A growing number of cities are green-lighting trials of smart parking systems that combine sensors, wireless communications and mobile apps. Tests in Los Angeles, San Francisco, Reno, Indianapolis and Washington, D.C., among other places, help drivers find available spots and could help cities optimize revenue. Some 950,000 on-street parking spots worldwide will be enabled with smart parking technology by 2020, predicts Navigant Research. Naturally, high-profile startups are vying for a piece of the action. Here are five companies.