The first observation here is that there is considerable variation illustrated in the data, leading to the
following average results as an order of magnitude:
• for passengers, in terms of passenger-km, road transport is 1 to 2 times more polluting than
aircraft, and 10 times more polluting than railways;
• for goods (in tonne-km), road transport is about 10 times more polluting than rail; and
• one tonne-km is slightly more polluting than one passenger-km.
The studies also make monetary evaluations, which lead to similar conclusions (Table 2.8).
Despite some variation in the data, Table 2.8 illustrates a few constants:
• the low value of pollution of railways and waterways, compared with roads and aircraft;
• roughly speaking, one passenger-km is as polluting by car as by aircraft; and
• one tonne-km is 1 to 2 times more polluting than a passenger-km, both by road and by rail4
.