A lot of accidents involve others and our reactions to others. It's a case of if someone was to pull out from a side road and you had to brake suddenly would you stop quicker if you were wearing very high heels or would it make no difference.
Suppose that car pulling out was actually a child running into the road, would that make your answer any different?
Or suppose it was you suddenly crossing the road, would you want to be wearing very high heels and/or would you be praying that the driver of the oncoming car was wearing very high heels?
The only thing I would say here is that as a pedestrian you are taking a personal risk, you are very unlikely to cause injury to anybody else by going for a walk in high heels.
As a car driver, it's like alcohol, drugs, smoking, mobile phones etc... if it's going to make you less effective at controlling the vehicule don't do it. And before anybody comments I'm not trying to say that driving in very high heels is comparable to drink driving etc ...