>>Without trucks being subsidised we'd have much more expensive groceries on our shelves.

This doesn't make any sense. If total subsidy is X and you remove it then prices should at most raise by X (in aggregate). It is at best an even trade. In reality it's worse because it creates perverse incentives.

If you want to help people then you can spend the same money by subsidizing groceries directly. You can also lower taxes for workers or 101 different things that help lower income people without promoting one way of doing transport.