Why would traffic actually be slower? Have you driven much in busy cities? Usually in California for example, when traffic conditions allow such, the actual advice given in things like drivers ed is to keep with the flow of traffic including in situations like freeway driving where traffic might be going much faster than the posted speed limit. Certainly ai might drive slower in places flagged as actually necessitating it, but if anything I'd think the advantage of a fleet of ai drivers is that cars could go even faster than before because suddenly there's nobody stubbornly slowing a lane down unnecessarily out of either panic or simply obstinance that they're in the right because they're going the posted speed limit.