Speeds would be slower and the air would actually have a wonderful fragrance to it much of the time.
I'd probably still seek out alternate afternoon routes away from the bicycle traffic jams.
There would be businesses built around cycling, just like they are around motoring. Not just bike shops as we know them, but food and rest stops tailored to small trips, hotels, hostels, and B&B's centered around longer trips, more and better accomodation on transit such as air and rail, bike parking everywhere, huge bike accessory businesses,
lots of bicycle racing and competetion, etc.