Most of us probably haven’t tried it, but driving a tram along busy inner-city streets can’t be the easiest of jobs. Sometimes it’s hard enough in a road vehicle. Other drivers just seem to do the daftest things, don’t they? Stopping dead for no reason, jumping the lights, pulling out without warning, doors being flung open – it’s all a bit tricky. And at the same time there are all those pedestrians and cyclists weaving about.

Every tram driver is equipped with the highly effective Mk I eyeball of course, and light rail vehicles do have very effective braking systems, but what if the driver is distracted for a moment?

Let’s not overplay it – light rail is a very safe means of transport. In fact, a 2009 report by the International Association of Public Transport (UITP) showed that accidents per kilometre are more than four times lower for trams than for cars. Safety remains a top priority for operators, but even so, driving a tram on sight for a whole shift can be challenging, especially at night or during poor weather.

A study in Hannover undertaken from 2005 to 2010 revealed a significant rise in the number of collisions. These involved impacts with road vehicles, other trams and with buffer stops. In Hannover alone the cost of accidents involving light rail vehicles is estimated to have been €6 million over the past ten years.

Early warning

Driver assistance systems (DAS) that can provide early warnings to drivers are one way to try to reduce traffic accidents. Indeed, some vehicles now on our roads use them for distance warning or collision avoidance. It hasn’t been very affordable until recently, though, to develop them specifically for light rail use due to the relatively low volume. However, interest is growing and requirements for their introduction are becoming clearer. Because their operating environment is partially unsegregated from other forms of transport, trams and light rail vehicles are emerging as a key field for the application of DAS.

Now Bosch Engineering Group has developed a new ‘Forward Collision Warning’ system for light rail vehicles that combines a radar sensor with a video sensor to detect cars, buses, nearby rail vehicles and other obstacles on the tracks. The idea is to use the system to identify potential accident situations and give drivers reliable warnings of dangerous situations. This should increase safety, prevent downtime and avoid accident-related costs.

Cost effective

Taking into account the annual repair costs of collisions, it has been calculated that a DAS installation could pay for itself within just two years. The systems work by alerting drivers to potential collisions, allowing them to gain precious reaction time. According to data provided by Stephan Lewisch, leader of the UITP group on safety in light rail, it is estimated that the braking distance can be reduced by 20% to 40%. As well as providing alerts for obstacle detection and collision avoidance the DAS systems can also prevent driver errors, such as over-speeding and SPAD (signal passed at danger) incidents.

In developing DAS systems there has been collaboration between operators and suppliers. This has been especially important to ensure the efficiency and effectiveness of the human machine interface. If the system is not designed properly there is a danger that false or unnecessary alerts could either confuse or irritate drivers instead of providing support.

The Bosch Forward Collision Warning system has been trialled in Frankfurt by tram operators Verkehrsgesellschaft Frankfurt am Main (VGF) and in Hannover by üstra Hannoversche Verkehrsbetriebe AG (üstra).

Both operators had started looking into the potential of DAS in 2012. A feasibility study was successfully completed in the summer of 2013 and Bosch, VGF and üstra agreed to cooperate. Field testing of the Bosch Forward Collision Warning system in vehicles for passenger service began at the start of 2014.

Double vision

Simultaneously, an alternative system based on the use of stereo cameras was tested by VGF in Frankfurt. Bombardier, together with research partner AIT (Austrian Institute of Technology), has produced a DAS based on a specially-developed optical 3D sensor system. The device has a set of three video cameras of high spatial resolution mounted at the top of the windscreen. One camera recognises the rails and a stereoscopic pair can detect and range any object that might foul the envelope of the vehicle.

Both VGF and üstra have decided to go forward with the Bosch system. However, following successful testing in passenger service, the Frankfurt Transport Authority has also decided to equip 74 bi-directional Flexity vehicles with 148 of the Bombardier driver assistance systems while the Bosch system will operate on the 38 Siemens R-vehicles.

Data from the trials has helped Bosch Engineering with further refinement of its system in preparation for the start of series production from the end of 2015.


It became apparent at an early stage that DAS hardware used in the automotive sector could be applied to light rail, albeit with some modifications. Bosch has made use of pre-existing automotive technology by using a radar sensor to detect the movement of road and rail vehicles, and the presence of obstacles on the tracks ahead.

In the light rail system, a video camera is used in addition to detect the rails and thus the path that the tram will take. A high-performance rail control unit running application-specific software calculates the trajectories and speeds of the detected objects relative to the tram’s own speed and predicted path. Within defined parameters, the potential for a collision will cause a driver alert to be raised. If the system detects that an object is coming dangerously close, or that the vehicle needs to brake to avoid an object in its path, it gives the driver a visual and acoustic warning.

The precise nature of the alert can be user defined, but in the trials it was found that combining a sounder with one or two lights was effective. Taking into consideration the speed of the light rail vehicle, one light illuminates for far objects and two lights if more urgent action is needed. The nature of the alert therefore enables the driver to make decisions about braking early enough to avoid a possible collision.




The radar operates on a frequency of 76-77 GHz and has a beam spread of 70°. The effective radar range is 160-metres, but only information from the first 80-metres is used. Beyond that distance, it is thought that there are too many variables for reliable predictions to be made. Bosch Engineering recognised that false or unnecessary alerts could very much dilute the effectiveness of the system. It needs to provide valuable assistance to the driver, rather than being an annoyance.

For this reason, the system ignores radar returns from moving pedestrians – human behaviour is deemed to be just too unpredictable! Interpretation of the radar returns is software defined and can be tailored to suit specific requirements.

During the trials, the radar scanner was mounted externally, immediately above the front coupler, as it had been found that there was too much attenuation of the microwave RF signal by the carbon fibre body panelling for the scanner to be mounted inboard. The video camera was mounted behind the windscreen. Production versions will have these items mounted either as a retro-fit kit, or incorporated into the design of new vehicles.

The Bosch system has been well received by both VGF and üstra. “It is already clear that we are at the beginning of a promising technical innovation to increase the safety of railway and especially LRT systems,” commented Michael Rüffer, head of rail operations and safety manager at VGF. “We chose the Bosch camera- radar combination because it was important for us to know that the components are taken from large-scale automotive production. Now we are working together to refine the system in our prototypes, applying the know-how and experience of our tram driving instructors.”

As part of this process, VGF is optimising the functions of the system so that they support the drivers in every situation to avoid collisions and accidents.

Driverless trams?

It has been legally possible in Germany for the driver assistance systems to be trialled on light rail passenger vehicles in the busy city streets of Frankfurt and Hannover because they don’t affect the principle of driving on sight – the driver is always ‘in charge’. With further technical progress anticipated within the automotive sector, particularly through the development of driverless cars, the question inevitably arises as to whether we can expect driverless trams in the future.

Currently, DAS activation will generate vigilance warnings and can lead to the automatic activation of any safety device such as applying the service brake or sounding an internal warning or the external horn. In the VGF prototype, DAS activates the loop of the driver’s safety device, but this can be over-ridden within two seconds by depressing a button on the master controller. If driver intervention doesn’t take place within that time, a safety braking sequence (as opposed to emergency braking) is initiated. This system is in effect a query of the driver’s vigilance.

The integration of the driver assistance system into the loop of the driver’s safety device makes it simpler to implement DAS braking into existing vehicles. In new vehicles, DAS could easily be integrated into the vehicle control software.

This would allow the implementation of speed limitation, interruption of the traction power, activation of different braking systems and, of course, the generation of optical/acoustic warnings.

It would be possible for further developments to increasingly take control away from drivers, but the recommended strategy seems to be to take a step-by-step approach. In any case, without segregation from other traffic, moves towards a high level of automation would require a radical change in the legal environment.

In the meantime, Forward Collision Warning has proved its worth to VGF and üstra and now looks set for wider adoption. The technology is readily available and has become commercially attractive, so it’s highly possible that it could become an industry standard before long.

Its adoption and the realisation of its further potential might even be providing us with a vision of what the future holds for light rail operation in our city centres.