Police say the incident started on Hwy. 401 and involved a car and a cargo van. One driver is said to have cut off the other.
Sgt. Kerry Schmidt says it's believed the driver of the van followed the other vehicle for a distance.
At a stoplight on Steeles, the van driver stepped outside and confronted the other driver.
The second driver refused to get out of his car.
That's when police believe the van driver took out a knife and slashed one of the car's tires.
He then got back in the van and drove off.
Police have information on the vehicle and the driver, but have not been able to find him.