The 40 odd cars...
Coming back to an earlier bit; whilst the differential between the two methods is neglible, consider this -
Each car has four tyres which also require energy to manufacture them
Each car creates greater congestion and hence decreases the efficiency of the roads (assuming they don't all decide to drive in the middle of the night)
Each car is also using 40 x lubricants (oil!)
Getting everyone on one mode of transport therefore makes sense (hence if they went on two coaches the difference would be yet another number crunching nightmare)