Helmets are required for bicyclists in all of King County (this includes the City of Seattle). The King County helmet law was enacted in 1993 and was updated in 2003 to include the city of Seattle, according to the Washington State Department of Transportation.
While some helmet laws throughout Washington state only apply to people of certain ages (for example, the city of Poulsbo's helmet law only applies to those under the age of 18), King County's helmet law applies to everyone who rides a bicycle.
Currently, there is no official state law that requires bicyclists to wear a helmet when they ride, which leaves the decision of whether or not to enforce helmet laws up to individual counties and cities. A full table of bicycle helmet laws by location throughout the state can be found below:
Penalties for Not Wearing a Helmet in King County
The violation is a civil infraction (ticket) and the base fine is $30. There are additional court costs of $51 added to the base fine amount, so the total is $81. All law enforcement officers have the authority to enforce this code -- police officers, sheriff's deputies, state patrol troopers, etc.
In addition to the financial costs, a bicyclist's failure to wear a helmet can come at an even greater cost. According to the Insurance Institute for Highway Safety, 91 percent of bicyclists who were killed in collisions in 2009 were not wearing helmets at the time of the crash.
The CDC says there were approximately 800 bicyclist fatalities in 2010 and another 515,000 bicycle-related injuries. Overall, it is estimated that the indirect financial costs for injuries to unhelmeted bicyclists is approximately $2.3 billion each year.