We have a few avenues, but like a lot of transport infrastructure in Toronto, they're stubs of an inconsistent length. Take College: it's a proper avenue of 4 traffic lanes, 2 bicycle lanes and on-street parking between Elizabeth and Manning. This section is very urban, and all traffic - including streetcars - moves quickly and consistently. Out of nowhere, the street narrows at two minor side streets rather than at two major arterials (University and Bathurst) where a reasonable amount of traffic could be diverted. Of course, this leads to a traffic jam at these two pinch points at rush hour and this is where most streetcars begin to bunch up.
Another maddening thing about Toronto is how much traffic is constricted by delivery vans illegally parking in traffic lanes, taxis making three point turns or people generally treating the road as a place to stop and run into a convenience store to buy a pack of smokes. There should be a huge fine slapped on drivers who do this because they, more than any bicyclist or streetcar, are really what's holding up a lot of traffic in the downtown area.
Out in the suburbs, a major cause of traffic is the 2km spacing between major arterials - among the widest supergrids of any North American city I know. You can immediately see how much quicker traffic flows on the north-south arterials in Scarborough and Etobicoke (eg. Brimley, Birchmount and Martin Grove are almost never jammed except if there's an accident), whose townships had the wisdom to locate major concession roads roughly 1 km apart almost two centuries ago. Traveling east-west you are really screwed, especially when an accident forces you to make at least a 4 km diversion (2 km there and 2 km back to the next arterial north or south of you).