Re: What about the downsides?
"Road tax" (VED) - nets about 5-7 billion per year
Fuel duties and taxes net about 70 billion per year
Only about 5-7 billion is spent on roads each year and the vast majority of damage is caused by vehicles in excess of 7 tons (damage goes up with the 5th power of axle weight and the square of speed)
A fleet of self-driving 6-8 seat minibuses would be _far_ better for the roads than any kind of double decker bus and the interesting thing is that they'd probably give better speeds for passengers.