Transportation Terms#
15-Minute City#
An urban planning concept where residents can reach daily necessities – work, school, shopping, healthcare, parks – within a 15-minute walk or bike ride. Popularized by Paris Mayor Anne Hidalgo and professor Carlos Moreno.
Why it matters: Seattle’s Office of Planning and Community Development has explored the 15-minute city as a framework for comprehensive planning. A 2021 study found only 44% of Seattleites can walk to basic amenities, with large gaps in West Seattle, Northeast Seattle, and other car-dependent areas. The concept connects zoning, transit, and land use goals.
See also: Complete Streets, Mode Share
Learn more: Seattle OPCD: One Seattle Plan | The Urbanist: Seattle’s Quest to Become a 15-Minute City
BAT Lane (Business Access and Transit)#
A dedicated lane that prioritizes buses while still allowing other vehicles to use it briefly for right turns and to access adjacent driveways. BAT lanes are less restrictive than fully bus-only lanes, making them politically easier to implement in commercial corridors.
Why it matters: BAT lanes are King County Metro’s primary tool for speeding up RapidRide service in suburban corridors. Debates around corridors like Route 40, RapidRide K Line, and RapidRide I Line frequently center on whether BAT lanes will be implemented or watered down due to business and driver opposition. Understanding BAT lanes helps advocates push for meaningful transit priority.
See also: RapidRide, Transit Signal Priority
Learn more: King County Metro: RapidRide | The Urbanist: Push to Water Down Route 40 Transit Upgrades
Bicycle Master Plan#
Seattle’s long-range plan for building a citywide network of bike infrastructure, including protected bike lanes, neighborhood greenways, and trail connections. The plan envisions 100+ miles of protected lanes and 250 miles of greenways.
Why it matters: The Bicycle Master Plan sets priorities for which bike routes get built and when. Progress has been slower than planned – the Move Seattle levy promised 50 miles of protected lanes but delivered far fewer. Understanding the plan helps advocates push for specific route improvements.
See also: Protected Bike Lane, Neighborhood Greenways, Vision Zero
Learn more: Seattle SDOT: Bicycle Master Plan | The Urbanist: Five Key Takeaways from Updated Bike Master Plan
Complete Streets#
A street design approach that considers all users – pedestrians, cyclists, transit riders, and drivers – not just cars. May include wider sidewalks, protected bike lanes, bus-only lanes, and traffic calming.
Why it matters: Seattle adopted a Complete Streets policy, and it shapes how SDOT designs road projects. When a street gets rebuilt, advocates can push for complete streets elements using this framework.
See also: LOS, Vision Zero, Mode Share
Learn more: Seattle SDOT: Complete Streets | Seattle Streets Illustrated
Curb Extension (Bulb-out)#
A traffic calming measure that extends the sidewalk into the street at intersections or mid-block crossings, shortening the distance pedestrians must cross and making them more visible to drivers. Also called neckdowns, bump-outs, or curb bulbs.
Why it matters: Seattle uses curb extensions as a core Vision Zero tool. They reduce pedestrian crossing distance, slow turning vehicles by tightening the turning radius, and prevent illegal parking near crosswalks that blocks sightlines. SDOT installs curb bulbs as part of intersection safety improvements, with costs ranging from $2,000 to $20,000 per corner depending on drainage needs.
See also: Daylighting, Vision Zero, Complete Streets
Learn more: Seattle SDOT: Crosswalks and Curb Bulbs | FHWA: Curb Extensions
Daylighting#
The practice of restricting parking near intersections and crosswalks to improve visibility between pedestrians and drivers. Typically involves “no parking” signs, painted or concrete curb extensions, bike corrals, or other infrastructure within 20-30 feet of intersections.
Why it matters: According to FHWA, restricting parking near intersections is one of the most effective and low-cost ways to reduce pedestrian crashes. Seattle’s daylighting program is funded by the Transportation Levy, with plans to daylight over 20 intersections in 2025. Daylighting doesn’t remove legal parking – it enforces existing laws that prohibit parking within 30 feet of stop signs.
See also: Curb Extension, Vision Zero
Learn more: Seattle SDOT: Intersection Daylighting Safety Improvements | FHWA: Curb Extensions
Diagonal Crossing (Pedestrian Scramble)#
A signal phase that stops all vehicle traffic and allows pedestrians to cross an intersection in any direction, including diagonally. Also called a “Barnes Dance” after traffic engineer Henry Barnes, who popularized the concept in the 1950s and 60s.
Why it matters: Pedestrian scrambles eliminate conflicts between turning vehicles and crossing pedestrians – a leading cause of pedestrian injuries. They’re most effective at high-pedestrian-volume intersections where turning conflicts are common. While Seattle has some pedestrian scrambles, they’re less common than Leading Pedestrian Intervals, which are easier to implement at existing signals.
See also: Leading Pedestrian Interval, Vision Zero
Learn more: FHWA: Pedestrian Safety Guide | SSTI: Return of the Barnes Dance
HAWK Beacon#
A pedestrian-activated traffic control device used at uncontrolled crosswalks, officially called a “High-Intensity Activated crossWalK beacon.” When a pedestrian presses the button, the beacon progresses through a sequence of flashing and solid red lights that require drivers to stop.
Why it matters: HAWK beacons are more effective than standard crosswalk markings at getting drivers to yield. FHWA research shows they reduce pedestrian crashes by up to 69% and driver yielding rates exceed 90% (compared to ~30% at traditional crosswalks). They’re typically installed at mid-block crossings or intersections where a full traffic signal isn’t warranted but pedestrian volumes or speeds (often 35+ mph) require more than paint. Seattle primarily uses RRFBs for uncontrolled crossings, but HAWKs are an option for higher-speed arterials.
See also: Rectangular Rapid Flashing Beacon, Vision Zero
Learn more: FHWA: HAWK Signals | Seattle SDOT: Vision Zero
High Injury Network#
The subset of streets where the most serious and fatal traffic crashes are concentrated. In Seattle, about 12% of streets account for over 50% of serious injury and fatal crashes. These corridors are the focus of Vision Zero investments.
Why it matters: Seattle’s High Injury Network is heavily concentrated in South Seattle, with Rainier Avenue, Aurora Avenue (SR 99), Lake City Way (SR 522), and MLK Jr Way being the deadliest corridors. 56% of fatal and serious pedestrian crashes occur in Council District 2 (Rainier Beach, Columbia City, Beacon Hill, Chinatown-ID). SDOT uses the High Injury Network alongside the city’s Race and Social Equity Index to prioritize safety investments.
See also: Vision Zero, Road Diet
Learn more: Seattle SDOT: Vision Zero | The Urbanist: Accountability in Vision Zero
Induced Demand#
The phenomenon where adding road capacity generates additional traffic, typically erasing congestion benefits within a few years. Research shows a 10% increase in lane capacity correlates with roughly 9% more traffic.
Why it matters: Induced demand is frequently cited in debates about highway expansion, including proposals to widen I-5. Understanding induced demand reframes transportation debates from “how do we reduce congestion” to “how do we reduce driving.”
See also: VMT, Mode Share
Learn more: WSDOT: Congestion & Mobility Reports | The Urbanist: The Law of Induced Demand
Leading Pedestrian Interval (LPI)#
A traffic signal timing strategy that gives pedestrians a 3-7 second head start to enter the crosswalk before vehicles get a green light. The walk signal activates while all vehicle signals remain red, making pedestrians more visible to turning drivers.
Why it matters: LPIs are one of Seattle’s most effective and widely-deployed pedestrian safety tools. At intersections where LPIs were installed from 2009-2018, Seattle saw a 48% reduction in turning collisions involving pedestrians and a 34% reduction in serious injury and fatal pedestrian crashes. By 2024, Seattle had installed LPIs at over 700 signalized intersections – nearly three-quarters of traffic signals citywide. SDOT continues to expand LPIs with federal, state, and local funding.
See also: Vision Zero, Diagonal Crossing
Learn more: Seattle SDOT: Leading Pedestrian Intervals | SDOT Blog: LPI Rollout Acceleration
LOS (Level of Service)#
A grading system (A through F) for traffic flow, where A means free-flowing and F means gridlock. Traditionally used to evaluate intersections and roads from a car driver’s perspective.
Why it matters: LOS has historically been used to block dense development or transit projects because they’d “worsen traffic.” Urbanists argue that LOS is a car-centric metric that shouldn’t drive land use decisions. Washington state passed reforms allowing cities to use alternative metrics.
See also: VMT, Complete Streets
Learn more: MRSC: Transportation Planning (GMA multimodal LOS reforms)
Mode Share#
The percentage of trips taken by each transportation mode (driving, transit, biking, walking). Seattle’s goal is to reduce the car mode share and increase all other modes.
Why it matters: Mode share is how we measure whether investments in transit, bike lanes, and walkability are actually changing behavior. Seattle’s downtown has seen significant mode shift away from cars, but most neighborhoods are still car-dominant.
See also: VMT, Complete Streets
Learn more: Commute Seattle: 2021 Mode Split Survey | Seattle SDOT: Commute Trip Reduction Program
Neighborhood Greenways#
A network of residential streets with traffic calming, wayfinding signs, and crossing improvements designed to prioritize walking and biking. Seattle has built about 100 miles of greenways, typically on low-traffic streets parallel to busy arterials.
Why it matters: Greenways are a lower-cost way to expand the bike network compared to protected lanes on arterials. They work well for confident cyclists but critics note they require navigating intersections with busy streets. The greenway network connects to the broader Bicycle Master Plan vision.
See also: Bicycle Master Plan, Complete Streets
Learn more: Seattle SDOT: Greenways Program | The Urbanist: Seattle Bike Network Popularity
Protected Bike Lane (PBL)#
A bike lane physically separated from car traffic by barriers, planters, parked cars, or grade separation – not just paint. Also called “cycle tracks” or “separated bike lanes.”
Why it matters: Protected lanes dramatically increase cycling rates and safety compared to painted lanes. Seattle has been slowly building PBLs on key corridors like 2nd Avenue downtown, Pike/Pine, and MLK Way S. The city’s levy proposals have fallen short of promised mileage, making PBL advocacy a persistent focus.
See also: Bicycle Master Plan, Vision Zero
Learn more: Seattle SDOT: Protected Bike Lanes | The Urbanist: Pike/Pine Protected Bike Lanes
RapidRide#
King County Metro’s bus rapid transit (BRT) network featuring frequent service (every 10 minutes or better), branded stations, off-board payment, and transit signal priority. Currently includes lines A through H, with more planned.
Why it matters: RapidRide lines trigger additional zoning capacity under HB 1110 – areas within a quarter-mile of RapidRide stops can allow up to 6 units per lot. The network is also key to Seattle’s transit-oriented development strategy. Metro’s long-range plan envisions 26 RapidRide lines (A-Z) by 2050.
Learn more: King County Metro: RapidRide | The Urbanist: Metro Launches RapidRide G
Rectangular Rapid Flashing Beacon (RRFB)#
A pedestrian-activated warning device installed at crosswalks, featuring two rectangular amber LED lights that flash in an alternating “wig-wag” pattern when a pedestrian presses the button. RRFBs are mounted on crosswalk signs at uncontrolled crossings.
Why it matters: RRFBs dramatically increase driver yielding rates at crosswalks – from around 15-20% with standard signs to 80-95% with RRFBs. FHWA research shows they reduce pedestrian crashes by up to 47%. They’re a cost-effective tool for improving safety at crosswalks that don’t warrant full traffic signals or HAWK beacons, and were added to the federal Manual on Uniform Traffic Control Devices (MUTCD) in 2023.
See also: HAWK Beacon, Vision Zero
Learn more: FHWA: Rectangular Rapid Flashing Beacons | Seattle SDOT: Vision Zero
Road Diet (Rechannelization)#
A street redesign that reduces the number of travel lanes, typically converting a four-lane road to three lanes (one in each direction plus a center turn lane), with the reclaimed space used for bike lanes, wider sidewalks, or parking. Also called “rechannelization” or “lane reduction.”
Why it matters: Seattle has implemented 26+ road diets since the 1970s, with documented safety improvements. The Stone Way road diet (2007) reduced pedestrian collisions by 80% and top speeders by over 80%. The Nickerson Street road diet (2010) reduced aggressive speeding by 93%. FHWA considers road diets a “proven safety countermeasure” with crash reductions of 19-47%. Seattle’s deadliest corridors (Rainier Ave, Aurora Ave) are frequently cited as candidates for road diets.
See also: High Injury Network, Vision Zero, Complete Streets
Learn more: FHWA: Seattle Stone Way Road Diet Case Study | The Urbanist: Rainier Ave Road Diet Data
ST3 (Sound Transit 3)#
The $54 billion regional transit expansion plan approved by voters in 2016. Includes light rail extensions to Ballard, West Seattle, Tacoma, Everett, and other areas. Full build-out expected by 2041+.
Why it matters: ST3 is the biggest infrastructure investment in the region’s history. Delays and cost overruns have been controversial. Understanding the project timeline and station locations is essential for following development patterns.
Learn more: Sound Transit: ST3 Plan | Sound Transit: Realignment
Stride#
Sound Transit’s bus rapid transit (BRT) brand, consisting of three lines (S1, S2, S3) running along I-405 and SR-522 corridors. Stride features battery-electric buses, off-board fare payment, all-door boarding, and 10-15 minute frequencies. The combined program budget is $2.35 billion.
Why it matters: Stride represents the next major Sound Transit opening after the 2 Line light rail, with lines scheduled to open between 2028-2029. The S1 connects Burien to Bellevue, the S2 connects Bellevue to Lynnwood, and the S3 connects Shoreline to Bothell. These lines provide a faster alternative to ST Express buses and integrate with Link light rail. Understanding Stride helps track regional transit expansion beyond Seattle city limits.
Learn more: Sound Transit: Stride Bus Rapid Transit | The Urbanist: Sound Transit Breaks Ground on Stride Bus Base
TOD (Transit-Oriented Development)#
Higher-density, mixed-use development concentrated near transit stations. The idea is that people living/working near good transit will drive less.
Why it matters: With Link light rail expanding, TOD around new stations is one of the biggest development opportunities in Seattle’s history. Station area planning shapes whether these areas become walkable, mixed-use neighborhoods.
See also: ST3, HB 1491, Upzoning
Learn more: Sound Transit: Transit-Oriented Development | Seattle OPCD: Equitable TOD
Transit Signal Priority (TSP)#
A traffic signal control strategy that gives priority to transit vehicles at intersections. When a bus approaches an intersection, the system can extend a green light or shorten a red light to reduce delay. TSP is a key component of bus rapid transit and RapidRide service.
Why it matters: Transit signal priority is one of the most cost-effective ways to speed up bus service without building dedicated lanes. Seattle and King County Metro have been expanding TSP along RapidRide corridors, though implementation has been slower than planned. Advocates push for TSP because even small time savings at each intersection add up to significant trip time reductions across a route.
Learn more: SDOT: Transit Program | The Urbanist: Buses Slowed by Traffic
Vision Zero#
A policy commitment to eliminate all traffic fatalities and serious injuries. Seattle adopted Vision Zero in 2015.
Why it matters: Vision Zero gives advocates a framework to demand safer street design, lower speed limits, and enforcement. The city has committed to this goal and can be held accountable for progress toward eliminating traffic deaths.
See also: Complete Streets
Learn more: Seattle SDOT: Vision Zero | Seattle Vision Zero Action Plan (PDF)
VMT (Vehicle Miles Traveled)#
Total miles driven by vehicles in an area. An alternative to LOS for measuring transportation system performance that doesn’t privilege car throughput.
Why it matters: Washington now allows cities to use VMT instead of LOS for evaluating development impacts. This is a significant policy shift that can unlock denser development near transit.
See also: LOS, Mode Share
Learn more: WSDOT: VMT Targets Report (PDF) | MRSC: Transportation Planning
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