Does putting my automatic in neutral when stopping at traffic lights or coasting down hill save petrol?

answer number two- i highly doubt that's the case with those being traffic violations. it is common practice to do as such in a manual car, and they teach you that in driver's ed and programs of the sort. also, ever heard of a brake? that should solve the whole backing into the car behind you issue.

Modern engines will cut fuel consumption when rolling down a hill when being in gear. Put the engine in neutral, and it will idle, thus consume a bit of fuel.

Stopping in front of a traffic light and put the gear in neutral will indeed gain nothing.

Well, sorry, but each of your suggestions are illegal traffic violations. stopping at a traffic light and shifting to neutral actually accomplishes nothing, except backing into the vehicle behind you. and, coasting down a hill in neutral is illegal. if the engine stopped, your power brakes would not function properly.

Best advice to save gasoline is to use the air conditioner, on the interstate, instead of opening the windows. the drag on the wind entering the cars window, uses more gasoline than the air conditioner. slow down, but not too slow to impede traffic. going from 70 mph to just 60 mph, saves at least 2-3 mpg. do not have a heavy foot on the gas pedal. treat the pedal as thought you have a chicken egg under it and baby that pedal. avoid jack rabbit starts and stops.

Yes, but not a whole lot.