Innovations to boost safety on U.S. roadways may look different depending on the region. In some parts of the southwest United States, factors like climate change, land use, farming practices, and desert environments have combined to create a serious problem—major dust storms that can block visibility for miles and make driving unsafe.

In the desert regions of Arizona, factors like these have increased the severity and number of dust storms, which can emerge as a side effect of thunderstorms. Thunderstorms frequently produce downbursts and straight line winds, which can blow loose sand and dirt from the ground, causing a large wall of dust and debris, or a dust storm, to form. These dust storms, sometimes called “haboobs,” are unpredictable and unexpected and can reduce visibility to near zero in seconds, resulting in deadly multivehicle crashes on roadways. The National Weather Service’s research finds that dust storms are the third deadliest weather event in Arizona. According to its report, 157 people died and 1,324 were injured in 1,521 “dust incidents” on Arizona highways between 1955 and 2011. Since 2000, dust has contributed to 1,207 collisions resulting in 40 fatalities and 1,136 injuries.

In order to address this issue and mitigate the impact of dust storms, the Arizona Department of Transportation (ADOT) developed a state-of-the-art, fully automated dust storm detection and warning system along a critical 10-mile (16-kilometer) segment of I–10 between Tucson and Phoenix—an area prone to sudden blowing dust events. The system uses multiple intelligent transportation system technologies to detect events, warn drivers, and modify driver behavior using a dynamic variable speed limit system and dynamic message signs. The primary objective is to enhance safety and driver awareness during blowing dust events.

This solution is an excellent example of a weather-responsive management strategy (WRMS). WRMS is a Federal Highway Administration initiative focused on deploying methods to mitigate the impact of weather on the transportation system.

“One of the primary benefits of developing WRMS is that those strategies can be used for a myriad of weather events, including dust storms and low-visibility issues, that impact our roadways and public safety,” says David Johnson, the team leader for the FHWA Road Weather Management Program. “ADOT’s dust detection system offers a perfect example of a regional approach to mitigate weather impacts through WRMS.”

The ADOT Dust Warning System 

ADOT installed 13 sensors—12 that measure visibility and 1 that measures both visibility and precipitation—in strategic locations on I–10 from milepost 209 to 219. The instruments are installed 8 feet (2.4 meters) off the ground to record the quality of visibility that drivers are experiencing. Placed about a mile (1.5 kilometers) apart, the spacing gets tighter closer to milepost 214, which is used as the center, or ground zero, for the system. The instruments measure the number of particles in the air and then calculate the visibility in feet.

“We chose these sensors because we have used them in the past; they are reliable and easy to maintain and calibrate,” says David Locher, P.E., ADOT’s manager for Transportation Systems Management and Operations (TSMO) Systems Maintenance.

In addition to the ground sensors, the system also uses an X-band (between 8.0 and 12.0 gigahertz) radar with a 60-mile (97-kilometer) radius. During the project design phase, ADOT discovered that the two major weather radars nearby, one in Phoenix and one in Tucson, did not sufficiently detect the area of concern. The two existing radars could determine high-rising thunderstorms but could not spot dust events closer to the ground. To complement the real-time ground observations the visibility sensors provide during an event, ADOT installed X-band radar near the new intersection of SR–87 and I–10. This system provides advance notice and warning of approaching storms.

The radar, the first and only to be owned by a State DOT, has the capability to rotate 360 degrees, and the current scanning pattern is designed to see both rain and dust. With each rotation, the radar increases one degree in elevation, starting at 0.5 degrees and going up to 5.5 degrees. The change in elevation enables ADOT to create 3-D images of a storm within a 60-mile radius. Leveraging the FHWA Road Weather Management Pathfinder program, ADOT can also send the radar imagery directly to the National Weather Service, where this data could be used in research and development to potentially better forecast sandstorms in the future.

Adjusting Speed Limits for Safety 

The system also contains a variable speed limit component, which includes a series of programmable speed limit signs that can display changes to the legal speed limit from a maximum of 75 miles (120 kilometers) per hour to as low as 35 miles (56 kilometers) per hour. Eight of these units are tightly spaced, around 1,000 feet (300 meters) apart, on the I–10 corridor, four on the eastbound side at milepost 209 and four on the westbound side at milepost 219. Additional variable speed limit signs lie at 2-mile (3-kilometer) intervals approaching the corridor. As the sensors begin to measure a reduction in visibility, the data (visibility measured in feet) are analyzed and, when appropriate, messages are sent to the system to reduce the speed limit in increments.

Locher explains further: “For example, if the visibility sensors record a visibility between 900 and 1,200 feet [270 and 370 meters], we have designed the system to automatically lower the speed limit to 45 miles [72 kilometers] per hour for that degree of visibility. We did not want drivers to see a speed limit sign go from 75 to 45 miles [120 to 72 kilometers] instantly, so we designed the series of four close-spaced signs. The four consecutive signs may [indicate] 65, 65, 65, 65; then 30 seconds later, 65, 55, 55, 55; then another 30 seconds, 65, 55, 45, 45. If we need to reduce to 35, it takes a total of 2 minutes to do the steps.”

The system goes back to its normal status of 75 miles (120 kilometers) per hour in a 5-minute interval. The slower return is intended to make sure the dust event is over.

In addition to warning motorists of conditions using variable speed limit messaging, ADOT installed four new dynamic message signs within the corridor and uses existing ones outside the corridor. The signs, or “boards,” are preprogrammed with messages that automatically change, depending on the sensed conditions, to prepare motorists for the expected conditions they will experience.

ADOT’s Traffic Operations Center can monitor the health of the system and measure the impact it has on driver behavior through additional system components. Embedded loop sensors in the pavement measure and report traffic speed and volume. The data can be used to measure the change in driver behavior and response to the changing speed limit. Six CCTV cameras can be accessed by the center to monitor both the health of the system and dust events.

 The radar provides advance warnings while sensors measure visibility. When the visibility goes down, the system reduces the speed limit and populates the dynamic signs with messages indicating the severity of the dust event.

“All of it can be monitored and managed by the Traffic Operations Center, but almost everything is fully automated, hands free,” says Locher.

Overcoming Design Challenges 

ADOT hired a firm to design the system in 2016 and added it to a widening project that broke ground in December 2017. The system used crash data from 2010 to 2015 to identify the most effective location for deployment. While building the system, ADOT ran into several challenges. Any system like this requires solid communications, and the quality of the available communications at the device locations often wasn’t optimal. Integrating multiple devices from multiple vendors also proved to be demanding, but the project overcame this obstacle with consistent communication and a collaborative approach with the contributors. Finally, designing and implementing the algorithms that determined what speeds the variable signs should be changed to relative to what observation analytics were available was difficult. However, the system design includes flexibility and scalability so that as it grows and evolves, these algorithms and analytics can be modified and become even more effective. The system, including the X-band radar, cost $6 million; it was completed and operational in the summer of 2020.

The Dust Warning System is still operationally new, and the monsoon season following its debut produced fewer than average events, making initial evaluation of its functionality difficult to determine. Even so, the system has garnered attention and accolades, including a nomination for an American Association of State Highway and Transportation Officials Operational Excellence award. In addition, the National Weather Service has named ADOT a 2020 Weather Ready Nation Ambassador of Excellence for the agency’s work on this project and others.

Looking to the Future 

As transportation-related technologies continue to advance, new challenges related to the effects of climate change and other related issues will arise and need to be addressed. For example, the increased deployment of connected and autonomous vehicle technology will require intelligent transportation system solutions to provide real-time, relevant information to equip vehicles to lessen the impact from weather events. Weather-responsive management solutions will continue to be a cornerstone to meeting those challenges. The ADOT Dust Warning System is the perfect combination of proactive, collaborative, and forward-thinking development and deployment of new sustainable technologies, solutions, and strategies that create systemic resiliency.

Tony Coventry is a transportation specialist with FHWA’s Road Weather Management Program. His current focus area is developing climate change impact mitigation and resiliency strategies for the Nation’s surface transportation system.

Kevin Duby is the statewide road weather manager for ADOT, coordinating the road weather information system networks and supporting winter operations. He has been with ADOT since 2002. He holds a master’s degree in emergency management from Arizona State University.

For more information, contact Kevin Duby at kduby@azdot.gov.