More go with the flow

Qinetiq is working with the Highways Agency to develop two new fibre-optic sensor systems to better measure road usage and improve traffic flow on major routes.

The systems are designed to integrate with the MIDAS (Motorway Incident Detection and Automatic Signalling) network, which sets variable message signs and advisory speed limits on the UK’s main roads.

Currently, a key component of MIDAS is a set of induction loops in the road surface that gather traffic flow information. These are arranged at intervals of 500m or closer and measure speed, vehicle count and occupancy. Each pair is connected to a MIDAS outstation housed in roadside furniture, which needs power and a method of transmitting to a central processing system.

Qinetiq’s sensor techniques aim to overcome many of the problems associated with inductive loops. Roger Crickmore from Qinetiq’s Winfrith Technology Centre described them in a paper he presented to the Intelligent Transport Systems and Services Congress.

The first solution is an axle detector that consists of a coil of optical fibre wrapped round a polymer tube embedded in the road. As a vehicle moves over the sensor, the coil of fibre is compressed and this can be detected by an attached interrogation system.

David Hill, a capability leader at Qinetiq’s Winfrith Technology Centre, who worked on the project, said: ‘This system employs a technique which detects a phase change in light due to the stresses of the weight of the vehicle that goes over it.

‘The benefit of this system over the existing method is that we can multiplex many sensors onto the same fibre or pair of fibres. It also removes a lot of roadside furniture. We could potentially deploy the sensors every five metres and collect the data every few kilometres.’

The fibre acts as both the means of sensing and transmitting the data back to the interrogation system. Using passive fibre-optic components means there is no need for any electronic equipment or power at the sensor sites, greatly reducing the roadside infrastructure needed.

‘We don’t see it as a replacement of MIDAS, but it is an evolution. The new system will integrate as another layer within MIDAS,’ said Hill. ‘It will be used in Active Traffic Management, such as the variable speed limits on the M25 and the M42 experiment, which allows traffic to the hard shoulder at certain times.’

The sensors have a Weigh In Motion capability which measures a vehicle’s mass as it passes over them. Each pair can also accurately measure speed using an algorithm to determine whether the axles belong to the same vehicle, even when the traffic is congested, which was difficult using inductive loops. The output is processed in real time.

‘For the latest stage we ran a pilot using an array of fibre-optic axle detector sensors set up along the M6. We were extremely pleased with the results,’ said Hill.

Traffic flow is currently recorded using an interrogation system only a few hundred metres away using a dedicated fibre link. However, Qinetiq plans to install sensors at a second site several kilometres away and measure these using fibre cable already installed as part of the National Road Telecommunication System.

Qinetiq’s second system, which is currently under development, could detect what happens between the sensor sites by installing fibre-optic cables longitudinally along each carriageway. Using these, a single Roadside Interrogation Unit could monitor up to six miles (10km) of road.

The system transmits pulses of light which are reflected back by a process known as Rayleigh backscatter. By measuring the characteristics of the reflected light, disturbances on any section of the fibre can be detected and localised.

Qinetiq has been demonstrating this technique using two 300m fibre cables running along the middle of each lane on one of its sites. It can give an indication of a vehicle’s speed and size, but the information is less accurate than the discrete axle sensors.

Qinetiq has been researching fibre-optic sensors for about 15 years, starting off in military applications then spinning out to civilian applications. The highway sensor project has been running for about six years. One previous application was underwater fibre-optic hydrophones used in the oil and gas industry.

‘The main challenge for the road application was to desensitise the equipment sufficiently,’ said Hill. ‘The hydrophone needed to be especially sensitive as it was essentially an underwater microphone designed for very quiet sounds, not a 40-tonne truck. Like many of our products, this builds on Qinetiq’s role of taking traditional military technology and exploiting it.’