The 'Ghost Car' on Mount Chiliad isn't a ghost at all — it's a glitch that turns bike race paths into highways for random traffic.

In GTA: San Andreas, a vehicle can rarely spawn driving down Mount Chiliad's bike track because the game accidentally uses the bike race path files for regular traffic. The car warps through wooden boardwalks and jumps because it's following invisible race waypoints meant for bicycles, not cars.

Source: Mount Chiliad Ghost Car — CC BY-SA