Playing in traffic may not be such a bad idea for a young company. After three years and $2 million in development, SmartRoute Systems, in Cambridge, Mass., now sells customized traffic reports to corporate subscribers and NYNEX mobile-phone customers in Boston. Employing slow-scan cameras, air patrols, and contracted road-reporters, the company can deliver route-specific, up-to-the-minute traffic information by phone or by fax. CEO John Liebesny expects that commuters, corporations, fleets, and cellular-phone companies will pay for his advice. Expansion is planned in 20 metropolitan markets before 1995.