Subject: How much gas does a car use when idling, how much used to start the engine?

In my own work on the regulation of digestive performance, I would like use 
a simple analogy of the costs and benefits of leaving your car idling vs 
turning your car's engine off and then restarting it.  What I would like to 
know is how much gas does is needed to start a car's engine and how much is 
used when it is idling.  I know that there is a lot of variation in gas use 
among cars - I'm just looking for some ball park values.  I would like to 
say something like that if you are stuck at a train crossing for 4 minutes 
then you will save gas by turning your engine off, even thought there is 
the added gas needed to restart your engine.  Any information will be 
greatly appreciated.  Thanks.

Re: How much gas does a car use when idling, how much used to start the engine?