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How to increase the safety of bridges in road traffic

Every day, numerous vehicles cross bridges to reach their destination. But how robust are the bridges in our road network? Speed limits are often introduced as a preventive measure to protect the infrastructure. Jenoptik develops systems for speed enforcement, which are used to check speed limits on bridges. This helps to increase the safety of the structures. Two examples from Germany illustrate the advantages of this approach.

, Sabine Elbers

Whether short or long, with beam, arch or suspension – bridges are a key component of our infrastructure and represent outstanding architecture in many places. Bridges offer us a very practical way to cross valleys and rivers and thus often shorten journeys.

According to the Federal Ministry of Transport (BMVI), there are almost 40,000 bridges in the network of federal highways in Germany alone. This corresponds to a length of more than 2,100 kilometers, which are managed by state-owned companies or road and traffic authorities. It is important to protect the structure of every one of these road sections in good time to ensure repairs can be kept to a minimum, to maintain the safety and stability of the bridges, and to guarantee traffic safety.

The majority of the existing bridges in Germany were built between 1960 and 1985, meaning most of them are well over thirty years old. Damage to bridges is often caused by problems with the material or environmental factors.

Moreover, the number and type of vehicles crossing the bridges place a strain on the structure over the long term. Particularly the speed and weight of the respective vehicles is decisive for the strain on a bridge. Vibrations are caused by speeds that are too high, which can lead to bridges needing to be repaired more quickly.

Not only has there been a significant increase in truck traffic in recent years, the permissible total weight of trucks has also been raised several times. This is because road transportation of goods is increasing steadily, along with the weight to be transported. Cars are also becoming increasingly heavy. This likewise has an influence on the condition of bridges.

Did you know that just one single truck causes a similar amount of wear to a bridge as around 40,000 to 50,000 cars?

Periodic inspections of bridge structures are designed to ensure that construction and load-related abnormalities can be detected at an early stage. Immediate repair or renewal is required from a (quality) condition grade of 3.5, in accordance with the relevant directive. Moreover, measures to repair damage, warning signs to maintain traffic safety, and restrictions on use are also required.

The aim of state and local authorities is, therefore, to keep harmful vibrations and congestion to a minimum to ensure that repairs, reconstruction and road closures can also be kept to a minimum. Consequently, local authorities often decide to introduce measures such as speed limits or driving bans, which need to be constantly monitored to ensure compliance.

Introduction of speed limits – preventive easing of the load on the structure of the bridge increases safety

Speed enforcement is generally used to monitor speed limits at accident black spots. Moreover, speed monitoring systems can also be used to protect constructions such as bridges. How does this work?

Among other things, the measuring systems also calculate the total weight of passing vehicles. So-called piezo sensors help to classify vehicles and determine their weight. Traffic monitoring systems have been further developed over the past decades so that they are now able to differentiate between cars and trucks and thus also take into consideration different speed limits. Based on piezo sensor technology, violations of a truck driving ban are detected by classification and processed for further prosecution.

Piezo measurement is understood to be the physical effect in which an electrical voltage is generated by the application of force. Piezo sensors convert mechanical energy into electrical energy during this process. These sensors are embedded in the road surface, where they are supplied with power. If a vehicle drives over the piezo sensors within a defined measuring range, the pressure between the cables of the sensors changes, initiating the measurement process. The system calculates the speed of a vehicle based on the distances between the sensors according to the distance-time law.

At the same time, the system identifies whether the vehicle is a car or a truck. Indicators of the type of vehicle are, for example, pressure or the time spent on the sensor. If a vehicle exceeds the permitted speed limit, a photo flash is triggered. Jenoptik uses piezo technology in the TraffiStar S330 measuring system, which enables precise measurement results to be achieved.

How does the measuring system know which speed limit applies? The speed enforcement system is linked via an interface to variable message sign systems (VMS), which display changing permissible maximum speeds – and can also adjust them dynamically. The camera and flash are triggered if a vehicle exceeds the permitted speed limit.

Variable message signs are designed to control streams of traffic to guarantee smooth and safe traffic flow at all times. The individual determination of maximum speed is an essential component of controlling the flow of traffic. It is essential that the maximum permitted speed is observed.

The type-approval certificate issued by PTB (Physikalisch-Technische Bundesanstalt, Federal Metrology Institute) Braunschweig for the TraffiStar S330 stationary speed enforcement system, which is linked to variable traffic signs, was the prerequisite for its unrestricted use also on road sections with dynamic traffic signs.

Best practice: The introduction of speed limits on German bridges helps to increase the safety of structures

The Leverkusen Bridge (A1) in North Rhine-Westphalia is considered an important route to cross the Rhine. More than 120,000 vehicles use this hub every day. It was originally designed for up to 40,000 vehicles, which is why the bridge has been reinforced and repaired several times over the years. A speed limit was introduced in order to preserve the benefits of these repairs for as long as possible.

Several TraffiStar S330 systems have thus been enforcing speeds on the Leverkusen Bridge since 2014. At the time, a total of twelve systems were installed, calibrated and commissioned within six weeks. Since then, one camera and one flash unit for each lane in both directions of travel have been monitoring both the speed of the vehicles crossing the bridge and the total weight in two measuring cross-sections. This is because there has been a permissible total weight limit of 3.5 tonnes per vehicle on the Rhine bridge since 2014. To enforce the traffic ban, three parallel piezo sensors were installed in each lane at a distance of one meter at right angles to the roadway.

Speed Enforcement Leverkusen Bridge

Another example of how the structure of a bridge is preventively protected can be found on the A7 freeway in Germany. Speed monitoring systems are used here along the Rader Hochbrücke, which is almost 1,500 meters long. After a short test phase, four speed enforcement systems in towers were put into operation in 2015 and have been monitoring around 62,000 cars and trucks every day since then.

The bridge can be protected above all by ensuring that 7.5-tonne trucks do not exceed the prescribed maximum speed of 60 km/h. Other limits apply to passenger cars due to their weight. For example, passenger cars are generally allowed to drive 100 km/h. A lower speed only applies for storms or strong winds. To ensure better compliance with the speed limits in wind and storm conditions, two additional measuring systems monitor these regulations. The special feature of these measuring systems is that they reliably identify what limit currently applies by linking to the variable message signs.

Are you also looking for a solution that offers preventative protection of your infrastructure? Allow our experts to advise you! They will help you find the right technology to suit your application.

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Sabine Elbers

About Sabine Elbers

Sabine Elbers has been working in the communications department of the Jenoptik Light & Safety Division since 2001. A graduate in linguistics, she uses her many years of experience for the internal and external communication of topics relating to public safety and traffic safety. As a commuter between home and work, she experiences the dangers of mobility in a conurbation on a daily basis and is certain that it would not work without rules and their enforcement.

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