2 Expressways Proposed Near Diamond Bar

Peak-hour traffic congestion through Diamond Bar would be alleviated by the construction of two expressways linking San Bernardino, Los Angeles and Orange counties, a consultant’s report shows.

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

They would be expected to siphon rush-hour congestion from the Pomona (60) and Orange (57) freeways, as well as from 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 a variety of 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 levels. The three counties have not made any decisions about whether to build the roads or how to finance 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 compared 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 erecting 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 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, as many as 88,000 cars would use the portion 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, a portion 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.