It's not that though that determines traffic speed. I don't suppose point to point average speeds have changed that much but rather than wheezing along at a constant slow speed as you could/ had to in the past, it's all point and shoot these days. Cars (and bikes) are so much quicker these days on the bits where you can use the power. The difference between the Mini we had in the 60's and my wife's current one is staggering, and my 60's Lotus is a slow lane plodder these days.
The fact that they're just rushing from jam to jam seems to be lost on white / brown / black van man, Audi / BMW power ranger executive or (evenings and weekends) Max Power in his Golf GTI. Even mum+kids in her Range Rover on her way to Waitrose will push you off the road. The last actual collision I had was when a woman with a baby on board sticker on her car cut in so fast when overtaking me she hit my pannier. It really is a case of if you can't take the heat get out of the kitchen.
My rule of thumb is that compared to (say) 50yrs ago you need a bike with twice the power now to have the same 'relationship' with the traffic you used to.