A smart cruise control would be a fantastic feature for many, and would save a lot of fuel. RH77 modified his so that it could only pull a certain amount of extra throttle when encountering hills, but nobody has done exactly what you're talking about.
FYI:
What features on your ULTIMATE fuel economy instrumentation?
I bet there are lots of other small things you could do to improve MPG at either end of your drive on the crowded highway.
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