Road Runner, the nation's No. 2 cable modem service, finished 1999 with 550,000 subscribers. The high-speed Internet access company started the year with 180,000 users. A partnership primarily between Time Warner and MediaOne, Road Runner trails Excite@Home, which topped 1 million customers in December. Road Runner began offering service in 15 new markets in 1999, and plans to launch in five additional markets--in North Carolina, Texas, Minnesota and Ohio--during the first quarter.