The "No Vacancy" sign is not up yet, but scenic Zion National Park is becoming so popular that the day may come when there is no more room.

"That may be in the future; however, we've had no finite studies to give us the day or year when that will happen," said Larry Wiese, assistant superintendent. "But we know we're close in the summer.""From Easter through September, it's often so crowded many motorists can't get a parking space," Wiese said. "Some tourists will just drive in and through and out of the park because of the very heavy congestion. In the summer now it's hard to tell the midweek days from the weekends."

Zion, about 300 miles southwest of Salt Lake City, is Utah's oldest national park and its most popular, attracting 2,174,508 tourists during 1988, the fifth consecutive year with a record increase in visitors.

The rugged and narrow canyon area, called Yosemite with Technicolor by the rangers, has reached a point where a mass-transit system must replace private motor vehicles, said Wiese, "and we're zeroing in on bus transportation. It's the most economical."

During a five-day period in August 1988, the service used a bus system to shuttle visitors to the park's attractions.

"We got a very positive response, with 90 percent of the people involved in the study saying they liked the bus ride," Wiese said. "And 70 percent said they wanted the Park Service to run it rather than a concessionaire."

"Based upon this very successful venture, a more permanent transportation system is being studied," he said.

But, because it will probably be up to three years in the planning, he said, "we have to do something now. We're looking at banning large vehicles, such as motor homes, from such areas as the Temple of Sinawava and Weeping Rock parking areas, thus creating more parking for cars."

The service also is considering requiring hikers who plan to leave their cars for several hours to park the vehicles in the headquarters area rather than in one of the smaller lots.

"We want everyone to be glad they came here," Wiese said. "But, with a 5 percent to 12 percent tourist increase each year, we need to solve the growing congestion problem. We may even end up with parking lots outside the park, with tour-staging areas in one of the nearby communities."