Collaboration Across the Road Weather Enterprise: The Pathfinder Project

CHAPTER 1. UNDERSTANDING WEATHER'S IMPACT ON THE NATION'S ROADWAY SYSTEM

THE WEATHER IMPACT

Weather has a significant impact on the operations of the nation's roadway system year-round. Rain reduces pavement friction; winter weather can leave pavements snow/frost covered or icy; fog, smoke, blowing dust, heavy precipitation, and vehicle spray can restrict visibility; flooding, snow accumulation, and wind-blown debris can cause lane obstructions. These weather events translate to changes in traffic conditions, roadway safety, travel reliability, operational effectiveness, and productivity.

Weather affects traffic conditions in a variety of ways. Weather events may prompt travelers to change departure times, cancel trips, choose alternate routes, or select different modes of transport. Slick pavements, low visibility, and lane obstructions lead to driving at lower speeds or with increased following distances. These changes in driver behavior can affect the operation of signalized roadways, where traffic signals are timed for clear, dry conditions, resulting in reduced traffic throughputs, increased delays, and increased travel times.

Weather affects roadway safety by increasing exposure to hazards and crash risk. Weather also affects the operational effectiveness and productivity of traffic management agencies and road maintenance agencies through increased costs and lost time.

It is, therefore, an important responsibility of traffic managers and maintenance personnel to implement operational strategies that optimize system performance under such conditions by mitigating the effects of weather on the roadways. The operational approaches these personnel use dictate their needs for weather and road condition information. Accurate, timely, route- specific weather information allows traffic and maintenance managers to better operate and maintain roads under adverse conditions.

The U.S. Department of Transportation (USDOT) Federal Highway Administration (FHWA) Road Weather Management Program (RWMP) has defined three types of road weather management strategies that can be employed in response to rain, snow, ice, fog, high winds, flooding, tornadoes, hurricanes, and avalanches: advisory, control and treatment.

Advisory Strategies

These strategies provide information on prevailing and predicted conditions as well as impacts to motorists.

A variety of approaches are available to traffic managers to advise travelers of road weather conditions and weather-related travel restrictions (such as road closures resulting from fog, smoke or flooding). Strategies include posting warnings on dynamic message signs (DMS), broadcasting messages via highway advisory radio (HAR), providing road condition reports through interactive traveler information (TI) systems such as websites and 511-phone systems, and Public Information Officer (PIO) interaction with media.

Control Strategies

These strategies alter the state of roadway devices to permit or restrict traffic flow and regulate roadway capacity.

To control traffic flow during adverse weather, traffic managers may regulate lane use (such as lane reversals for evacuations), close hazardous roads and bridges, restrict access on particular roadways to designated vehicle types (e.g., tractor-trailers during high winds), implement variable speed limits (VSL), adjust freeway ramp metering rates, or modify traffic signal timings.

Figure 1. Photo. Plowing is One of Many Treatment Strategies for Adverse Weather.
(Source: United States Department of Transportation)

Treatment Strategies

These strategies apply resources to roadways to minimize or eliminate weather impacts. Maintenance managers use road weather information and decision support tools to assess the nature and magnitude of winter storms, determine the level of staffing required during a weather event, plan and execute proactive and reactive road treatment strategies (e.g., plowing, sanding, chemical applications), and activate anti-icing/deicing systems.

Road Weather Observations

Road weather observations are a critical aspect of any forecast system. They allow for vital understanding of weather impacts and situational awareness of current conditions. Observations also provide a basis for value added applications including nowcasting/forecasting systems and supply a ground truth for assessment of forecast quality. For example, road weather observations can be used to understand when forecasts deviate from reality and allow for an understanding of forecast accuracy. Finally, observation information provides an excellent basis for road maintenance performance measurement.

Did You Know?
Beyond winter weather, maintenance managers are also concerned about the impacts of other events such as sand storms and wildfires that may reduce visibility and create hazardous driving conditions.