Truth Needle: Can a light-rail system carry 16,000 hourly riders through its busiest places? Perhaps, if people are extremely crammed.

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The claim: In television ads supporting this fall’s Proposition 1, better known as Sound Transit 3, University of Washington professor Mark Hallenbeck says: “One more regular freeway lane moves only 2,000 cars per hour. But a single light-rail line carries 16,000 people per hour.”

The same numbers are cited in speeches by transit-board Chairman Dow Constantine, and by the Mass Transit Now website.

What we found: Mostly true.

Historically, rail campaigns in the U.S. have used freeway comparisons to woo frustrated drivers.

Before considering the numbers, keep in mind Hallenbeck is talking about how many people trains can move — not how many people will actually hop aboard.

Other factors affect transit use, including station locations, population density, trip times, fares, parking costs, sporting events and the economy. The sound bite should say “could carry” instead of “carries.”

The $54 billion ST3 proposal features 62 miles of light rail in seven extensions, plus more bus-rapid transit and commuter rail, to be completed by 2041.

First, consider the freeways.

Two thousand vehicles an hour is the ideal throughput at 45 mph, says the Washington State Department of Transportation’s Corridor Capacity Report. The number of people traveling is higher.

Toby Nixon, a Kirkland City Council member opposed to ST3, says the 2,000 figure “excludes carpool, vanpools, buses. It’s very misleading; it borders on lying.”

State data say a bus-carpool lane of Interstate 5 passing Northgate moves 5,000 people in the busiest hour, or 3.9 people per vehicle, compared with 1,700 people per general lane, or 1.3 per vehicle. During stop-and-go traffic, WSDOT says, a general lane carries as few as 700 vehicles per hour.

Now, consider the trains.

Each railcar has 74 seats but holds a maximum 200 people, crowded shoulder-to-shoulder. This happens after a ballgame, or after a rush-hour train delay. Station platforms are long enough for four-car trains, which could hold 800 people. (Currently the longest trains have three railcars.)

ST3 proposes a train every six minutes from West Seattle to downtown Everett, sharing trackway with trains between Redmond and the Mariner/128th Street Southwest station. Where these overlap, a train would pass every three minutes between Mariner and International District/Chinatown stations at peak times.

At that frequency, 20 trains times 800 riders equals 16,000 per hour per direction.

“This will never happen in Seattle,” says opponent Mark Ahlers, of Lynnwood, a self-described transit geek and analyst for the No on ST3 campaign. “People will wait for the next train, long before that happens. It’s a failure for the people riding it.”

When Sound Transit CEO Peter Rogoff released his plan last December, transit staff favored a lower planning target of 150 riders, generating a more sustainable 12,000 passengers per hour. Half would be standing.

Hallenbeck, whose specialty is traffic research, said even the 16,000 figure is credible, for segments just north of downtown. People in London ride this snugly, he said. “It’s not fun,” he said, “but it also beats the heck out of sitting in your car going nowhere.”

There’s more.

ST3 would add another tunnel beneath downtown parallel to the existing transit tunnel; otherwise there’s not enough room to handle trains arriving from five directions, according to Rogoff. In 2035, trains from SeaTac and Tacoma are supposed to use that east tunnel, then veer to South Lake Union and Ballard, every six minutes.

In an online pamphlet for voters, the agency goes so far as to list a theoretical 64,000 figure, counting travel in both north and south directions. That also presumes east-downtown trains show up every three minutes, which requires more railcars and routes than the six-minute service offered by ST3.

It also presumes everything flows smoothly. Clogged station platforms, logistics mistakes or mechanical breakdowns all lower capacity. And though all extensions in ST3 would be grade separated, surface crashes with people or cars in Sodo and Rainier Valley can cause delays that spread north.

To sum up, Link light rail can haul most of the advertised 16,000 riders, while a 2,000-car figure for general traffic is defensible.

Foes say maximum loads are irrelevant for outlying areas like Fife, downtown Everett and Issaquah, which would attract only a few thousand daily train riders.

Regionally, they predict self-driving cars and buses will boost freeway capacity in the near future.

The greatest fallacy with train vs. lane comparisons lies not in the math, but that some voters, inundated by footage of gridlocked freeways, will assume ST3 would take cars off the road so people can drive faster to work.

That sort of claim would be false.