To cut through the fog we need to rely on some technical language. We must distinguish between average cost, marginal cost, and average marginal cost. The average carbon cost of travelling by car or bus is the total emissions divided by the number of passengers: these are the numbers that are unflattering to buses. The marginal carbon cost is the extra emissions caused by one additional passenger. For planes, trains and buses this is low – unless, that is, the passenger is the straw that breaks the camel’s back, and causes an additional bus, plane or train to be scheduled in future, in which case the marginal carbon cost of that passenger will be gigantic.