You'd need to raise the price of diesel to avoid a sudden shift to diesel-powered automobiles, but you'd also need to provide assistance for truckers; their salary is not indexed to the price of the fuel they burn (or it wasn't in the late 1990s, last time I had to go to traffic school and wound up chatting with a trucker). (Though you could also not tax biodiesel the way you tax petroleum diesel.) Right now, IIRC, there's also a tax break for the cost of transporting goods, which would be good to slowly eliminate as a way of encouraging more commerce with local providers.

DM, the science backing carbon offsets is currently rather iffy; I'd much rather see actual conservation measures than carbon indulgences.