I have done some more research on an RCTA as would be useful in the Walmart example:
A - I note that ultrasonic based systems only work to around 6'. That would be good for obstacle avoidance, but not really useful as an RCTA.
B - Microwave based systems seem the best, but most are over $700 just for the parts.
C - My current thinking is to use an optical system with 2 small side-looking cameras (one facing each way) located either below the bumper edges or on the license plate frame. They would feed a split screen monitor in the car. That method has the advantage of low cost and the ability to see far up and down the aisles of a parking lot. The driver then has good early warning, including the ability to easily notice animals, kids, people, cars. Disadvantages include increased information load on the driver, and clutter in the cabin. I'm currently looking at parts to build a small prototype.
Cheers,