Head toward the big red bridge between San Fierro and Las Venturas. Just before you get onto it there is a gray pavement going downhill to your right. Follow this and down a stair case and there is a big sign there that reads:

Bridge Facts

length - 150.7 m

height - 60.3m above sea level

16000 polygons inc. LODs

600m draw distance

11 textures

Takes up a staggering 1.27mb of disc space

There is also a display that says "an actual piece of cable" with a big red piece of cable just down from there.