I think that's part of the promise of self-driving cars -- they'll ostensibly be safer, so can drive closer together, and can cooperate at intersections to increase throughput, no one needs to stop, just adjust speed to allow a gap.
Though based on my car's adaptive cruise control ability, we've got a long way to go before we can have cars that drive within a few feet of each other at highway speeds, and even longer before we could have intersections without signals (or stops).
I don't know if this promise will ever be real in that extent. Even if you assume perfect drivers(automatic or not) with instant reaction speed there can be situations where a road can become impassable quickly due to external influences, and this alone makes short distances between cars unsafe.
All it takes is one rock dropping onto the road heavy enough to immediately stop a car in its tracks, and what would follow would be a series of collisions affecting a large number of cars.
Then you need to leave gaps for new arrivals from a different road, and in the end I don't think the efficiency would be that much higher.
Presumably autonamous cars would maintain proper separation - I try to maintain a three to five second gap between myself and the next card, but drivers in my area tend to be quite aggresive and will force their way into the gap - I have nearly been hit or run off the road multiple times.
I think the benefits are more that the car strictly obeys road rules, as opposed to humans being unaware, ignoring, or straight up willfully breaking (e.g. speeding) the rules.
<rant>
On my path to and from work there is a "trouble nexus" - you have a few main roads intersecting, a relatively short stretch of road with a couple of entrance/exits towards a public transit hub (trains, buses, and a large carpark converging - so there's a lot of road traffic), then a bridge, then another major intersection and exits onto a highway.
People are always slowing down to create a gap in the traffic flow and let vehicles from the transit hub onto the road leading up to the bridge, in an effort to be kind towards other drivers, but this has disastrous effects on the traffic behind it - interrupting the traffic flow of the major roads.
If people did not make space for vehicles from the side roads, the main flow of traffic would pass fairly quickly and then the sideroad traffic would be able to get on noramlly.
</rant>
The point I am trying to make is that autonamous cars would let traffic planners adjust e.g. road speeds, traffic light timings, etc. closer to a global optimum without having to account for human selfishness.
Though based on my car's adaptive cruise control ability, we've got a long way to go before we can have cars that drive within a few feet of each other at highway speeds, and even longer before we could have intersections without signals (or stops).