Data help Scottsdale Transportation improve driving safety

Allstate recently published a reportlinks to external site ranking Scottsdale 15th in the nation for safest drivers. The report compares collision frequency based on Allstate claims data for the 200 largest U.S. cities. 

Since 1990 the median collision rate for intersections in Scottsdale has held steady at approximately 0.5 collisions per million vehicles, or, in other words, only one out of every million vehicles traveling through Scottsdale intersections is in an accident.

While driver caution and attentiveness play a big part in road safety, so do safe driving conditions created through proper road design based on data. Since 1986 the Transportation Department has collected collision and traffic volume data for all major intersections and road segments throughout the city. They use the data to identify locations with the highest rates of collisions. For the top 20 to 25, they thoroughly investigate the police reports of each collision to look for trends and causes. 

Increased traffic in an area may seem a likely culprit for increased collisions, but sometimes it might be a change in technology, such as increased use of cellular and electronic devices while driving. Even decreased traffic or increased pavement width in an area can lead to more accidents if it enables people to drive faster than the speed limit.

Data also help prevent overreaction to any given accident or to slight deviations from the norm. In 2018, for example, there were 19 fatal collisions in Scottsdale, which was a relatively high number. Analysis of the accidents individually and as a group, however, determined that they weren’t part of any larger trends. The accidents occurred throughout the city and had a variety of causes. While some had common causes, such as speeding, failing to yield the right-of-way, crossing the centerline or making improper turns, others included more atypical causes, including medical events unrelated to driving and pedestrians crossing the street where prohibited. (For more information on fatal accidents in 2018, please see the presentation to the Transportation Commission from March 21, item 6, 2:00:00.

 

Data-backed traffic modifications

When the Transportation Department determines a specific reason or trend for less-safe driving conditions, they identify potential engineering or infrastructure solutions, also backed by data. 

Following are data-backed transportation infrastructure and systems that help create safer driving conditions in Scottsdale:

Roundabouts and traffic circles

Because all traffic must move around a raised median, roundabouts eliminate head-on and right-angle (T-bone) collisions, typically the most serious accidents. Roundabouts are also designed to encourage traffic to travel 15 to 25 miles per hour. Studies have shown that roundabouts reduce collisions by approximately 30 to 50 percent, injuries by 60 to 70 percent and fatalities between 80 and 90 percent when compared to traditional traffic signals. Scottsdale currently has three multilane roundabouts and 21 single-lane roundabouts. To learn more about Scottsdale’s roundabouts, including tips for navigating them, visit ScottsdaleAZ.gov, and search “roundabouts.”

 

Photo Enforcement

Photo enforcement helps reduce speeding, the most common factor in both minor and serious collisions. Multiple studies have determined that photo enforcement reduces both the number and severity of accidents not only at locations where it’s deployed, but also at nearby locations. 

Raised Landscape Medians

One of the most common causes of serious collisions is a driver drifting across the center line into oncoming traffic. Raised medians prevent that. The Scottsdale standard for major streets includes raised medians, and several current transportation projects include implementing them. To learn more, visit Scottsdaleaz.gov, search “city construction projects,” and click on the transportation tab.

Lagging left-turn arrows

Unlike most other nearby cities, most of Scottsdale’s intersections have left-turn arrows that follow rather than precede green through lights. Data show that cities with lagging left-turn arrows have lower rates of all intersection collisions and of left-turn collisions specifically. Drivers are less likely to squeeze through a short gap in traffic when they know they will soon get a left-turn arrow. It’s also easier for cross traffic to see a car finishing a left turn than it is for drivers to judge whether a car approaching from the opposite direction is going to go or stop as a light turns red.

 

Couplets

Data show that vehicles next to each other traveling at different speeds cause collisions. Scottsdale implemented its couplet--Drinkwater and Goldwater boulevards--to route traffic trying to quickly get through town around the lower speed limits and the many pedestrians, cyclists, cars and traffic lights in Old Town.

 

Quick, responsive safety solutions

Sometimes solutions may require only minor modifications to a street or intersection, such as adding or extending a right- or left-turn lane. For many cities this requires identifying and funding the modification through a capital improvement program, often a long process bound by an annual cycle. The Scottsdale Transportation Department, however, maintains a fund of approximately $800,000 annually that can be used for alterations that cost less than $250,000. 

Continued improvement

The Transportation Department is continually trying to improve the process of collecting, analyzing and responding to data. The city recently funded a collision statistics analysis methodology specific to Scottsdale that can identify locations where the actual number of collisions is more than expected. The statistical model will also be able to predict whether recommended improvements will likely reduce collisions.  

While the number of accidents in Scottsdale may be relatively low, every accident is important and consequential for those involved. The Transportation Department continues to improve its methods for collecting and using data to find ways to make driving in Scottsdale even safer.  

bloomberg philanthropies announces scottsdale as What Works Cities 2019 silver certification

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