In fall 2000, the Colorado DOT widened a stretch of mountainous highway from two to four lanes to better accommodate hordes of eager skiers traveling to and from Interstate 70 and the City of Aspen. The expansion did the trick — as long as pavement temperatures stayed below freezing.
When they rose above that, emergency crews responded to crashes along 3-1/2 miles through Snowmass Canyon, a narrow valley in the heart of the Rocky Mountains, much more often. On site, they noticed that drivers entering the accident zone had trouble slowing down. The situation was puzzling because accidents increased during nice weather.
DOT engineers and maintenance employees were called to investigate.
A black ice situation
Mountainous regions often necessitate building two roadways — one virtually on top of the other — along a steep grade. Also, geologically unstable slopes required the department to use a terraced system of retaining walls to minimize environmental impacts.
There are several blind curves with the new concrete barriers along one side of the accident-prone stretch. When crews plow along the barriers, melting snow drains from the super-elevated curves across the road. After 2 p.m., the road becomes shaded and pavement temperatures fall below freezing, creating black ice.
On bad days, people drove cautiously and crashes were less frequent. But on nice days, when the pavement just before the problem area was safe and dry, drivers entered the curves at the prevailing rate of speed (50 mph). Or more.
The road drains properly and no ponding occurs on the surface, but the department can’t push snow back any further because of concrete barriers. The solution was to warn drivers about pavement conditions before entering the dangerous section.
Buried telephone wasn’t available, so in 2011 the department bought a Skyline Products Inc. variable message sign (VMS) and invested $75,000 in a Vaisala road weather information system, consisting of:
- Remote Processor Unit
- Non-Intrusive Surface State Sensor
(Vaisala Model DSC111)
- Non-Intrusive Surface Temperature Sensor
(Vaisala Model DST111)
- Air Temperature/Relative Humidity Sensor
- Precipitation Identifier and Visibility Sensor
- Ultrasonic Wind Sensor
- Pan-Tilt-Zoom Color Camera
- Relay Device Control
- Wireless Device Control
“We wanted something that gave us and the approaching vehicle real-time information about the curve,” says CDOT Region 3 Project Manager Mike Curtis, PE. His team selected sensors that use lasers and infrared technologies to detect and measure pavement information from the side of the road. These “non-intrusive” sensors are easier and safer to maintain and, thus, less expensive in the long run than sensors embedded in the road surface.
The system is configured to calculate friction, or how much traction the pavement supplies, based on how wet the surface is at any given time and temperature. The sensors pick up and transmit this information to two destinations:
- DOT computers via cellular modem as part of a statewide system for winter response decision-making; supervisors dispatch crews only to icy areas
- A weather station that crunches the data and, depending on which predetermined thresholds it detects, sends a signal via spread spectrum radio to the VMS upstream. The sign then displays, in real-time, either “Wet Road Ahead” or “Icy Road Ahead.”
“An automated solution is nice because it’s quicker to activate and there are fewer chances for human error, like a manual sign,” Curtis says. “The maintenance folks believe the weather station and sensors are depicting the road condition at that location.”
Jon Tartleton is the roads marketing manager for Vaisala. E-mail firstname.lastname@example.org. Visit www.vaisala.com.