Witnesses say a truck hauling construction vehicles appeared to clip the bridge Saturday on the London-bound M20.
Ambulance officials say the injured man was a motorcyclist who came off his bike as the bridge started to collapse in front of him, but his injuries were not life-threatening.
The southeast motorway, which links London with the major cross-Channel port of Dover and the nearby Eurotunnel terminal, was closed in both directions.
The shutdown caused massive traffic jams on what was already an exceptionally busy three-day weekend for holidaymakers heading to and from France.
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