Taxis driving slowly in bus lane on M4

[Dom Tuk]

Is there a reason why black cabs carrying passengers from Heathrow into London, drive usually at an irritating 40 mph on the lane reserved for taxis and buses on the M4. They are seen pelting back at 70 mph on the return journey if they have no passengers. Does driving slowly have any effect on the final fare if it is metered. And if there is a difference is it substantial enough in terms of petrol costs etc. Bloody irritating though.

[WoWo]

No idea why they do it but... Yes driving at 40mph is much more economical than 70mph given the amount of miles a taxi driver does it would be significant enough bearing in mind there is somebody in the cab. As I understand it a cabs meter is linked to the odometer and speedometer, when the car is going it's charged on distance but when its stopped in traffic it flips over to time. Therefore driving slowely doesn't make alot of difference fair wise. They may rush back because they have a fair to pick up from a plan.