Mostly it makes it impossible to drive in Paris. When I come to Paris from the UK, I now try to fly because gare du nord is surrounded by a gigantic moat of traffic jams. Not sure it is a win for the environment.
I wouldn’t seriously consider driving to St Pancras either barring a taxi for an early trip.
Note that the removal of the other 50% of traffic would easily shift the road from a traffic jam (Level of Service E or F) to a free-flowing road (LOS C). Even if only a fraction of the overall traffic is removed, traffic would easily get down to LOS D.
This is exactly why encouraging other modes of transportation (cycling, walking, bus, train, etc) actually causes big improvements for all road users.
I probably shouldn't have called it rush hour. The traffic is busy from 7am until 7pm on weekdays.