Hmm? This shouldn't be surprising at all given the rather large assumption that one car passes a fixed point each two seconds. Go as fast as you like... an observer on the roadside must still wait two seconds for each car to rocket (or crawl) by. Imagine having an ethernet card that could only send one frame a second to a party on the other side of the globe: would you care if the additional network latency due to switches and the speed of light were 1msec or 200msec?
Where we may see great improvements in road throughput is in driverless cars that greatly reduce that gap.
It'll be really interesting to see how driverless cars negotiate things like natural merges (a freeway losing a lane), and how that affects traffic flow. Ostensibly they'll be as efficient as possible, but if two lanes become one, there's going to be some slowing regardless. Furthermore, in a vehicle that also ostensibly wants to get you to your point of interest in as timely a manner as possible, how does it decide whether to subject itself to staying in a right lane where it might be subject to moreane merges, or does it ever decide to go around a particularly bad or congested spot for the sake of getting you to the destination?
Where we may see great improvements in road throughput is in driverless cars that greatly reduce that gap.