Washington, D.C., area commuters battled heavy traffic in cars, on bicycles and aboard crowded buses Wednesday as the second-busiest U.S. subway system shut down for emergency safety checks.

Some of the estimated 700,000 people who ride the Washington-area Metro system on a typical weekday complained that they had little time to prepare after the unprecedented shutdown was announced late Tuesday afternoon. It is intended to give safety crews time to inspect 600 underground cables that could pose a fire risk.

The shutdown also raised concerns about the safety of a system that has long been plagued by smoky tunnels, breakdowns and deadly accidents. It was no windfall for the city's cab drivers, many of whom said the heavy traffic was hurting their business.

Frederick Gunther said he left his home in Laurel, Maryland, about 20 miles (32 km) northeast of Washington, at 6 a.m. to get to work downtown by 8 a.m., thinking that would be ample time.

"But if I'd left any later, I would've definitely been late," the 55-year-old security guard said. "The closer I got into the city, the crazier traffic got."

The Washington Metropolitan Area Transit Authority shut down the 119-mile (230-km) system serving the U.S. capital and its Maryland and Virginia suburbs at midnight, its normal closing time, and said it would reopen at 5 a.m. Thursday.

U.S. government offices, Congress and most schools remained open, although the Office of Personnel Management, which oversees the federal workforce, gave employees the option of taking unscheduled leave or working from home.

U.S. Transportation Secretary Anthony Foxx said the shutdown was necessary for a system where 18 people have been killed in rail accidents in the past 34 years.

"WMATA has a long, well-documented list of safety issues and needs to work aggressively to fix them," Foxx said in a statement. "While this shutdown is inconvenient, they are doing the right thing by putting the safety of their passengers and workers first."

By midday, WMATA officials said that more than half the inspections had been completed.

"This has proven to be a worthwhile endeavor," spokesman Dan Stessel said. "The inspections are finding things that need to be rectified."

He declined to say whether the shutdown could be extended due to the conditions inspectors found, saying that the agency would announce its decision around 5 p.m. EDT.

Taxi Troubles 

More than 60 people were lined up waiting for taxis at Union Station, the Beaux Arts long-distance passenger rail hub a few blocks north of the Capitol.

Ted Cox, a 62-year-old immigration lawyer from New York, was worried about making a court appointment in Arlington, Virginia, and then getting back to the station in time for his Amtrak train home.

"What kind of country are we in that the nation's capital can't run a metro system?" he said. "It's sort of on a par with the infrastructure in the rest of the country. Hopefully this will get the attention of legislators to deal with infrastructure in general."

Cab drivers said the traffic was limiting the number of trips they could take.

"Everywhere is crowded," said Asamenew Tesfaye as he drove a yellow cab in from suburban Alexandria, Virginia. "It looks busy for everybody, but we make less money than the other days."

Some commuters took to Twitter to express frustration at delays and crowding on buses, making #MetroShutdown the top-trending hashtag in the United States on Wednesday morning. One user joked that the city should flood the subway tunnels to the level of the platforms and rely on Venetian gondolas rather than trains.

With cloudy skies and temperatures approaching 60 degrees Fahrenheit, the morning weather was conducive to cycling and walking, but forecasters warned of possible afternoon thunderstorms that could make for a messier trip home.

Buses were running normally Wednesday, and parking was free in Metro-owned lots and garages.

Ride service Uber said it would cap surge pricing in the Washington area at 3.9 times base fares during the shutdown. It said it was expanding uberPool coverage across the area.

San Francisco-based rival Lyft said it was expecting high demand and offered new customers $20 off their first ride.

Rania Hassan, a 41-year-old graphic designer, said she normally bicycles to work, but Wednesday's commute was tougher than usual.

"There are more cars on the road, way more cars," Hassan said. "And they don't know how to drive."