They could start with putting trackers in taxis, and issuing fines/disqualifications for speeding. After about an hour the streets would be empty.
I had a rare reason to rent a car and drive today. I was going at 20mph on a 20 limit street, and the private hire vehicle following me was flashing his lights and driving much too close :-/
I did get to cheer on the commuter cyclists (legally) overtaking me, though.