Ride 100 provides an excellent demonstration of this even just by closing -some- roads for -one- day.
cf this graph of pollution on Putney High Street:
I expect there are some people who do make other arrangements and take a bus, so it's a net gain, but there are also those who take their car on Monday since they couldn't on Sunday.
EDIT: I looked up how RideLondon works, map at the end of this PDF shows road closures https://d1ffaecguugkl4.cloudfront.net/ridelondon/live/upload...
So it's not even time-shifting necessarily. People can drive around that and just move the pollution from one street to the next. But yes, the pollution measurements on one of the closed roads would look better that day.
Yes, I know, it was an illustration of how much pollution is caused by traffic in London and how much could be gained by reducing those traffic levels overall. I am baffled that this is causing confusion to people.
My own commute times nearly doubled with the Olympic lanes. It would have been intolerable permanently.
It's already been successfully done, there is just a lack of political will to make such measures permanent.