Sensor technology of road ice information systems

Photo-credit: LianeM – Fotolia.com

“We expect the unexpected”, the motto of a hotel in the US where I stayed recently.

Our meteorological winter applications follow a similar principle. Will it come or not? Early or late? Does the old farmer’s wisdom still apply? Will the cost of a ton of salt again rise to a record of over €300 because the need was underestimated and the price increases dramatically when demand is strong?

We’re now in the 4th quarter of 2012 and there’s already frost in early October in our local microclimate, the first snow is expected in Moscow any day now, and new road ice information systems are being hectically installed along the eastern seaboard of the US.

In central Europe, we’ve come to expect good driving conditions on roads in just about any weather conditions. Postponing events for winter storms is done only in dire situations. The high expectations we have of public winter services requires know real-time traffic weather conditions not only on highways, but also in cities and on main roads. This data is then used to make reliable short-term forecasts (now casting) for all highway stretches. This is the pre-requisite for planning preventive winter road maintenance to avoid slippery conditions caused by frost, snow, and ice.


» SWIS

Measured data from all sixteen German states from more than 1,200 traffic weather stations, found predominantly on highways, is transmitted in a uniform (TLS-) format to roads offices around the nation. The data centers exchange data with the German Weather Service in Offenbach, which determines the forecast for the specific locations and reports it back to the states. Each state has a data center (MDSS = Management Decision Support System), which maps the respective organization and triggers alarms and actions.

In addition, there are many independent monitoring networks with warning systems at the local level not linked to the SWIS System.


» SWIS External Data Collection Technology

In the past, meteorological sensor technology to measure wind, air temperature, humidity, calculated dew point temperature, precipitation type and intensity, and visibility was found only in sensors built into the highway surface, but recent years have seen the introduction of a new technology which works without actually touching the surfaces. The similarities and differences will be briefly discussed below.


» Built-In Sensors

Whether passive or active sensor technology or all-in-one (active and passive combined), the sensors are usually relatively compact, with a diameter of about 10-12 cm and an installation depth of about 5 cm. In Germany, it is recommended this sensor technology be installed in the center of the fast lane where wintery road conditions remain critical the longest. The installed sensors measure from the bottom up.

Passive sensors can take care of all measuring tasks in one unit, with several sensor elements installed in the open facing the road to be evaluated by a smart sensor microprocessor.

Typical results include the road surface temperature, one or more readings of temperatures below the surface,  water film level, road conditions, and the temperature where the road surface will freeze. Passive sensors use the salt content in the surface precipitation to calculate the current freezing temperature. If different salt mixtures are used (NaCl, MgCl, CaCl), the use of active road sensors is recommended. This applies also to liquid de-icing agents at airports, which frequently work without salt components.

Active sensors are heated and cooled with a Peltier element. Every twenty minutes, the sensor determines the freezing temperature and repeats this cycle as long as the surface road conditions are not dry. Since the active sensor changes the surface conditions with this procedure, the other measuring tasks (temperature, water film) must be performed by a second sensor removed at some distance from the active sensor.
In both cases, the freezing temperature cannot be determined if the road is dry.

Both active and passive procedures are often used in combination today.


» Contact-Free Sensors

When a built-in sensor on the highway becomes inoperable during the winter period, replacing it is time consuming and expensive. Granted, sensor technology can be easily exchanged nowadays without having to dismantle the entire unit, but the highway would still have to be temporarily closed to accomplish this.

This has led to a global trend to avoid building sensors into road surfaces, not only for collecting data about weather conditions, but also about vehicular traffic, which is frequently done with contact-free radar technology these days.

The non-intrusive sensors for road temperature and road conditions operate on the optical/spectroscopic principle. In contrast to the bottom-up measuring process of built-in sensors, these sensors work top-down from an average distance of 6-15 meters above the road.

These sensors “see” the road conditions like a camera or a scanner; in winter, there is often a mix of water, snow and ice particles on the road. The upper layer is always the critical one because that is the one where vehicles drive. This condition is not only shown as dry/moist/wet/ice/snow/critically wet. Contact-free sensors rate the “grip” of the surface on a scale of 0 to 1. Values above 0.8 show perfect grip, values below 0.6 indicate increasingly critical grip. Because of their high levels of consistent results (sensors in the same position calculate the same grip value), these could be used similar to a traffic light in winter applications of the future. Green for a grip of > 0.x, red for a grip of < 0.y and yellow in-between. Yellow can be interpreted as a start signal for preventive road treatment.

To replace the functions of the built-in sensors in full, the contact-free sensors also calculate surface freezing temperatures. As soon as the first ice particle is measured (which is not yet a critical situation), the freezing temperature is reported. This allows the instructions previously used to continue to be used with the contact-free sensors and the grip provides important additional information for making time-critical decisions.


» Tests (BAST – Federal Highway Research Institute in Bergisch Gladbach)

The function of Stiftung Warentest in traffic technology is made possible by BAST in Bergisch Gladbach. BAST tests, compares and helps make technologies more suitable for their application and improve their technical aspects.

In collaboration with the Technical University of Munich, a testing field for meteorological traffic sensor technology (moisture, visibility, temperature, etc.) has been in operation for the past few years on highway A92 near Munich Airport. In addition, BAST is currently building another testing field to compare and test traffic weather sensor technology. The focus is on checking suitability for winter road maintenance applications.

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