Right. The Exit/Onramp problem is simply solved with AV's, because you do not need traffic lights and 8-feet of extra space on either side to accommodate idiot drivers, and you can have walls and tunnels and narrow passages because you don't have to worry about people panic-braking because the lane got narrower, etc.
Essentially, if we had SmartCar-sized AV's, we could fit TWO lanes, side-by-side, in the space currently taken by ONE lane of idiot-operated cars.
Every single road is something like 80% wasted space and design, to account for human error, and the peculiar foibles of our sensory systems. When you eliminate those vectors of uncertainty from the calculation, you can fit FAR more vehicles into the current traffic pattern, with little/no impact on the current traffic flow and control mechanisms.
Let's look at a detailed example...
Off-Ramps and AV's
Because AV's are smaller and narrower than a regular car, you have TWO lanes of AV's operating in your current HOV lane.
Off- and On-ramps are on the LEFT side of the freeway, and ONLY accessible to AVs.
They do not need to be built to support a 40-ton semi tractor trailer. They need to support a series of 1800-pound AVs. The off-ramp also doesn't need to be flat, or provide forward visibility, or a clear line of sight, so angles, turn radii, and crash zones don't need to be designed into the off-ramp design. (Imagine AVs turning off the freeway into a simple tube, that bends down away from the freeway in a sweeping arc, towards the road below.)
The outside of the tubes can become public art spaces... paint them like snakes, or dragons, or whales, or whatever.
Below the freeway, the tube merges with regular traffic in a standard merge lane, or - more likely - adds a lane to the road that is dedicated to AVs.
AVs enter the freeway the opposite way, entering the freeway from the left, and merging in with other AVs.
Because all the AVs are talking to one another constantly, the flow of traffic in these lanes is smooth. All the vagraries of acceleration, object avoidance, jammed up off-ramps, etc, are spread amongst ALL of the vehicles in the area, so each one backs off an inch (for example) and the cumulative effect over 60 vehicles is enough space for a new vehicle to merge in.
With two AV lanes, the left one is the slower lane, with traffic entering and exiting the freeway, and the right lane is the "fast" lane for vehicles going further.
The AVs essentially become person-sized train cars, wirelessly connected, which can dynamically "connect" and "disconnect" from the cars in front and behind them, and flow in with vehicles in the other lane.
The Fun Stuff
The best part is once people get past their hangup on the "Trolley Problem" strawman argument, and the software designers get to begin thinking of how to make things flow like water, rather than road designers having to design to accommodate that Escalade with one person in it, who is also on the phone, and riding on bald tires with brakes that should've been serviced 20k miles ago.
What if an AV gets a flat tire? I could see the AVs in front and back "teaming up" and sandwiching the one with the flat tire, and helping it get safely to a turnout or Repair-de-sac (something I just made up: a little turnout that can fit 3 AVs... the "bad" one, the "replacement" one that picks up the rider immediately and continues their journey, and the Repair vehicle that carries the dude who fixes the broken one.)
When we get to this point, London reduces the number of vehicles allowed in The City even further. Taxicabs in two lanes, AVs in their own four lanes. That provides the financial base and incentive for the AV companies to develop and built resilient systems, and tweak the code in a live environment, before rollout to progressive cities worldwide... Sao Paolo, Vancouver, Toronto, Dubai, Stockholm, Berlin, Nagoya, etc.