[mikewarot]

City streets are already congested, the proposed remedy would increase congestion slightly by making many trips longer.

Making all the streets in a city core one way would make many of the left turns the only possible turn, and thus improve safety.

[Someone]

Assuming the trips made stay the same (probably a reasonable assumption, but who knows?), it would increase the number of cars on the road, but not necessarily congestion. Road capacity goes down if cars move at varying speeds, as happens when traffic has to stop for cars making left turns.