Skid Risk, Accident, Fog

61 years ago a pioneering experiment was launched on the M5 - and now, for the first time, we can share pictures of the enormous signs that lit up the Worcestershire countryside.

Ten years ago last month, we first published Mixed Signals, our extensive history of the development of variable message signs - a field where the UK led the world in the 1960s, producing the most ambitious and most advanced signs ever seen, and pushing at the boundaries of what technology could offer.

When in use, the signal could show up to three messages - SKID RISK, ACCIDENT and FOG - and when any of those were lit, the signal would also show the word SLOW and its flashing beacons would be switched on. For maximum performance, each letter had its own electric lamp, with a stencil in front of the lens.

The sign was designed so that the lamps, and the lenses covering them, were angled in favour of approaching traffic, so the best and clearest view of the letters was from the road. The brightness of the lamps was carefully calibrated so that they were bright enough to be visible in the day but not dazzling in darkness.

Mounted on the legs of each signal was an equipment cabinet, and this was where AEI’s cutting edge technology was found. The cabinet was a Type “T” Field Control System, and it was a treasure chest of mid-century analogue electronics.

T party

The system itself was called FDM, or Frequency Division Multiplex. It used two spare wires in the emergency telephone cabling that ran down each side of the motorway to carry an electrical circuit. The wires passed through each Field Control System box, where the electronics listened for the signal they carried.

The system used 18 distinct sonic frequencies to encode its messages. Mechanically tuned reeds would vibrate when a specific frequency passed down the line, triggering a switch. A single pair of wires could therefore control fifteen signals: 15 of the frequencies were used to address an individual signal, and the remaining three frequencies corresponded to the messages Skid Risk, Accident and Fog.

Messages simply instructed a single sign to switch a single message on or off (“sign 12, Fog”). A sign would then stay doing whatever it had been told to do until another message gave it a new instruction. In this way any number of signs could show any number of messages, but only one command could be sent to one sign at a time.

The spare telephone wires were connected to a control room at Hindlip Hall, the headquarters of Worcestershire Police, where motorway patrols were coordinated. The control room was rather charmingly basic by modern standards.

In the middle was a control panel for the emergency telephone system. Multiple phones were connected to a single line, so there were switches to effectively pick up and speak to groups of ten phones at a time. Since it was unlikely there would be simultaneous calls from more than one phone this was considered sufficient, though experience over the years has led to modern systems being much more sophisticated.

On the left was the control panel for the new AEI signalling system. It had a mimic of the M5 in Worcestershire down the middle, and three individual switches for each of the 22 signals. Instructions for a single sign to switch a message on or off could be sent by flicking the appropriate switch up or down.

On the right the picture shows a police officer holding the telephone receiver, but since none of the switches on the telephone control panel are set, he is probably just posing for the camera. Pinned to the wall behind him is a small booklet titled Home Office: Motorway Emergency Telephone Operating Procedure at Police Stations. It looks like it’s been well thumbed.

Clearing the way

AEI’s new signals went into service in the summer of 1964. The UK had just had its first winter of terrible motorway pile-ups, and as the weather turned cold again, a second winter of fog panic began.

The M5 wasn’t actually particularly prone to fog or pile-ups. The focus of media attention, and the ensuing public panic, was the M1, which passed through more industrial areas and carried far more traffic. But the M5 had something the M1 didn’t: it two spare wires in its telephone system that could be used to carry messages to motorway signals without laying miles and miles of new cabling. It was the only reason the new signals - hoped to save the public from horrifying accidents in bad weather - appeared on the M5 first and not in the place they were most needed.

AEI never got to expand their pioneering FDM technology to the rest of the motorway network. By the end of 1964 emergency battery-powered Motorwarn signals had been deployed to the entire English motorway network to deal with further negative publicity about the Ministry’s failure to get a grip on the fog problem.

By the time permanent electronic warning signs were being installed elsewhere, the final design had changed considerably. AEI’s clever system of sonic frequencies had not worked as hoped: experience on the M5 was more or less OK, but on the railways it was found to be unreliable and was only ever installed in a couple of places. They were not involved in the creation of the clever Motorway Signal, which was manufactured by Plessey and used light matrix technology to allow dozens of different symbols and messages to be displayed on a much smaller and more pleasing sign face.

But the M5 trial wasn’t a waste of time. It proved that electronic light signals could work on motorways, and the value of having signals operated from a central control room and not by waiting for police patrols to attend each sign to turn it on or off.

The experience of lighting up the Worcestershire countryside with messages of Skid Risk, Accident and Fog helped inform the next generation of signs - and, for a number of years afterwards, continued to serve the M5 until it too got light matrix signs.

You can read the full story of the UK’s groundbreaking development of electronic roadside signals in our article Mixed Signals, including the makeshift Motorwarn and the revolutionary Motorway Signal.