Durning finds the evidence in The Guardian:

Did Seattle’s mandatory helmet law kill off its bike-share scheme? 

Fundamentally, low ridership killed Pronto. The system had 500 bikes at 54 stations. In its first year of operations, there were 142,832 trips or an average of just 0.78 rides per bike per day. According to the National Association of City Transportation Officials, the national average for US bike share systems is 1.8 rides per bike per day. New York City’s CitiBike system gets nearly 3.8.

If you ask five people why Pronto had such low ridership, you’ll get as many answers. Some say Seattle’s helmet law discouraged use. Others say the system was too spread out and never got the expansion it needed. Some say it lost its political support both inside and out of city hall. More still think would-be riders were discouraged by the lack of bike infrastructure in downtown Seattle or the city’s notorious rain and hills.

In truth, all those theories are at least partially correct. It was a series of compounding problems that spiralled over time until the mayor had little incentive to fight to keep Pronto alive. …

Few advocates think the helmet law alone killed Pronto, but Russell Meddin, co-founder of The Bike-Sharing Blog, is among those who think helmets hurt the spontaneity that makes bike share successful. “Helmet laws stop the serendipity of using the system,” he says.People want convenience. The more convenient a system is, the more it’s used.” …

One of Pronto’s biggest flaws was its lack of station density. According to Meddin, successful systems need 20 to 28 stations per square mile. Seattle’s had closer to 12. …

In February 2016, as the city council debated whether to acquire the system, Pronto’s financial woes came to light. Ridership had fallen short of projections, leaving the system with less fare revenue than planned. On top of that, the non-profit had stopped seeking new sponsorships in 2015 because they’d expected the city to take over sooner than they did. When it came time to vote, Pronto was nearing insolvency and the city needed to spend $1.4m to acquire it and pay off its debts. …

The final straw came when it emerged that Seattle was going to have to scrap all Pronto’s equipment to move forward with its expansion plans. All the leading proposals from the tender process called for starting fresh with new stations and bikes, and the winning bid from Bewegen called for an all-electric assist bike share fleet. “That definitely frustrated some of the more fiscally conservative members of the council who felt like we were had. Bad press leads to bad policy leads to bad outcomes and it spiralled,” explains Padelford. …

Some cycle experts in the city believe new Chinese private bike share companies will set up in the city. Operating on an Uber-like model of implementing first, asking forgiveness later, they have been launching without permission in cities around the world: BlueGoGo launched in San Francisco recently, while Ofo is headed for Cambridge.