I entirely disagree. Why would people want to move onto tighter, more roundabout routes which would also be 20MPH? As for being too slow, all one is doing on main roads like LL and DH is moving from one set of traffic lights, zebras etc to the next. Driving at 30MPH just gets you to the next stop point quicker so you spend longer sitting with your engine running. I observe this empirically almost every day: cycling at an average of 15mph, I can stay with cars from Denmark Hill all the way to Clapham, Tooting and beyond, if they're going my way. They shoot off into the distance at 30MPH+, but I inevitably catch up with them again at traffic lights. Driving at 30MPH on roads with frequent lights and junctions doesn't get you to your destination any quicker than driving at 20MPH, but you will be a lot less able to react effectively to emergencies and you will do a lot more damage if you have an accident; if you hit a pedestrian at 30MPH, they are seven times more likely to die than if you'd hit them at 20MPH. Two thirds of car trips in London are for distances under three miles (a disgrace but we'll put that to one side just now); imagine the utopia (for drivers) of a completely traffic free road with no blocks in terms of lights etc: at 30MPH you'd cover three miles in six minutes, at 20MPH you'd do the trip in nine minutes. Three minutes out of your day to be seven times safer in the event of a collision, seems a pretty good exchange to me.