Buses: You have questions; King County Metro has answers.

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Bus ridership has continued to grow in the Seattle area, and recently, readers have become more interested in the inner workings of the bus system.

In this week’s Q&A, Traffic Lab reached out to Jeff Switzer, a spokesman for King County Metro, to answer questions readers sent in about buses.

Some questions have been lightly edited for clarity.

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, CenturyLink, Kemper Development Co., NHL Seattle, PEMCO Mutual Insurance Company, Sabey Corp., Seattle Children’s hospital and Ste. Michelle Wine Estates. Seattle Times editors and reporters operate independently of our funders and maintain editorial control over Traffic Lab content.

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Q: Our bus-route numbers go at least as high as the 700s, possibly higher. I know there aren’t 700 different routes in the system. How are route numbers determined? Do they mean anything?

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— Donna Howard, Maple Leaf

A: Bus-route numbers are correlated with the agency providing the service or the community where the majority of the route operates.

• 0-99 numbers are Seattle city routes.

• 100-199 are South King County routes.

• 200-299 are Eastside routes.

• 300s serve North King County.

• 400s are Community Transit routes that serve Snohomish County.

• 500s are Sound Transit routes.

• 671-676 are Metro’s numerical designations for RapidRide A-F routes.

• 773 and 775 are the shuttles serving the King County Water Taxi.

• 800s are Metro’s custom bus routes.

• 900s are Metro DART (dial-a-ride transit) routes.

The process to designate new routes looks at what numbers are available, the history of that route and nearby service.

For example, if route 62 were canceled, planners would want to wait a few years before introducing a new route with that same number to prevent confusion among riders.

King County Metro retired route 359 after Mark McLaughlin was killed while driving the bus. A passenger shot McLaughlin then fatally shot himself, causing the bus to plunge off the Aurora Bridge. Another passenger died and 32 others were injured.

Q: What is the purpose of the bus-only lane after 3 p.m. on Howell Street? The lane is almost always used by vehicles other than buses and is not enforced even while a police officer is directing traffic at Howell Street and Minor Avenue.

— Sara Martin, Edmonds

A: A transit-only lane on Howell Street was installed in 2011 in an effort to keep buses moving around traffic backed up from the southbound Interstate 5 ramp during evening commute times.

At the time, most traffic heading in that direction was coming from downtown Seattle and organized into the right lane, Switzer said.

Now, growth in South Lake Union has changed traffic patterns, and more drivers enter Howell Street from Minor and Boren avenues. Cars often block the bus lane, preventing bus operators from merging to the right to access the I-5 ramp.

Enforcement of the bus-only lane is up to the Seattle Police Department (SPD). The traffic officer regularly stationed at Howell Street and Minor Avenue is employed by the Metropolitan Park towers to facilitate vehicles exiting their parking garage.

Police do control bus-only lanes, but downtown is a “complex thing” and officers have to “juggle different elements,” while looking out for violators, an SPD spokesman said.

Switzer said Metro and the Seattle Department of Transportation are in talks to design a plan that would move the bus-only lane to the far left side of Howell Street, instead of in the center lane, to help further separate it from traffic heading to I-5.

Q: Why are empty buses that are “out of service” or “returning to terminal” allowed to use the carpool lane? There is only one occupant in these cases — the solo driver.

— Laurie Wylie, Shoreline

A: Allowing buses, even when empty, to use carpool lanes keeps them “moving to where they need to be” in order to “carry the most people possible, which also takes cars off the road that might otherwise be there,” Switzer said.

A terminal can refer to the start or end of a trip. Sometimes bus drivers are heading from the base to a starting point for a trip and use the freeway to get there quickly and efficiently, he said.

Other times, such as when the bus is out of service, a driver may have just completed all of his or her trips or finished a shift and is returning to base on the fastest route possible.

The Puget Sound region’s network of High Occupancy Vehicle lanes was designed to encourage carpooling by prioritizing the movement of large numbers of people over solo-driven cars. Those portions of roadway are reserved for cars typically carrying two or more passengers.

Under state law, WSDOT also allows public transit vehicles to use HOV lanes — even when the bus driver is the only passenger aboard. Certain motorcycles and emergency vehicles are permitted to use HOV lanes as well.

Got a question?

Do you have a question about transportation for Traffic Lab? We’d like to try to answer it. Send your questions to trafficlab@seattletimes.com or tweet us at @STtrafficlab, and we may feature them in an upcoming column.