connecting Windsor, Ont., and Detroit carries one-quarter of all trade between Canada and the United States.
In 2010, a reported 28,814 trucks crossed the privately owned bridge daily, with a trade value of almost $500 million U.S.
Border expert Bill Anderson of the University of Windsor said a four-delay can potential cause distribution problems for the auto supply chain, which relies on just-in-time delivery.
Fortunately, Chrysler-Fiat's Windsor Assembly Plant, which builds Chrysler minivans and is one of Windsor's largest employers, is shut down until May and was not awaiting auto parts for production.