Ah, but road transport isn't really shared between commercial and personal transportation. Road maintenance is in large part funded by fuel tax. Fuel use is roughly proportional to vehicle weight at low speed, or frontal area at high speed. While wear to the road is proportional to the fourth power of the axle weight.

So trucking is heavily subsidised by smaller cars. If the costs created by trucking were internalized, the market share in the shipping industry would likely shift considerably toward rail transport.