and prizes, using a professional tier system, began to be held throughout California, such as the California Free Former World Professional Skateboard Championships, which featured freestyle and slalom competitions. A precursor to the extreme sport of street luge, that was sanctioned by the United States Skateboarding Association (USSA), also took place during the 1970s in Signal Hill, California. The competition was called "The Signal Hill Skateboarding Speed Run", with several competitors earning entries into the Guinness Book of World Records, at the time clocking speeds of over on a skateboard. Due to technology and safety concerns at the time, when many competitors crashed during their runs, the sport did not gain popularity or support during this time. In March 1976, Skateboard City skatepark in Port Orange, Florida and Carlsbad Skatepark in San Diego County, California would be the first two large size US skateparks to be opened to the public, just a week apart. They were the first of some 200 skateparks that would be built through 1982. This was due in part to articles that were running in the investment journals at the time, stating that skateparks were a good investment. Notable skateboarders from the 1970s also include Ty Page, Tom Inouye, Laura Thornhill, Ellen O'Neal, Kim Cespedes, Bob Biniak, Jana Payne, Waldo Autry, Robin Logan, Bobby Piercy, Russ Howell, Ellen Berryman, Shogo Kubo, Desiree Von Essen, Henry Hester, Robin Alaway, Paul Hackett, Michelle Matta, Bruce Logan, Steve Cathey, Edie Robertson, Mike Weed, David Hackett, Gregg Ayres, Darren Ho, and Tom Sims. Manufacturers started to experiment with more exotic composites and metals, like fiberglass and aluminium, but the common skateboards were made of maple plywood. The skateboarders took advantage of the improved handling of their skateboards and started inventing new tricks. Skateboarders, most notably Ty Page, Bruce Logan, Bobby Piercy, Kevin Reed, and the Z-Boys started to skate the vertical walls of swimming pools that were left empty in the 1976 California drought. This started the "vert" trend in skateboarding. With increased control, vert skaters could skate faster and perform more dangerous tricks, such as slash grinds and frontside/backside airs. This caused liability concerns and increased insurance costs to skatepark owners, and the development (first by Norcon, then more successfully by Rector) of improved knee pads that had a hard sliding cap and strong strapping proved to be too-little-too-late. During this era, the "freestyle" movement in skateboarding began to