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Speed <br />* https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/813582.pdf <br />† https://wtsc.wa.gov/dashboards/fatalities-dashboard/ <br />‡ https://www.iihs.org/research-areas/speed <br />§ https://crashstats.nhtsa.dot.gov/Api/Public/ViewPublication/813582.pdf <br />¶ https://apps.who.int/iris/bitstream/10665/254760/1/WHO-NMH-NVI-17.7-eng.pdf <br />Figure 23 shows the speed limit on each evaluated <br />segment in the study area, as well as the 85th <br />percentile speed observed during the data collection <br />period compared to the posted speed limit. The <br />highest observed speeds during the weekday and <br />weekend data collection period in October were <br />recorded along two segments of SR 906: between <br />Exit 53 and Kearney Drive (Location 5) and between <br />Tanner Way and Exit 54 (Location 10). On these <br />segments, 85th percentile speeds exceeded the <br />posted speed limit by 14.9 mph northbound and <br />14.8 mph southbound on the first segment, and by <br />11.4 mph northbound and 17.5 mph southbound on <br />the second segment. <br />Higher vehicle speeds greatly increase both the <br />likelihood of a crash and the severity of the outcome. <br />In the U.S., speeding is a factor in nearly 30% of <br />traffic fatalities each year, and even small increases <br />in speed can dramatically raise the risk of death <br />or serious injury.* Between 2019 and 2023, 31% <br />of traffic fatalities in Washington State involved a <br />speeding driver, reflecting the same trend locally.† <br />For pedestrians, the chance of being killed if struck <br />rises from about 1% at 20 mph to nearly 20% at 35 <br />mph.‡ Motorcyclists are especially vulnerable, with <br />over one-third of fatal crashes involving speeding.§ <br />Globally, the relationship is consistent: an increase of <br />about 0.6 mph in average vehicle speed raises fatal <br />crash risk by about 4-5%.¶ <br />This underscores why high vehicle speeds <br />are a critical safety issue—and why <br />measures such as traffic calming, speed <br />enforcement, and roadway design can help <br />reduce speeds. <br />Why is the 85th Percentile Speed Important <br />in Traffic Analysis and Safety? <br />The 85th percentile speed is the speed at or below which 85% of vehicles are observed to <br />travel. It reflects the speed that most drivers choose when not constrained by traffic or <br />enforcement. <br />When the 85th percentile speed on a road segment is significantly higher than the posted speed limit, it <br />can signal a mismatch between driver behavior and roadway design or signage. This mismatch can lead to:* <br />* https://www.fhwa.dot.gov/publications/research/safety/17098/003.cfm <br /> ƒIncreased crash severity: Higher speeds result <br />in more kinetic energy during a crash, which <br />increases the likelihood of serious injuries or <br />fatalities. For example, a pedestrian hit at 20 <br />mph has a ~10% risk of a fatality or serious <br />injury, but at 40 mph, the risk jumps to 80%. <br /> ƒGreater speed variance: When some drivers follow <br />the speed limit and others drive much faster, the <br />variation increases the chance of collisions. <br /> ƒDesign misalignment: If most drivers exceed the <br />posted limit, it may indicate that the road feels safe <br />to drive faster than intended—suggesting a need <br />to reassess the speed limit or roadway design. <br />39 Snoqualmie Pass Comprehensive Safety Plan | Kittitas County