The Herndon Town Council last night was expected to approve a 90-day traffic study of the impact on the town of two major highway construction projects, the Dulles toll road now under construction and the Springfield Bypass, the proposed Fairfax County highway approved by state and county officials last fall.

The Dulles Access Road, which passes through about 2,200 feet of the town's southernmost section, has a partial interchange at Centreville Road. When the $57 million, four-lane toll road is completed alongside the Access Road in 1984 it will create a major interchange at Centreville Road.

The 35-mile, $200 million Springfield Bypass will be longer in coming--no state or federal funds are presently allocated for it--and will pass along the Herndon-Reston border, just inside Reston, with major Reston and Dulles toll road intersections.

The town is completing its own Herndon Parkway nearby and is concerned about the impact of future traffic from all the various projects, said town Planning Director Peggy Dubynin.

"Our concern is not only the roads, but the heavy development expected in eastern Loudoun County and western Fairfax County. There will be a lot of traffic in and around the town" from the development and the roads, she said.

The town is about to improve its main street, Elden Street, with turning lanes and traffic lights, Dubynin said. "We hope the traffic study can say you need to do A, B and C in order to improve the level of service on your streets," she said. None of the recommendations is expected to involve major road widenings, but may propose turning lanes and synchronized traffic lights, she said.