Basically there are two problems that are solved by putting skateparks underneath bridges. One is the "Not in my back yard" problem. No one seems to want a skatepark in their area. Fearing noise, and other problems people associate with skateboarders. Second is Rain. Skateboarding doesn't work in the rain. In fact, it's very dangerous if tried in the rain. Putting a skatepark under a bridge, like Burnside in Portland, and FDR in Philadelphia, and Washington Street in San Diego, solves both of these problems with one fell swoop. Freeway's and other streets are often noiser than skateparks, so the park isn't even noticed from a noise aspect. And these spots are largely away from houses, and finally, they are covered. Which would allow kids to skate even while it's rainingyou're your interested, here's a proposal I sent to the City of Seattle detailing exactly this idea.
Site in Lynwood: Under Highway 525 at Maple Road, Lynwood, near Alderwood Mall.  
Sites In Seattle: Under the Ballard Bridge.  

Under Interstate 5 for a couple of miles in Seattle along Airport way.  

Under the 12th Street Bridge, and Dearborn Ave.  

Under 45th street next to University Village  

Under Highway 509 at the intersection of Hwy 99 and 509 and W. Marginal Way.  

Under Hwy 99 at South Hanford Street  

Under Hwy 99 at South Atlantic Street  

Under I-5 at 40th Street  

Under I-5 and Spokane St.  

Under I-5 at Shelby St.  

I-90 next to Rainer Ave. and Goodwill  

Under I-90 near Safeco Field  

Under West Seattle Bridge at Duwammish street  
Site on Mercer Island: Under I-90 on Mercer Island (West side)  
Site in Bellevue: Under I-90 in Bellevue (West side)