What are the primary features of a gravity hill given the following description?

A gravity hill, also known as a magnetic hill, mystery hill, mystery spot, gravity road, or anti-gravity hill, is a place where the layout of the surrounding land produces an optical illusion, making a slight downhill slope appear to be an uphill slope. Thus, a car left out of gear will appear to be rolling uphill against gravity. Hundreds of gravity hills are recognized worldwide.

The gravity hill optical illusion is defined by having the following characteristics:

1. A downhill slope will appear to be an uphill slope
2.  Objects appear to be rolling uphill against gravity