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39 Snoqualmie Pass Comprehensive Safety Plan I Kittitas County <br />Speed <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, S5th percentile speeds exceeded the <br />posted speed limit bv 14.9 mph northbound and <br />14.8 mph southbound on the first segment, and by <br />11.4 mph northbound and t7.5 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 />ln the U.S., speeding is a factor in nearly 3O% 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 2OI9 and2023,3t% <br />of traffic fatalities in Washington State involved a <br />speeding driver, reflecting the same trend locally.t <br />For pedestrians, the chance of being killed if struck <br />rises from aboutT% at 20 mph to nearly 20%at35 <br />mph.+ Motorcyclists are especially vulnerable, with <br />over one-third of fatal crashes involving speeding.s <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 4-5o/5.n <br />This underscores why high vehicle speeds <br />are a critical safecyissue-andwhy <br />measures such as traffic calming, speed <br />enforcement, and roadway design can help <br />reduce speeds. <br />'hi-tLrs://.rashstats.nhtsa.dot.eov/Aoi/Puhlic/ViewPubli.ation/81358?.ndf <br />t httnq://wtsc.w.1.eov/dachboards/f atalities-dashboard/ <br />+ htlns://www.iihs.orelresearch-a reas/speed <br />5 httn.://crashstats.nhtsa.dot.eov/Api/Puhlic/ViowPuhlication/813582 odf <br />1T httns:,4aoos.who.int/irls/bitstream/10665/254760l1/WHO-NMH-NVl-177-ene.pdf <br />Why is the 85th Percentile Speed Important <br />in Tratfic Analysis and Safety? <br />The 85th percentile speed is the speed at orbelowwhich 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 />J <br />t_ <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 />. lncreased 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 -1O% risk of a fatality or serious <br />injury, but at 40 mph, the risk jumps to B0%. <br />' Greater speed variance: When some drivers follow <br />the speed limit and others drive much faster, the <br />variation increases the chance of collisions. <br />. Design misalignment: lf 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 />https://www.fhwa.dot. gov,/publications h esearch / saf ety / 77 098/0O3.cf m <br />'4'"' <br />..\