Travelling 100km by car produces about 10.8kg per person of carbon dioxide (assuming an average of 1.5 passengers per car). Doing the same by bus produces about 1.3kg, while taking a modern electric train produces about 1.5kg (based on the energy balance in the UK). What is remarkable is that shipping freight by truck produces 180 grams of CO2 per kilometre, while doing so by train produces just 15. Clearly, switching freight transport modes offers considerable scope for emission reductions (as does reducing the total amount of freight shipped).
When you factor in how much damage heavy trucks do to roads – as well as the expense and carbon emissions involved in rebuilding them – it seems pretty clear that disincentives to ship freight by road make sense. Yet another externality that road pricing and carbon taxes could help address.