When traffic lanes were eliminated on Spruce and Pine Streets from river to river in Center City five years ago to make room for bicycle lanes, it took more than just long strips of white paint to ensure a happy marriage between motorists and cyclists.

An expensive and years-long project to uproot the city's 1940s-era traffic signals in the central area and replace them with digital ones that can be operated remotely made it possible.

Through computer commands, officials were able to synchronize the lights so vehicular traffic could flow smoothly enough in one lane to allow bicyclists safe passage along the other. That put an end to hazardous stops and starts, which would have made the commingling of cars and bikes a tricky tango.

That project to replace the city's antiquated signals, which stalled for several years after the 9/11 attacks, is back in gear and spreading north of Center City to Callowhill Street.

The new digital signals, which are being installed along with curb cuts, traffic cameras, and fiber-optic cables, are key to helping cope with pressures associated with the surge of redevelopment near the city's core, officials said.

Crews recently began work on a $10.6 million job to replace 51 clicking electromechanical signals with digital ones, connected fiber-optically to a network that will transmit to an as-yet-unbuilt nerve center.

Bids to construct the $4.2 million nerve center are due Thursday.

"We're the last major city in the country to build a traffic operations center," said Streets Commissioner David Perri.

Curb cuts and other improvements also are planned for the quadrant bound by Callowhill and Market Streets and Broad and Second Streets.

The work to convert the old traffic signals in central Philadelphia before anywhere else in the city has proceeded in stops and starts since 1996. Funding issues have prevented continuous work on the project, officials said.

Though the results may not be evident to the untrained eye, they have affected pedestrians, motorists, and cyclists, particularly as the city has sought to make room on its narrow streets for bicycle riders during its housing renaissance.

Digital signals allow engineers to control car traffic and crosswalk signs in ways far less cumbersome than with older equipment, which must be readjusted manually at the site if they go off-line due to, say, snow or a power outage.

And so, after digital signals were installed along Spruce and Pine Streets west of Broad and began going up east of Broad five years ago, officials set the signals so traffic would flow at 20 m.p.h.

"Signal timing allowed the bike lanes to be installed," Perri said.

The digital signals are replacing midcentury electromechanical ones whose timers are set inside control boxes. They operate much like an automatic home lamp timer, telling the signal when to change from green to yellow to red.

"Imagine the same thing, except ours moves a lot faster," said Richard Montanez, the city's chief traffic and light engineer.

"You've got a piece of steel, and you stick it in a notch and it triggers a trigger," said the deputy streets commissioner, Michael Carroll.

So far, 500 miles of fiber-optic cable have been installed as sections of Center City have been upgraded. But there is no telling when the rest of the city might enjoy similar upgrades, given the unpredictable nature of federal aid.

The current project consists of 80 percent federal funding and 20 percent from the city.