COLLISIONS between light rail vehicles, and cars, bicycles or, in some tragic cases, pedestrians, are an unfortunate reality of operating tram networks in busy urban environments.

Overcoming this problem has long been a point of discussion in the industry, including between Dipl-Ing Michael Rüffer, head of rail operations at Frankfurt Transport (VGF), and his colleague at Üstra in Hannover, Dipl-Ing Steffen Kähler.

VGFRüffer recalls that following an accident on the Frankfurt-am-main network in 2012, he spoke to Kähler who told him that his boss had bought a new car which was fitted with a driver advisory system (DAS). This system alerts the driver if they are in danger of a collision, and Kähler said that the CEO asked why this could not be adapted to trams?

“I said I thought it would be too difficult to do,” Rüffer says. “However, a week later we both suffered an accident on the same day. At that point I called him and we agreed that we should do something to eliminate this problem.”

VGF and Üstra subsequently initiated a pilot project at the beginning of 2013 to test the capabilities of two systems: the first from Bosch Engineering, which was trialled by both VGF and Üstra, and Bombardier’s stereo camera system which VGF tested.

The Bosch system is based on its solutions for the automotive sector and comprises a video camera for detecting the rails and a radar to sense objects in the rail area and to measure the distance between the tram and the object.

Software in the DAS control unit subsequently evaluates whether a dangerous collision is likely, and if it is, the system triggers an alarm, alerting the tram driver using both a light and an acoustic warning. The driver has the option to override this within two seconds of activation. If they fail to respond in time, an emergency braking application is triggered.

In contrast, Bombardier’s stereo camera system is entirely reliant on cameras to scan for obstacles of more than 40cm in size in a 15-60m area ahead of the train and to measure the distance, with three cameras installed underneath the destination display to offer a three-dimensional view. Software in the DAS unit again evaluates whether a collision could happen with a 0.3s response time before triggering the DAS output signal for warning or automatic braking.


In Frankfurt, two trams were fitted with the Bosch system and five with Bombardier’s. Rüffer says that trials of both systems have shown that each application is effective and that both are similar in their application and behaviour. However, the radar system is more effective at night and in foggy conditions, whereas the stereo-camera system can decipher clearly between metal objects and people. “For this reason the stereo camera was the best system for us,” Rüffer says

Rüffer adds that while the Bosch system has evolved from a previous driver advisory system into an application suitable for rail, Bombardier has effectively reduced the capability of its system from a platform designed for automation to simple detection. “This could offer us big advantages in the future,” he says.

The decision was subsequently taken by VGF to install the Bombardier system on its 74 S-Type single-car LRVs and with the five test vehicles already in service, he expects the work to conclude in 2017.

As for installation of the remaining 38 R-type vehicles in its fleet, Rüffer says that given the age of this rolling stock it made no sense to retrofit them with the DAS system. “We are looking to procure new vehicles in the future and all of these will be fitted with DAS,” he says.

In both systems the driver is and remains the key element as they will continue to drive using line-of-sight. DAS does not exempt the driver from his or her duties, which also means that VGF has not had to seek additional SIL approvals to install the system.

Persuading drivers to buy into the system was critical to the success of the trials, and ultimately long-term adoption of DAS. Rüffer says that 15 participated and offered regular feedback on what it was like to use DAS and how they thought the system might be improved.

“From the very beginning we asked the drivers if they would be interested in participating in the trials, and we consulted their opinions throughout the trials of the system,” Rüffer says, adding that the system has proven to offer the added-benefit of improving driver behaviour.

“A special feature of the system is the drivers’ reaction to it,” Rüffer says. “When they are relaxed, they are not too stressed, and too far away from traffic, there is no beep. However, when they are driving aggressively, they will get a beep. It is no fault of the system, they are simply driving too close to the vehicle in front of them.”

Among the future improvements are the addition of a map, which would familiarise the DAS with specific features of the route and avoid any potential false alarms. For example, if there is a pole within the line of sight, but which is not going to impact operations, the DAS will recognise this and ignore it. Work is also taking place to improve performance through curves. Currently the cameras position and view means that the system can miss potential hazards not visible straight ahead of the vehicle.

Rüffer says VGF’s work on the DAS project reflects its desire to do innovative things to improve the performance of its network. The results from the tests have proven positive, and the hope is that avoidable accidents and driver error will be eliminated in the not too distant future.

“In Germany we are increasingly looking at automation in transport, and we don’t want to fall behind,” Rüffer says. “A lot of work has gone into this project and we will continue to work on it in order to innovate and offer a better and safer service.”