TTC vehicles were involved in 18,000 accidents over the last four years, roughly a third of which were deemed avoidable, according to a report released Friday. 

Half of the 18,000 accidents involved buses and 5,000 were considered avoidable — data that raises concern about the safety record of the city’s transit system. 

But TTC CEO Andy Byford says the numbers must be viewed in context, noting that TTC drivers cover 750 million kilometres per year. 

“Seventy per cent of those [accidents] the TTC is innocent and another party has run into one of our vehicles,” Byford told CBC News. "And even the 30 per cent of those accidents deemed preventable, that includes everything — if we scratch a mirror, if we run into a garbage pan — that goes into the statistics." 

The data shows some individual drivers are especially bad, and have been in multiple accidents. One driver had been in 30. Others have been removed from the road.

“The vast majority go through their careers without any incidents,” said Byford. “But when you get into dozens [of accidents] it’s very concerning and we have a responsibility to look into that.” 

The TTC says its insurance costs in 2013 were around $27 million.

The head of the TTC's union says conditions have also become more difficult around the city. 

"The traffic congestion we've been facing in the city of Toronto has been more and more difficult for our members to manoeuvre through the city while maintaining a schedule," said Bob Kinnear. 

With files from Jamie Strashin