SOME OF THE BEST laid plans of yesteryear in Bethesda have gone awry, leaving traffic going nowhere. When the planners looked up from their drawing boards at Wisconsin Avenue the other day, they discovered what any driver could have told them. Bethesda has grown too big IL,7P,85 for its arteries. The roads in the business district just can't handle any more traffic. The result is a decision by the Montgomery County Council to prohibit approval of most new commercial development in the Bethesda area. Had there been more modest thinking about development before, Bethesda's mess might not have become so awful.

No wonder Montgomery Council member Scott Fosler was slightly angry during the deliberations. Accusing the council of ignoring early indications that development in downtown Bethesda would lead to terrible traffic jams, Mr. Fosler called the latest development ban "a little Band-Aid." Considering the dimensions of the congestion, he's right. A study done for the council shows traffic nearing a calculated capacity of 15,408 vehicles an hour on the bigger roads. Add to this the traffic to be generated by buildings already approved but not completed, and the analysis shows these roads carrying 24 percent more traffic than they were designed for.

The development limit prohibits approval by the county planning board of any commercial or retail subdivision that would require more than five cars on Bethesda roads during peak evening hours. The limit is in effect until a newly ordered traffic study can be made by the planning board and examined by the council.

The troubles don't stop at the D.C. line, either. That's why the city's zoning commission, in its treatment of Wisconsin Avenue, should not allow deviations from the comprehensive plan that would increase the traffic along what is already a bottleneck rivaling Bethesda's. At the very least, the city could learn a lessen from its gridlocked neighbor to the north. Or will it be bumper-to-bumper from the Potomac waterfront to Battery Lane?