At Lakehurst Naval Engineering Center.
very impressive picture
The jet car test site has four one mile long tracks. Unmanned deadloads are propelled by jet cars powered by J57 engines to preselected speeds before entering the test area. Lakehurst engineers can configure test equipment and select thrust settings to simulate any service condition or energy level to test a variety of applications including seat ejection tests, antimisting fuel crash tests, barricade tests, and tire and runway traction tests. The site's remote location provides safety and a noise buffer.
http://deputy-dog.com/2009/04/record-breaking-jet-sleds-and-rooftops.html