This article is the first in a three-part series on the aftermath of a fatal traffic collision in west Wichita, and what the incident means for the future of mobility in our city.
By Alex Pemberton
On Monday we shared an article analyzing the deadly traffic collision on 119th Street, in which a 59-year-old woman was killed on Saturday after pulling her SUV over to the curb to let a funeral procession pass, and the forces that result in such carnage on Wichita's streets.
Her vehicle was struck from behind by the 31-year-old driver of a van; the cause of the crash is still under investigation. Though the immediate cause is not yet known, the stretch of 119th Street where the incident occurred has a reputation for danger. Multiple residents quoted in a newspaper article on the collision shared their perception that the corridor invites excessive speeding.
In this article, we will take a deep dive into the traffic data along the corridor and analyze the role street design may have played in the tragic crash. The lessons learned from 119th Street are instructive for understanding the relationship between street design and traffic safety citywide.
The Danger of Comfort
It is hardly a secret that speed kills. In a ten-year study of traffic fatalities, the National Transportation Safety Board (NTSB) found speeding was a factor in 31% of all deadly crashes; speeding causes about as many fatal traffic crashes as drunk driving. 78% deadly speeders were driving passenger vehicles like the ones involved in Saturday's fatal collision. The NTSB found that speeding increases crash risk by both increasing the likelihood of being involved in a crash, and the severity of injuries suffered in a crash.
Speed limits are most often set by statute. Locally, the standard for neighborhood streets is 30 m.p.h. and arterial streets are set at a default of 40 m.p.h. - however, these standard speed limits are often adjusted by the City of Wichita based on observed operating speeds for each road segment. This technique is known as the 85th Percentile Rule. The 85th Percentile Rule has many flaws, yet its application is a reflection of the influence street design has on driver behavior.
The rule essentially holds that speed limits should be set at a level at which 85% of drivers will drive at or below the speed limit. The rule is a reflection of how humans alter their behavior based on their environment - risk homeostasis. When there are narrow driving lanes, heavy traffic, lots of edge friction (like trees, bus stops, or close buildings), frequent intersections, and the presence of pedestrians, drivers respond by operating their vehicles at slower speeds - the risk of a mistake, running into a tree or lamp post or jogger, is amplified in these environments.
Conversely, environments lacking these street features - instead, often long, wide stretches of road with few intersections and far-away edges - instill in drivers a level of comfort that encourages them to boost their speed. The risk of hitting something seems minimal, so they feel more comfortable to adjust their risk threshold upward. In laymen's terms - they speed, and they pay far less attention to their surroundings.
Deadly By Design: 119th Street Edition
The segment of 119th Street in which Saturday's collision occurred typifies how Wichita's over-engineered streets lead to tragic outcomes.
From Kellogg to Maple, 119th Street is the epitome of a featureless, frictionless stroad. Recently expanded to five lanes, the street has far more capacity than is justified by its level of use.
Wichita's open data portal allows users to search map-based visualizations of public data, such as traffic counts and 85th percentile speeds. Our team analyzed this data for 119th Street, as well as similar low-traffic suburban arterials. The segment of 119th where the crash occurred, which is a posted 40-mile-per-hour zone, registered 85th percentile speeds at 52 m.p.h. - 30% over the posted speed limit. In short, the data backs up residents' perceptions and the theory of risk homeostasis - traffic speeds on this stretch of 119th are excessive and dangerous.
This data is significant because the velocity of car crashes has a tremendous impact upon their severity. A study conducted by the London Department for Transport on the relationship between speed and risk of fatal injury to pedestrians and car occupants found that as the change in velocity resulting from a collision increases above 20 m.p.h., the likelihood of fatalities increases rapidly. For reference, the study found that if a car driving 52 miles per hour made frontal impact with a stationary vehicle (which, due to the conservation of momentum, results in a change in velocity of 26 miles per hour for both vehicles), the likelihood of a resulting fatality is approximately 20%.
A collision like the one Saturday, assuming the driver was traveling at the 85th percentile speed, will kill someone one out of every five times and result in serious injuries 60% of the time - and that's the best scenario. A side impact - such as might occur with a car driving on 119th colliding with another driver pulling out from a side street - at that speed brings a 60% fatality rate and 95% likelihood of serious injuries. Head-on collisions at that speed are certain death.
Saturday's Death Was Preventable
Further analysis of the 119th Street corridor shows the fatal error of expanding the street from Kellogg to Maple - and provides lessons learned for a safer path forward.
The stretch of road where the collision occurred has been constrained in its development the Resthaven Cemetery, Auburn Hills Golf Course, Meadows Park, and West Millbrook Park; nearly 60% of the entire frontage of this corridor is dedicated to these uses and, thus, undevelopable.
North of Maple, however, the land adjacent to 119th had been developed rapidly - outpacing the city's ability to expand the roadway. The cost of acquiring additional right-of-way would make an expansion project unreasonably expensive, so 119th Street has remained a two-lane arterial from Maple north past the city limits. The low density development characteristic of the area has kept traffic manageable, though, with traffic counts ranging from 8,973 to 10,899 vehicles per day.
The nature of the street design on 119th Street north of Maple has contributed to limited speeding. While the 85th percentile speed south of Maple was twelve m.p.h. over the posted speed limit, the average for street segments north of Maple was less than one mile-per-hour over the limit. Virtually no speeding occurs north of Maple for two reasons:
- the narrower, two-lane roadway causes greater feelings of friction with both oncoming cars and vertical elements on the edges of the roadway, such as trees and fences
- a single travel lane in each direction causes blocking - traffic moves at the speed of the slowest vehicle
The safety implications for this reduction in speeding are so tremendous that it bears note: traffic counts on the two-lane and five-lane segments of 119th Street are virtually identical. And traffic flow is hardly encumbered by the reasonable speeds enforced by the two-lane design - a trip along the entire four-mile corridor (assuming you hit all the green lights) would take only 5:53 at 40.75 miles-per-hour, the average 85th percentile speed of the segment north of Maple. Driving the entire segment at 52 miles per hour would take four minutes and thirty-eight seconds.
It's time to have the difficult conversation - is a one-minute-shorter trip a quarter of the way across the entire city of Wichita really worth a life? What about the more than thirty lives we lose each year on our streets?
A Forward Vision
Sadly, for too long we as Wichitans have answered that question with a resounding, selfish "YES". To prevent tragedies like Saturday from occurring again, we need to rethink our priorities for the city's transportation system.
In the second and third parts of this series, we'll lay out a vision for improving traffic safety throughout the city, while shifting our infrastructure investments from prioritizing speed to creating platforms for productivity.