Montgomery County planners have put a tighter squeeze on future development along the traffic-choked Route 29 corridor by eliminating a popular loophole for developers of small residential subdivisions.

Since 1982, a section of Route 29 near the Prince George's County line has been designated at "zero threshold" by planners, meaning that the road cannot handle more traffic than would be generated from already-approved developments. This designation, in effect, put a moratorium on all approvals of future development along the Route 29 corridor, planners say.

However, much of the development that was approved before the 1982 designation never has been built. So planners, under what they call the "discount policy," have been allowing some developers of small residential subdivisions to build along the corridor, with the traffic expected to be generated taking the place of projected traffic from the other unbuilt, but already approved, developments.

The subdivisions were allowed up to 75 units of housing each under the discount policy, said Charles Loehr, a planner for the county.

This spring, the board voted to discontinue the policy because it no longer could assume that projects approved before the 1982 designation will not be built in the near future.

"They couldn't be sure that the rationale for the discount policy was still valid," Loehr said.

Last week, the board ruled that a handful of developers who had applied for permission to build subdivisions before the discount policy was discontinued can get approval for their developments. But these developers will be the last to build under the discount policy, Loehr said.

Although few new projects along Route 29 are being approved, planners are quick to point out that development is booming along the road as landowners put up office buildings, stores and subdivisions that were approved years ago. But there is still much land with development potential, and builders are pressuring the county to do something about Route 29.

What that something could be is a matter of debate among developers, the county and the state, which owns the road. Planners say Route 29 presents an interesting case study in the future of major arteries in areas of rampant development.

Development along Route 29, running from Howard County to the District, has boomed in the last decade because available land is abundant and because the county allows dense development in the area. Helping choke the road even more are outer county residents who use Route 29 to pass through Montgomery on their way to the District, said John J. Clark, planner for Montgomery County's department of transportation.

Montgomery has lobbied hard for state funds for the road but has been relatively unsuccessful, Clark said. The county is paying $2.6 million and the state only $1 million in a current project to widen Route 29 from four to six lanes along a four-mile stretch on either side of Randolph Road.

This paving project, to be completed in two years, will not raise the "zero threshold" of Route 29 because the project was approved years ago and was factored into the threshold limit, Clark said.

What is needed to lift the threshold for Route 29 is something much more extensive, such as eliminating intersections by building overpasses at each street crossing, he said.

This idea is still just a seed, and Clark estimates that it will be more than a decade before the state actually will fund the project. And while the county was willing to foot the bill for most of the lane-widening project, it cannot undertake a massive project on Route 29 by itself, according to planners.

"It's not unusual any more for the county to make a commitment and spend its own money on state roads," said Jeff Zyontz of Montgomery County's planning research division. "But there is a limit to that."

"Nothing is going to happen on this road overnight," Clark said. "We are talking about very-long-term projects."

In the meantime, developers and Montogomery County have been working out other programs that will alleviate the "zero threshold" limit. Specifically, the county is looking toward mass transit and car pooling as an alternative.

If a developer can come up with a project that will take away more traffic than it will generate from Route 29, then the county will approve new development, Zyontz said.

Under this program, the developer is required to set up van-pooling and car-pooling programs and even bus routes that will encourage area residents to generate less traffic on Route 29, Zyontz said. The developer posts a 10-year bond to ensure compliance with the transit program.

"We don't see the problem being totally alleviated by transit," Clark said. "But it's an excellent, innovative idea until something more can be done -- or even after something more is done."