With construction of the massive interstate highway system now 99 percent complete, it's time to get out of some old policy ruts.

Wisely, Transportation Secretary Samuel Skinner is trying to take the national transportation program in a new direction, with emphasis on road repair and gridlock relief rather than new highway projects.The 44,328-mile interstate system has been the world's most expansive - and expensive - public works proj-ect. And, it has been a popular proj-ect with politicians who have benefited from the monetary plums that came with the huge construction proj-ects. The interstate's massive concrete tentacles reach into virtually every corner of the nation.

While Skinner is preaching a new direction in national policy, the proposal still includes a significant federal contribution to highway construction with the emphasis shifting from the interstate system to what Skinner calls "highways of national significance" that are important to the nation's commerce and defense. Skinner also wants federal highway dollars made available to mass transit programs to spur more efficient use of existing highways and, for the first time, he wants to allow private developers to build toll-roads using some federal assistance.

As an incentive for state support, Skinner is also proposing to give states more local control over a second but lesser allotment of federal dollars. Skinner hopes the promise of more local control will rally support from state highway officials.

Major obstacles include the estimated $70 billion to $90 billion price tag and Congress' habit of sticking with the old and familiar.

One element of Skinner's plan should receive particularly high priority - road repair. It has taken 35 years to complete the interstate system. Many of the older sections of the system are showing their age. Heavier trucks and higher volumes of automobile use have taken a debilitating toll on these ribbons of concrete and asphalt.

If funds already spent on the interstate are not to be wasted, new money must be provided to keep the roads and bridges safe. This may involve the establishment of toll-collection booths in parts of the country where this fund-raising method has not been seen before. But so be it.

Time is short, though. The current spending authorization for federal highway, bridge and mass transit programs are due to expire on Sept. 30, 1991. In dealing with Secretary Skinner's new initiatives, Congress must move faster than usual.