Expressways Proposed Near Diamond Bar : Traffic: The extensions of Tonner Canyon Road and Soquel Canyon Road are expected to relieve congestion on the Orange and Pomona freeways.

Construction of two expressways linking San Bernardino, Los Angeles and Orange counties should help siphon commute-hour congestion from the Pomona and Orange freeways, a consultant’s report shows.

The expressways would be 10- and 12-mile extensions of Tonner Canyon Road and Soquel Canyon Road, respectively.

Aside from helping to ease traffic on the two freeways, the new strips should help relieve surface streets that freeway motorists frequently use as detours. The report says traffic in Diamond Bar along Diamond Bar Boulevard and Grand Avenue would be cut at least by half.

The feasibility study by Orange-based Parsons Brinckerhoff, Quade & Douglas Inc. lists varied options--from building no roads at all to constructing both as six-lane, 50-m.p.h. expressways--and shows the resulting effect on traffic. The three counties have made no decisions about whether to build the roads or how to pay for them. The report suggests a toll charge on either or both roads, and a later phase of the study will explore that option.

The $195,000 study, commissioned by Orange and San Bernardino counties, is scheduled to be released Thursday. A copy was obtained last week by The Times.

If the study findings become reality, a prime beneficiary would be Diamond Bar Boulevard, which often becomes clogged with bumper-to-bumper cars trying to bypass the 2 1/2-mile junction of the Pomona and Orange freeways. With the two expressways in place, the report says, traffic on Diamond Bar Boulevard would be reduced to about 20,000 cars a day, in contrast to 52,000 without the new roads.

Officials in Diamond Bar have been pushing for speedy construction of the proposed expressways, expecting an onslaught of traffic problems this September, when an extension of Grand Avenue through San Bernardino County is scheduled to open.

Grand Avenue will connect the rapidly growing community of Chino Hills with the Pomona Freeway via Diamond Bar. City officials, fearing the worst, had tried to delay the opening by building a fence and wooden barricades across Grand Avenue. The barricades went up, but Diamond Bar officials decided to remove them in September, after San Bernardino County promised $1.2 million in road improvements and cooperation in building new streets.

There would be trade-offs in reducing traffic along Grand Avenue and other surface streets. The report says the proposed roads would exacerbate congestion on Carbon Canyon Road in Orange County.

Twenty years from now, up to 88,000 cars would use the part of Carbon Canyon east of Valley View Avenue if the new roads are built. Without the roads, 42,000 cars would travel on Carbon Canyon Road.

In addition, part of Soquel Canyon Road would cross a corner of Chino Hills State Park, eliminating a habitat for the southwestern pond turtle. Tonner Canyon Road would pass through the Firestone Boy Scout Reservation.