Q: Does anyone know what roadside bar codes are for? Here in Cumbria they are about 80 centimetres tall and 30 centimetres wide and face onto the road, unlike normal road signs.
A: Roadside bar codes are fixed location reference points used by some highways authorities to help road maintenance workers find sites that have been identified for repair, such as potholes or broken street lights. By using a bar-code reader the maintenance workers can pinpoint accurately where, in relation to particular bar-code signs, the problem sites are.
Andrew Howard, Road Safety Department, The Automobile Association, Basingstoke, Hampshire
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A: Considerable curiosity has been raised by the appearance of bar-code plate signs on Cumbria鈥檚 motorway and trunk road network. However, their purpose is very down-to-earth. They are there to assist with highway maintenance. They provide a means of surveying and location referencing for road crews using the high-speed road monitor.
This innovative machine鈥檚 main claim to fame is its ability to measure the condition of the road while travelling at speeds of up to 100 kilometres per hour. An on-board laser checks the location of trouble spots by using the bar-code plates. The survey highlights stretches of road requiring further structural investigation.
Now is an interesting time for highway maintenance. The opportunities offered by various new technologies are considerable. The need to continually improve efficiency and reduce the inconvenience on traffic are paramount. Somehow, I don鈥檛 think that we have seen the last of the unusual signs on Cumbria鈥檚 roads.
John Mather, Senior engineer, Construction and Services, Cumbria County Council
A: The Highways Agency manages the motorways and all-purpose trunk roads on behalf of the Department of Transport. As part of this work, contractors working for the Highways Agency regularly survey different aspects of the condition of these types of roads.
One of these surveys is carried out using a high-speed road monitor (HRM). The HRM collects measurements of the longitudinal profile, average transverse profile and surface texture in the two wheel paths of the vehicle carrying out the survey. One half of the network managed by the Highways Agency is surveyed by the HRM each year.
In order to identify the position of the HRM on the road network, lengths of road, referred to as sections, have their start and end positions marked by two metal studs. These are 100 millimetres square and are sited approximately 175 millimetres apart. They are positioned on the road surface, midway between the wheel tracks of the nearside lane.
A camera on the HRM recognises the studs on the road surface and records their location in relation to the measurements of the road surface condition. This allows the road condition from one set of measurements to be compared to another set that has been collected at a different time.
The studs do not, however, provide a direct identification of the road section, only the start and end of any unidentifiable section. The large bar codes on the side of the road which face the road, identify the road length.
A bar-code reader on the HRM notes the bar codes as the machine passes and correlates the identifier from the bar-code plate with the condition measurements. This ensures that the actual route taken by the HRM matches the planned route and that the measurements are assigned to the correct length of road.
The bar codes are not present for every road section but, in general, are located on the HRM survey routes every three or four sections.
This places them typically at up to 7 kilometres apart in rural areas but as close as 3 kilometres in urban areas.
R. Abell, Transport Research Laboratory, Crowthorne, Berkshire
On the double
Q: If you travelled at the speed of light, would you get a light flash as in a sonic boom at the speed of sound?
A: A sonic boom does not occur at the speed of sound, but when the shock wave caused by an object going faster than sound passes the ear. Particles cannot exceed the speed of light in a vacuum, c. But if light is slowed to less than c, as it is in an optically dense transparent material then it is easy to exceed the speed of light. When a charged particle exceeds the speed of light it does emit light in a way analogous to a sonic boom, which may be seen as a flash. This radiation was discovered by Pavel Cherenkov in 1934. Cherenkov radiation causes the blue glow in a water-moderated nuclear reactor.
Howard L. Medhurst, Crawley, West Sussex
A: There is an analogue to the sonic boom for electromagnetic waves, but the particle must be travelling faster than the speed of light. This is possible because light travelling through a medium has a velocity less than its velocity in a vacuum. This lower velocity is given by v = c/n where c is the speed of light in a vacuum, and n is the refractive index of the medium.
The radiation of light analogous to the sonic boom, called Cherenkov radiation, is produced whenever the velocity of a particle exceeds c/n. The blue glow that emanates from water in which highly radioactive nuclear reactor fuel rods are stored is caused by the Cherenkov effect. Much of the radiation fuel rods emit is in the form of high energy electrons. The electrons travel through the water at a velocity greater than that of light in water and hence cause the characteristic 鈥淐herenkov glow鈥�.
The importance of the Cherenkov effect as a scientific tool lies in the connection between a particle鈥檚 momentum and the angle at which the Cherenkov photons are emitted. A measurement of the angle of Cherenkov emission provides an indirect measurement of the speed and direction of a particle. Cherenkov detectors are one of the important tools used by particle physicists to probe the ultimate small-scale structure of matter.
Ray Hall, Illinois, USA
A: Cherenkov radiation occurs when a charged particle travels through a refractive medium at a speed faster than the local speed of light. A refractive medium is necessary because nothing can travel faster than light in a vacuum. For particles of sufficiently high energy even air has enough refractive index to produce Cherenkov radiation.
Pete Bleackley, Leicester
This week鈥檚 questions
Vegetative state: Death in vertebrates is defined as the point at which brain activity ceases. In light of this definition in the animal kingdom, at what point is a plant considered dead?
Glenn Graham, University of Queensland
Sushi blues: Some Japanese restaurants serve hot drinks in small ceramic beakers. The beakers rapidly turn deep blue, and then return to normal as the beaker begins to cool. What is happening and how?