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That's not true though. Using more petrol isn't necessarily worse.
If you use less petrol but produce more emissions because the engine isn't run as efficiently at it's optimum rev range or whatever then you could create more pollution with city driving. You also wear car parts out faster, ie. brakes so there's more brake residue in town, you're contributing to the traffic so you're probably helping slow everyone else down (although as I said this probably applies to the M25 carpark too).Anyone actually done the numbers on this?
hippy-
Hmm, I'm not sure, I'm coming at this from an schoolboy chemistry perspective not a car-person perspective so you might know more about it than me. I assume that x litres of fuel is made of y grams of carbon all of which is converted directly into CO2 at 12:44 ratio by weight. In that case nothing else matters except the amount of carbon burned.
But yes, I am ignoring brake residue, PM2.5, NOx, incomplete combustion, whatever. Happy to accept that it's more complicated than what I said.
frankenbike
The simple answer is whichever one uses more petrol. Probably the 50 miles in my experience.
Edit: found some random ballpark numbers. Assuming idling consumes 0.6l/hr and your efficiency is 0.1l/km (~28 mpg) on the short route, and 0.05l/km (~56 mpg) on the long route, and you're idling 50% of the time on the short route, you're consuming 3.2l + 0.6l per 1 hour journey on the short route and 4l on the long route. So that's about the same in each case. I don't know how efficient your car is though.
If you've got a little computer in the car you could do both journeys at typical traffic conditions and try to record the fuel use in each case and report back.