This has actually been studied, and it depends on many factors like time of day + sun location, the weather, pavement conditions, and rigour during driver licen

This has actually been studied, and it depends on many factors like time of day + sun location, the weather, pavement conditions, and rigour during driver licensing. Optimal flow on a highway during sunshine is around 80km/h, where the gap between the cars is around a car and a half length. Under that and we would be better off having parking lot queues before onramps. So why don't we do this? Because usually people want to travel to the city from the suburbs. The only place that has room to build these parking lots are the suburbs, but then the city couldn't take the excess flow (since it all needs to empty out into city streets and parking lots anyway).