DF car chases

[The Colonel]

Someone always clips some bins in a car chase, so remember to put a tanners wagon or other dung cart in there.
Alternatively, people flinging waste out of houses could be an interesting developement.
A substantial gulley full of filth masquerading as a shallow puddle is a good place for one of the pursuers to end the chase - these were usually the result of incompetent attempts to drain the street and crop up occasionally in medieval cities (just ask anyone who has driven through Basra or Baghdad recently).
Two guys carrying a shutter or large painting on wood would probably be the best substitute for the sheet glass.
A friar preaching in the street would also make a good obstacle, as would pigs foraging in the street (not unknown in a medieval city) - horses and pigs don't really get on.