I have always assumed it was just due to the uneven distribution of weight by vehicles often being stationary in the same spot, so those points are subject to m

I have always assumed it was just due to the uneven distribution of weight by vehicles often being stationary in the same spot, so those points are subject to more compression forces than the surrounding road surface. If it were acceleration and deceleration I’d have expected the effect to be less localised, as breaking and accelerating happens over a much longer distance. But, I have no actual idea. It’s just probably not friction…