Closures, Openings Mark Freeway Work : Construction: Fourth Street bridges over Santa Ana and Costa Mesa freeways open today while 1st Street bridges over both will close Monday.

The 4th Street bridges over the Costa Mesa and Santa Ana freeways will open today as work progresses to add lanes to the two thoroughfares.

But come 8 p.m. Monday, the 1st Street bridges over both freeways will close for about 18 months as work begins to completely rebuild the interchange between the two freeways. The $68-million project began this week with the clearing of brush and will continue for the next three years.

During the interchange construction, some ramps and nearby surface streets will close and drivers will be inconvenienced, California Department of Transportation engineer J. Don Juge said Thursday. But when the work is done, gone will be the dreaded circular ramp that backs up northbound Costa Mesa Freeway traffic as drivers try to get onto the Santa Ana Freeway toward Los Angeles.

In its place will be “much smoother, high-speed connections,” Juge said, including a flyover ramp similar to one connecting the southbound Costa Mesa to the southbound San Diego Freeway.

Besides the closure of the 1st Street bridges in Tustin and Santa Ana, the road contractor this Saturday will temporarily close all but one ramp between the Santa Ana and Costa Mesa freeways to install concrete lane dividers and wooden screens to keep construction activities from distracting drivers. The ramp from the Costa Mesa Freeway to the Santa Ana Freeway will close after 1 a.m. Saturday, and the northbound connector from the Santa Ana to the Costa Mesa will close about 6 a.m.

All ramps should reopen between 10 a.m. and noon Saturday.

Other closures caused by the construction include:

* The Main Street tunnel under the Santa Ana Freeway will close next April for seven to eight months as the tunnel is lengthened to support a wider freeway above it.

* The transition from the northbound Santa Ana Freeway to the southbound Costa Mesa will close for up to 1 1/2 years beginning in mid-1993 to allow for construction of the flyover connector ramps and freeway widening. It is the least used transition ramp between the freeways, Juge said. Detours will be posted.

* The 4th Street off-ramp from the southbound Santa Ana will close in one to three months for most of 1992. A temporary off-ramp to 1st Street will be built so that motorists will face only a short detour, Juge said.

Once construction work is complete, the interchange will boast California’s only direct freeway-to-freeway transition for car-pool lanes, Caltrans spokesman Albert Miranda said.

The two-lane separate bridge will allow car-pool vehicles to switch from the northbound Costa Mesa Freeway to the northbound Santa Ana, and vice versa, without ever leaving the car-pool lane. The single flyover will handle traffic in both directions.

Currently, commuters in car-pool lanes have to fight their way across the freeway to get to the transition lane, then fight their way back across to the car-pool lane on the new freeway, Miranda said.

The new car-pool transitway “will eliminate all that weaving” which tends to back up traffic, he said.