You have to do a ton of overtaking. When I go to the UK, I stick to the motorway speed limit because it saves a huge amount of fuel. However, there are so many speed limited (56mph) lorries that you're constantly having to pull out to overtake lorries. The worse is lorries overtaking lorries on the M6 between Manchester and Birmingham. You have one lorry with a speed limiter at 55.99999998 mph, and another with a speed limiter at 56.00000001 and the faster one is overtaking the slower one, and it takes about 15 miles to complete the manuevre, and in the right lane you have a van overtaking the lorry overtaking the lorry, but the van is only doing 60 causing all the speeding repmobiles to suddenly slam on the brakes and slow to 60, causing a standing wave traffic jam in that lane.
If we could get autonomously driven lorries that can communicate and agree on a speed to drive so they never overtake each other, then it'll hugely increase the capacity of the M6.