"Attention HTV-01, you have entered a speed restriction zone. For your own safety, and ours, your COVAS unit will adjust your maximum speed and disable boosting. Thank you for your visit and please, enjoy your stay."
We know it would happen in a real world scenario because humans can't be trusted to be adults.

We would "boomtube" all the time; so governments would make it illegal to not have COVAS (as it is now), and would then install an override in COVAS to prevent us from speeding/boosting.
Whilst I personally don't mind that, as it would feel more realistic to me, I know it would infuriate a lot of players who are very time-sensitive and like to be able to decide how fast they're going - it would, however, make for a much better traffic flow I believe.
A thru-slot would also make more sense from an design perspective; not only for traffic flow, but for safety purposes as well. A single entrance/exit is just ... mind-blowingly daft when it comes to safety. Unfortunately Frontier didn't want to deviate too much from the original (which is a pity since a lot can change in over a century.. just not in the Elite universe it seems), so they had to stay with the one-slot design.