I read today they were tweaking the AI so the probability for the cars to chose an alternate route increases with the congestion level of the fastest route they are currently all trying to take. That should help with a lot actually, but extreme congestion points such as the motorway exit will not be solved by that, and neither will my problem with the airport as all the taxis want to go there and they can't chose any other way but the more direct once they are in a certain proximity.