This volt is the key to time travel discovered by Emmet "Doc" Brown. 1.21 gigawatts is the required voltage to power the Flux Capacitor, which in turn powers the time machine. Considered impossible to achieve with the materials in the 1950s, Doc devises a plan to harness the energy of a lightning bolt to power the time machine. The Easter egg is located on most power boxes throughout the game, mostly along the railroad.
Hill Valley is the name of the fictional town in the movie Back to the Future. The name itself is an oxymoron, which was employed along with gags in the movie. The sign is located in Pacific Buffs, Los Santos near the Vinewood Cemetery.
88 mph Sticky Note
In Lester Crest's house, there are sticky notes that can only be seen on next-gen versions of game. The sticky note reads 88 mph, TIME TRAVEL. There is then an image of a DeLorean car underneath. This is a reference to the exact scene from the movie, where Doc Brown discovers what speed he has to get the DeLorean to go.