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|Federal Highway Administration > Publications > Public Roads > Vol. 68 · No. 3 > Another Rain Delay|
Publication Number: FHWA-HRT-05-002
Another Rain Delay
by Paul Pisano and Lynette Goodwin
Roadway managers are tackling the problem of weather-induced congestion head-on.
Sandy wakes up to a wet morning, looks out the window at the light rain, and immediately knows that her drive to work will be miserable. A drive that normally takes her 30 minutes on a dry day will take longer—and it is anyone's guess how much longer—because traffic just cannot seem to handle the conditions. She knows she will see more fender-benders on the side of the road and will keep her fingers crossed that she does not get rea rended by a driver following too closely behind her and not accommodating for the poor driving conditions. If she hurries, she can leave a little early, but a quick check on the morning news reveals a morning commute that already is going from bad to worse. "It's just a light rain," Sandy says to herself, "why does it have to be such a mess?"
The reason for Sandy's frustration and why her commute will take longer is actually due to a number of complex and interrelated factors, including the weather, roadway environment, driver behavior, and technology. Although managers cannot change the weather, they can control the impact of weather on roadways, with varying degrees of success. Indeed, traffic managers are making great strides toward solving the problem of weather-induced congestion.
Rather than being reactionary—waiting for weather-induced congestion or crashes to occur before taking action—many transportation managers across the country have adopted proactive management practices to improve the safety of the highway system. In other words, these agencies have embraced the principle that weather events are nonrecurring incidents that can be predicted, observed, and mitigated.
How Weather Affects Roads
To understand the complex interactions between weather and roads, it is first necessary to consider the impacts that various types of weather have on the roadways, and the ways in which these impacts affect both traffic flow and related operations. Roadway managers typically divide weather impacts on roadways into four categories:
The first three impacts lie within the realm of maintenance and operational strategies, but infrastructure damage typically requires a significant reconstruction response.
Loss of friction, restricted visibility, and lane obstruction affect traffic flow in a variety of ways, causing a reduction in speed, an increase in speed variance, and a reduction in roadway capacity. Furthermore, these impacts also affect other operational aspects of overall system performance. Vehicle performance (traction), for example, affects the capability and behavior of the driver. Changes in travel demand may result from people deferring trips and changing departure times. On one hand, transit riders may opt to drive to avoid getting wet en route to the bus or train station, whereas snow may induce other drivers to take rail to avoid the hazardous driving conditions. And control devices such as traffic signals, which were designed for clear, dry conditions, may perform at suboptimal levels. As a result, weather not only hampers the performance of overall system operations, but it also jeopardizes safety and increases the risk of crashes.
What this means for motorists is that the trip from Point A to Point B is going to take longer. How much longer depends on a number of factors, especially traffic volume, weather intensity (including the length and severity of the event), and the amount of dirt and oil on the roads that has accumulated since the previous weather event.
When expressed in terms of statistics, the magnitude of the impact of weather on traffic flow becomes apparent: speeds may drop by 10 percent for light rain and 16 to 40 percent for heavy rain or snow; capacity can decrease by 11 to 19 percent; and delays can increase by 11 to 50 percent. Clearly these impacts are large enough to warrant action—and solutions exist to manage or reduce the impact of these factors.
To achieve performance-driven, 21st-century highway operations, roadway managers are learning to manage the system under all conditions, including adverse weather. The following success stories from North Carolina, New Jersey, Minnesota, and California represent just a sampling of the many strategies that road managers have at their disposal, including weather-responsive traffic control devices, anti-icing techniques, and pretrip and en route road weather information.
Weather-Related Signal Timing, Charlotte, NC
In North Carolina, the Charlotte Department of Transportation (DOT) manages 615 traffic signals with a computerized control system. In the central business district, the city uses weather-related signal timing plans at 149 signals to reduce traffic speeds during severe weather. Signal timing also can be employed at more than 350 intersections controlled by closed-loop systems.
System Components: The traffic signal control system comprises signal controllers located at city intersections, a closed-circuit television (CCTV) surveillance system, twisted-pair cable and fiber-optic cable communication systems, and a signal system control computer in the traffic operations center. Images from more than 25 CCTV cameras on major arterial routes are transmitted to the operations center and displayed on video monitors. Traffic managers can select and download various timing plan patterns (stored in the computer) to field controllers via the communication systems.
System Operations: System operators assess traffic and weather conditions by observing CCTV video images and reviewing weather forecasts. Forecast data are available through radio and television broadcasts, the Web site for the National Oceanic and Atmospheric Administration's National Weather Service, and a private weather service vendor. When operators observe heavy rain, snow, or icy conditions, they access the signal computer and manually implement weather-related timtiming plans. To slow the speed of traffic, these signal timing plans increase the cycle length—which is typically 90 seconds—while offsets and splits remain the same. During offpeak periods operators also may select peak-period timing patterns, which are designed for lower traffic speeds.
After initiating the weather-related signal timing plans, operators monitor traffic flow on the roadways. If warranted by field conditions, operators can increase cycle lengths to further reduce traffic speeds. When weather conditions return to normal, operators access the central computer to restore normal time-of-day and day-of-week timing plans.
Transportation Outcome: When weather-related signal timing is engaged, travel speeds decrease by 8 to 16 kilometers per hour (5 to 10 miles per hour). By selecting signal timing plans based on prevailing weather conditions, traffic managers improve roadway safety by reducing speeds and minimizing the probability and severity of crashes.
Speed Management, New Jersey
While retiming traffic signals can help reduce speeds in an urban center, an advanced traffic management system (ATMS), coupled with variable message signs, is a common tool for monitoring road weather and adjusting speed limits accordingly on highways and interstates.
The New Jersey Turnpike Authority operates an ATMS to control 237.9 kilometers (148 miles) of the turnpike—one of the Nation's most heavily traveled freeways. Various subsystems monitor road and weather conditions, manage traffic speeds, and notify motorists of hazardous conditions. Speed management and traveler information techniques are helping the turnpike authority improve roadway safety in the presence of fog, snow, and ice.
System Components: ATMS control computers are located at the turnpike traffic operations center in New Brunswick, NJ. A wireless communication system using Cellular Digital Packet Data technology facilitates data transmission between field components and the central control systems. A vehicle detection subsystem, composed of inductive loop detectors and remote processing units, collects speed and volume data and detects traffic congestion. A CCTV subsystem enables operators to verify road conditions visually.
The turnpike's road weather information system includes 30 environmental sensor stations deployed along the turnpike to gather data. Nine stations detect wind speed and direction, precipitation type and rate, barometric pressure, air temperature and humidity, and visibility distance. Pavement temperature and condition data are collected at 11 sites, while 10 other stations simply monitor visibility distance.
The turnpike authority conveys traveler information to motorists through 113 dynamic message signs, 12 highway advisory radio transmitters, and a variable speed limit (VSL) subsystem. More than 120 VSL sign assemblies are positioned along the freeway at 3.2-kilometer (2-mile) intervals. Sign assemblies include speed warning signs, which display messages like "REDUCE SPEED AHEAD" and the reason for speed reductions, noting "FOG," "SNOW," or "ICE."
System Operations: Traffic and emergency management personnel in the traffic operations center monitor environmental data to determine when to reduce the speed limit. When reductions are warranted, sign assemblies are manually activated to decrease speed limits in 5-miles-perhour, mi/h (8-kilometers-per-hour, km/h) increments from 50, 55, or 65 mi/h (80.4, 88.4, or 104.5 km/h) to 30 mi/h (48.2 km/h), depending on prevailing conditions. System operators also may disseminate regulatory and warning messages via message signs and highway advisory radio. State police officers enforce the lower speed limits by issuing summonses to drivers exceeding the posted limit. When the vehicle detection and road weather information subsystems indicate that traffic and weather conditions have returned to normal, the original speed limits are restored.
"Effective coordination between the operating agency and State police is critical," says Solomon Caviness, assistant traffic engineer with the New Jersey Turnpike Authority. "To ensure credibility, the agency and police department should optimize communication to provide motorists with up-to-the-minute, clear, and concise information and guidance on appropriate speeds during adverse weather conditions."
Transportation Outcome: According to Caviness, speed management and dissemination of traveler information improve safety by reducing the frequency and severity of weather-related crashes.
Anti-Icing/Deicing System, Minnesota DOT
Other technologies that can help improve the safety of roadways during inclement weather are antiicing and deicing systems. Several Minnesota DOT districts installed fixed maintenance systems on curved and super-elevated bridges that are prone to slippery pavement conditions. On Interstate 35, the department installed an automated anti-icing system on a 594-meter (1,950-foot), eightlane bridge near downtown Minneapolis. The bridge deck was susceptible to freezing due to moisture rising from the Mississippi River below. On average, 25 crashes occurred on the bridge each winter, causing significant traffic congestion.
System Components: The automated anti-icing system includes storage tanks, a pump and delivery system, environmental sensors, four motorist warning signs with flashing beacons, and a control computer located in the district office. An enclosure houses the pump, an 11,734-liter (3,100-gallon) chemical storage tank, a 379-liter (100-gallon) water storage tank, and control mechanisms. Liquid potassium acetate is pumped through the delivery system to 38 valve bodies installed in the median barrier. The valves direct the anti-icing chemical to 76 spray nozzles. Sixty-eight nozzles are embedded in the bridge decks of both northbound and southbound lanes. The nozzles are installed in the center of travel lanes at a spacing of 16.8 meters (55 feet). Eight barrier-mounted nozzles are located at the north end of the bridge to spray approach and exit panels.
Minnesota DOT installed two types of environmental sensor stations on the bridge. The first is equipped with air and subsurface temperature sensors, pavement temperature and condition sensors, and precipitation type and intensity sensors. The second sensor station includes only pavement temperature and condition sensors. The environmental sensors determine whether the pavement is wet or dry and whether the pavement temperature is low enough for surface moisture to freeze.
System Operations: The control computer continuously polls the environmental sensors to gather data used to predict or detect the presence of black ice or snow. When predetermined threshold values are met, the computer automatically activates flashing beacons on bridge approach ramps to alert motorists. The computer also checks the chemical delivery system for leaks and initiates one of 13 spray programs. Each program activates different valves in various spray sequences and at different frequencies based upon prevailing environmental conditions. An average spray cycle dispenses 128.7 liters (34 gallons) of potassium acetate (that is, 45.4 liters or 12 gallons per lane mile) over 10 minutes. Conventional treatment strategies (like plowing, sanding, and salting) supplement automated anti-icing when slush or snow accumulates on the bridge deck.
At the end of each winter season, Minnesota DOT staff inspects the anti-icing system and reconfigures it to spray water instead of potassium acetate. Over the summer, the system is manually activated on a monthly basis to ensure proper operation of the pump and delivery. Department staff reinspects the system in the fall before reconfiguring it for winter operations.
Transportation Outcome: In the first year of operation, the automated anti-icing treatment significantly improved roadway safety, achieving a 68-percent decline in winter crashes. Mobility improved as well because fewer crashes translated into reduced traffic congestion. Installing the bridge antiicing system also improved productivity by lowering material costs and enhancing winter maintenance operations throughout the district.
"We've been very satisfied with how well the anti-icing system works," says Christine Beckwith, maintenance research engineer at Minnesota DOT. "Following the successful installation on I-35 West in Minneapolis, we've moved out of the research phase and have accepted anti-icing systems as a technology that we will use again in the future. In fact, we're installing a new system on I-35 East in St. Paul in the fall of 2004."
To view the department's final report, visit www.dot.state.mn.us/metro/maintenance/Anti-icing%20evaluation.pdf.
Flood Warning System, Palo Alto, CA
In February 1998, several days of heavy rainfall caused the San Francisquito Creek to overflow its banks, flooding the city of Palo Alto, CA. Because residents and emergency managers were caught off guard, the event prompted the city to develop a flood warning system. Later that year, Palo Alto launched a Web-based warning system that has become an integral part of the city's emergency management operations system. When flood conditions exist, emergency managers use automated surveillance techniques to inform the public.
System Components: Water level sensors, a rain gauge, flood basin detectors, tide monitors, and a CCTV camera help road managers assess field conditions. The city installed ultrasonic sensors at five bridge locations to detect high water or flood conditions. The sensors use acoustics or sound waves to measure the distance from a transducer to the water surface. Water level readings are transmitted to the water, gas, and storm drain Supervisory Control and Data Acquisition (SCADA) system via the city's telephone and radio communication networks. A digital subscriber line transmits still video images from one bridge site to the emergency operations center.
System Operations: Real-time and historical water level data and video images are posted on the city's "Creek Level Monitor" Web site, where road managers and residents alike can easily access the information. Current water levels, 12-hour water level trends, 24-hour rainfall, annual rainfall, current temperatures, and tidal data are updated every minute on the SCADA system and posted on the server for Web site updates every 3 minutes.
Emergency managers access this information to plan response actions and to alert residents. In the event of a flood threat, an automatic telephone warning system at the emergency operations center dials all city residents and businesses in threatened areas to advise them of potential flood conditions.
Transportation Outcome: Prior to installing the flood warning system, emergency management personnel had to travel to bridge locations to monitor the storm drain system visually and check water levels by hand. Drain system status and water level readings were radioed to the emergency operations center every 20 minutes. By eliminating the need for field measurements, the monitoring system enhances the productivity of city staff and provides timely access to traveler information to improve public safety.
Says John Ballard, supervisor of public works with the city of Palo Alto: "The success of the program is incredible. This is the Number 1 looked-at link on the Web site for the city of Palo Alto in the winter. We literally get thousands of hits."
The flood monitoring system is so popular with the community, Ballard says his team continuously updates and improves the Web site and monitoring equipment. "With interest from engineers at nearby Stanford University as well as public school students and teachers, we have made dozens of modifications to improve the system, like adding rain gauges, expanding the scale of the trends showing water depth, and applying more user-friendly colors and icons on the Web site."
These examples, along with research conducted by FHWA (such as a report Test and Evaluation Project No. 28: Anti-icing Technology, Field Evaluation Report, FHWA-RD-97- 132), show that the benefits accrued from road weather solutions are well documented and often outweigh the costs.
Despite these and other advances in managing roads during inclement weather, much remains to be done. Today's road weather information systems have demonstrated the ability to provide relevant information, but coverage is haphazard in terms of the overall highway network. Many state-of-the-practice solutions used today tend to be spot-specific, typically focusing on repeatable weather problems.
To achieve 21st-century operations, however, requires a more holistic, systems approach. That means reaching beyond the case-by-case problems to observe and predict traffic over the entire network and under all weather conditions. To do so requires timely, accurate, and relevant information about weather and road conditions.
Several efforts are underway to build the systems that meet these information needs. Likewise, institutional achievements are underway, such as building a working relationship between the transportation and weather communities. As the technologies and the relationships mature, these systems will become ubiquitous and serve as the basis for a host of tailored products and services that improve the operation of the Nation's roadways.
FHWA is working actively with State partners and other Federal agencies, such as the National Oceanic and Atmospheric Administration, to advance the practice and state of the art of road weather information systems. Empirical traffic flow studies, for example, will help quantify the impacts of inclement weather on the transportation network and feed new weather-responsive traffic algorithms. This work also will support FHWA's Next Generation Simulation Program, which is developing behavioral algorithms that will improve the quality and performance of simulation tools, providing superior accuracy for nonideal conditions.
Also underway is a study to integrate weather information into traffic management centers, which will document the different types of road weather information received by the centers, including how they obtain their information and how operational responses change depending on the type and severity of various weather events. The Missouri DOT is creating a prototype weather response system to demonstrate decision- support tools that will be tailored for different types of users, including traffic managers, transit agencies, maintenance supervisors, and law enforcement agencies. Another tool, the Maintenance Decision Support System prototype provides winter maintenance managers with recommendations for road treatments.
Finally, because accurate forecasts at the earth's surface are critically important to managing adverse weather impacts on roads, FHWA recently defined a new initiative called the Nationwide Surface Transportation Weather Observing and Forecasting System. The objective is to design, demonstrate, and deploy an integrated observational network and data management system for road weather. The system will combine observations from fixed sensors on the roadside, in vehicles, and in remote locations such as satellites. The data will be assimilated and Signs like this one notify motorists in Minneapolis, MN, when anti-icing activities are in progress on local bridges. made available to public and private providers of weather information, who will then process the data to create forecasts appropriate for road users. The 5-year effort will begin by demonstrating the system's capabilities on a regional level and then refine the solutions for eventual deployment nationwide.
Sandy wakes up to a wet morning, looks out the window at the light rain, and immediately knows that her drive to work will be just like any other morning, taking her about 30 minutes. Through her home computer, which is linked to the city's road weather information system, she can access updates on weather conditions and drive times.
On her way to work, she will see fewer crashes because drivers effectively staggered their trips and are driving in a more uniform manner, which keeps traffic moving smoothly. Dynamic message signs provide appropriate speed limits and travel times, and the traffic signals automatically account for the changes in driver behavior under the adverse conditions. If by chance she encounters another vehicle encroaching on an intersection, then she also knows that her vehicle-based control system will provide the appropriate alerts to help avoid a collision. "Sure, this rain is a mess," Sandy says to herself, "but at least I know my commute will not suffer for it."
Paul Pisano is the team leader for the Road Weather Management Team in the Office of Transportation Operations at FHWA. Pisano has worked in several offices at FHWA over the past 19 years, and in his current capacity, he is responsible for the program that addresses the impacts of weather on all aspects of the highway system, including winter maintenance, traffic management, and traveler information.
Lynette Goodwin is a lead transportation engineer in the Intelligent Transportation Systems Division of Mitretek Systems. She currently supports the FHWA Road Weather Management Program. Goodwin holds a bachelor's degree in civil engineering from Howard University and a master's degree in engineering management from The George Washington University.
For more information, visit http://ops.fhwa.dot.gov/weather/index.asp or contact Paul Pisano at 202-366-1301 or email@example.com.
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