The Clydevale bridge over the Clutha River will have speed bumps installed at each end during a two-month trial to slow motorists, after a survey revealed drivers regularly speed across the structure.
The bridge has a speed limit of 70kmh for light vehicles. Heavy trucks can only cross at 10kmh.
The bridge is 22km northwest of Balclutha and has been subject to speed and weight restrictions since November last year, when an inspection found the structure was never intended to support class 1 (44 tonne) vehicles.
In order to preserve the bridge until it can be strengthened and brought up to a class 1 structure, vehicles have been limited to 70kmh for light vehicles and 10kmh for heavy vehicles, with a permit system in place for loads exceeding 50% of class 1 (23 tonnes).
Mayor Juno Hayes was vexed by the survey data, recorded between January 27 and February 24, which was presented to the Clutha District Council District assets committee last week.
It showed heavy-vehicle drivers were travelling at speeds well above the 10km allowed, and while most light-vehicle drivers crossed the bridge at the recommended 70kmh or less, there were still some who ignored the signs.
Fifty-one light-vehicle drivers were recorded crossing the bridge at more than 100kmh, with one driver crossing at 140kmh.
Mayor Hayes said it was "plain ignorance" for people to disregard the speed signs, and he intended to make sure anyone caught flouting the speed restriction was prosecuted.
"If you exceed the speed limit by 45kmh, you will lose your licence for 28 days," he said.
Clutha District Council chief executive officer Charles Hakkaart said speeding vehicles could have a detrimental effect on the bridge.
"Speed is a major factor when it comes to the cause of damaged bridges," he said.
"Many motorists would assume the quicker they get off the bridge the less wear would occur. However, this isn't the case."
Mr Hakkaart said the overweight permit system would remain in place until the structure had been strengthened, which could be another six months.
As well as the speed bumps, the 10kmh speed limit for heavy vehicles will be extended beyond the bridge.
