Late-night drivers may legally cross the lower West Seattle swing bridge, while the city considers opening the two-lane span to more users.

The change takes effect immediately. It will relieve some drivers from a 4-mile detour using the First Avenue South Bridge. The Seattle Department of Transportation (SDOT) also announced public and private school buses may travel the swing bridge at all times.

That slight loosening of the rules is a prelude to adding other groups this fall, subject to automated camera enforcement that records license-plate numbers, the agency says.

Traffic Lab is a Seattle Times project that digs into the region’s thorny transportation issues, spotlights promising approaches to easing gridlock, and helps readers find the best ways to get around. It is funded with the help of community sponsors Alaska Airlines, Kemper Development Co., NHL Seattle, PEMCO Mutual Insurance Company and Seattle Children’s hospital. Seattle Times editors and reporters operate independently of our funders and maintain editorial control over Traffic Lab content.

All vehicles will be allowed on the bridge from 9 p.m. to 5 a.m., seven days a week.

For the last three months, the low bridge was limited to public transit, freight, longshore workers and emergency vehicles. Police typically patrolled the bridge only at peak commute times, but issued more than 500 citations.

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Traffic is a mess because of rapid cracking on the seven-lane, high rise West Seattle Bridge, which closed March 23. SDOT restricted low-bridge use to prevent a flood of traffic that would block emergency vehicles.

Based on recent trip data, the swing bridge and its five-spoke westside intersection can absorb another 160 vehicles per hour each direction, before reaching a 500-vehicle density that could slow fire and police response, according to Adiam Emery, a city intelligent transportation system engineer.

Competition is underway for those prized 160 spaces, discussed Wednesday by a city task force representing bridge neighbors and interest groups.

Heather Marx, SDOT mobility director, said the city is considering employer-operated private buses, van pools, essential health-care workers and maritime workers for priority access.

The winners can avoid backtracking on industrial West Marginal Way Southwest along the Duwamish River, where traffic has nearly tripled, or cutting through residential areas in Highland Park and White Center.

Vigor Industrial seeks swing-bridge access for commuting shipbuilders, said task-force member Jill Mackie, Vigor’s senior vice president for public affairs. But they and other Harbor Island workers, by themselves, could exceed all 160 vehicles.

Task force co-chair Greg Nickels, a former Seattle mayor, asked Emery to gather data on how firefighters and police used the low bridge since March.

He wondered if SDOT’s traffic policies create “a very incremental public safety benefit” and could perhaps be loosened. Specialty firefighting trucks crossing the bridge could “create that access in a relatively short time, before they actually get to the incident that already has a significant response,” Nickels said.