I used to have one of these Road Runners, a 1969 model that looked nearly identical to the yellow one in the pic, minus the hood scoop. It had a bored out 440 engine with a big cam, big four-barrel carb, traction bars, battery in the trunk, headers, etc. I miss the acceleration, but I don't miss the 6mpg or smelling like exhaust all the time thanks to leaky headers. I especially miss the "Beep-beep!" horn.
In 1969 or 1970 there was a also a limited edition Plymouth Superbird and Dodge Charger Daytona available. These things cleaned up in NASCAR races. NASCAR eventually outlawed them, they were too successful.
Thank God this never happened. According to Wikipedia, "A prototype 1981 Road Runner was built on the Plymouth Reliant body, but it never made it to production".
It was all an homage of course to Wile E. Coyote and The Road Runner.
The Plymouth Road Runner from the Stock Mopar site:
"It cost Plymouth $50,000 for the rights to use the Road Runner name and other bells and whistles ( beep - beep horn ). Plymouth's main goal in producing the Road Runner was to be able to run 13s at the race track and still be able to sell the car for under $4,000."