On August 10, 1966, the partially completed south span of the Heron Road Bridge collapsed into the Rideau River below. The accident killed nine workers and injured fifty-five others in the worst single workplace accident in Ontario’s history. An inquest placed the blame for the collapse on the use of green lumber and the lack of diagonal bracing on the wooden support form for the poured concrete. It had been the second such collapse in the city that construction season. In March, a building on the corner of Elgin Street and Gilmour, across from Minto Park, had collapsed, killing one worker and injuring three others.

After an inquiry into the bridge collapse, the Association of Professional Engineers of Ontario disciplined three of its members and the construction company, O.J. Gaffney Limited of Stratford, Ontario, was fined the maximum penalty of $5,000. Ontario subsequently rewrote the provincial construction safety standards.

Just to the west of the bridge, there is a plaque honouring the nine workers killed that day:

Lenard T. Bairs
Clarence Beattie
Jean-Paul Guerin
Omar Lamadeliene
Edmund Newton
Lucien Regimbald
Raymond Tremblay
Joas Viegas

In 1987, the Canadian Labour Congress placed its national monument to workers killed and injured each year at work within sight of the Heron Road Bridge.