4 Roadway Conditions
4.1.3 Bridge De-icing Systems – Wisconsin
Impetus for the Activity
The problem that sparked Wisconsin DOT to consider a technological treatment was slide-off crashes on icy bridges. A technological treatment was chosen over a more conventional treatment due to: the potential for faster response to frost situations; the potential for this new method to be more cost effective in the long run; and the availability of the technology. The expected result of deployment of the system is improved safety.
Criteria for System Selection
Wisconsin DOT has two bridge de-icing systems in place. In both cases, they were approached by the vendor. For the first system, FHWA funding was available, and for the second system, the vendor offered to provide and install the system free of charge.
Description of the System
Both systems are currently installed and active. The first system was installed in fall/winter 1997-1998, and became operational in January 1998. This system is an Odin system. The second system became operational in January 2000. This second system is called “Freeze-Free” and comes from Energy Absorption Systems.
The Odin system is installed on a rural superelevated bridge with a curve and approach and exit ramps as part of the bridge. The Freeze-Free system is installed on a bridge at the bottom of a hill. Both sites have a history of having a lot of slide-off crashes.
The objective of the systems is to prevent crashes and have a faster de-icing response time than conventional methods.
The systems use pavement temperature and precipitation sensors to obtain system inputs. The inputs are measured constantly. System output involves spraying magnesium chloride through 11 nozzles at the first location and 6 nozzles at the second.
System Performance
While no system performance problems have been experienced to date, there has not been much chance to use the systems under weather conditions. Two factors that could affect system performance are the accuracy/calibration of the activation system and the location of the precipitation sensor. This sensor is located on the roof of the chemical storage building, and the fear is that the sensor might activate the system for dry blowing snow, which is undesirable.
It was noted that both systems are considered to be still at the test and evaluation stage. The Freeze-Free system was only recently installed during a very mild winter, and the Odin system has not been operational when needed due to changes being made to the system.
A procedure is in place to formally evaluate the performance of the systems. An evaluation form is to be filled out each time the systems are activated. These forms rely on visual observations and collect information on the functioning of the communications link and hydraulics and driver response. It is noted that they will need to have the systems functioning for a whole winter without changes to have an acceptable evaluation.
System Effectiveness Evaluation
A formal comparison evaluation between the effectiveness of the automatic de-icing system and conventional treatments is planned. A nearby conventionally treated bridge will be used for comparison.
Perceived “Hot Spots” for Rural ITS Deployment
A few areas were identified as “Hot Spots” for rural ITS deployment aimed at improving safety in rural areas. They included:
- All bridge decks subject to freezing,
- Areas of shaded pavement that never get solar thermal warming, and
- Sharp curves subject to icing.
Obstacles and Lessons Learned
It was noted that this technology is very new and that the vendors are still working to get the systems to operate properly. It was also noted that there is work to be done on the self-activation system so that it activates on time, but does not over-activate.
There is much interest in deploying these systems on a wider basis, but there is a concern that the systems are still at the research/test and evaluation stage. It is also felt that the benefit/cost ratio needs to be evaluated before widespread deployment happens.