Road traffic signalling system

A system for signalling individually to a vehicle driver in a flow of traffic that he is too close in relation to has speed to the vehicle ahead. The system comprises a succession of interconnected electronic signalling units of the "cat's eye" type positioned at intervals along the road. Each signalling unit detects and times the passage of vehicles past the unit, determines the distance to the vehicle ahead and communicates with adjacent units. Signalling to the driver may be direct by light signals emitted from units in front of his vehicle, or indirect by transmitting a local signal from each unit for detection by vehicle-borne receivers.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a road traffic signalling system, and in 
particular to a system for signalling individually to vehicles in a flow 
of traffic. 
2. Description of Related Art 
A major factor influencing road safety is the difficulty drivers of 
vehicles face in assessing distance, in particular a safe distance to the 
vehicle ahead. In conditions of poor visibility, such as fog, the problem 
is exacerbated by the lack of visual reference points from which a driver 
can judge his speed and it is well established that multiple crashes are 
often caused when successive vehicles in a flow of traffic become too 
closely spaced in relation to their speed. 
International (PCT) Patent Publication No. WO-88/07560 describes a vehicle 
guidance and proximity warning system in which a series of "cat's eye" 
units embedded in the road are interconnected by optical fibres. The light 
received by any one unit from the headlights of an approaching vehicle is 
transmitted to neighbouring units. By transmitting light in a forward 
direction (relative to the direction of travel) the path of the road ahead 
is lit up. Alternatively, by transmitting light in a rearward direction, a 
warning is provided to a following vehicle of the vehicle ahead. Although 
the rearward lights provide an improved indication of traffic ahead in 
conditions of poor visibility, a simple passive system of this type can 
make no assessment of vehicle speed--an essential factor in determining 
the safe distance between vehicles. 
A more complicated system, described in United Kingdom Patent Specification 
No. 1,090,091, provides for the illumination of lights in "cat's eye" 
units both ahead of and behind a vehicle. Two forward lights are arranged 
to come on after respective fixed delays from the detection of a vehicle, 
so that if the vehicle is exceeding a speed limit determined by the time 
delay and the separation of the light units, the vehicle will literally 
"over-ride" the lights and they will not be visible to the driver. Thus, 
although vehicle speed is not itself measured, an indication of speed in 
excess of a preset limit is provided. Each vehicle detected also causes to 
be set up a fixed pattern of "tail" lights to the rear of the vehicle. 
These tail lights are colour-coded according to the distance from the 
vehicle, so that a following driver receives an indication of his distance 
from the vehicle ahead as he closes up on it. However, the distance 
signals are fixed and take no account of vehicle speed. Thus, in order to 
judge a safe distance from the vehicle ahead, and assuming he does not 
over-ride his own forward lights, a driver has to take account of his own 
speed and the distance to the vehicle ahead, based on which of the tail 
signals associated with that vehicle are visible to him. There is no 
direct indication of safe distance as determined from measurement of 
vehicle speed. 
SUMMARY OF THE INVENTION 
It is an object of this invention, therefore, to provide an improved road 
traffic signalling system to warn vehicle drivers of unsafe traffic 
conditions, in particular in relation to vehicle position and speed. 
According to the invention there is provided a road traffic signalling 
system for signalling individually to vehicles in a flow of traffic, the 
system comprising a succession of electronic signalling units positioned 
at intervals along a road, each unit comprising: 
(a) a detector for detecting the local presence of a vehicle, 
(b) timing means for determining vehicle speed past the unit, 
(c) communicating means for communicating with adjacent units, 
(d) coding means for transmitting back a coded signal indicative of a 
vehicle detected at a unit ahead, 
(e) signalling means for signalling to vehicles approaching the unit, and 
(f) a processor responsive to signals from the local detector and the local 
timing means and to a said coded signal received by the communicating 
means, to control the signalling means in response to traffic conditions. 
For respective pairs of adjacent units, the timing means of the forward 
unit is preferably controlled in response to the successive detections of 
a vehicle at the rearward unit and the forward unit to give a time 
interval indicative of vehicle speed past the forward unit. Alternatively, 
the timing means may be adapted to determine vehicle speed by measuring a 
time interval during which the output of the detector exceeds a 
predetermined threshold value. 
The coded signal is initiated by the detection of a vehicle at a unit (the 
originating unit) and is transmitted back from unit to unit by means of 
the communicating means. Preferably, the coding means of each unit 
modifies the coded signal so that, at any unit, the coded signal is 
indicative of the distance to the originating unit. Alternatively, the 
coded signal may carry an identification code representative of the 
originating unit, the processor of each unit being adapted to determine 
from the identification code the distance to the originating unit. 
According to a feature of the invention, the processor of each unit is 
adapted to predict from said distance and said time interval, the time 
lapse before a vehicle at that unit will reach the originating unit of the 
coded signal, and to control the signalling means of neighbouring units to 
that unit in dependence upon the predicted time lapse. The neighbouring 
units preferably comprise the units ahead as far as the originating unit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 two vehicles A and B occupy one traffic lane 7 of a road; The 
direction of traffic is indicated by the arrow 6. Embedded in the centre 
of the lane 7 and positioned at regular intervals along the lane is a 
series of electronic signalling units, of which five, 1 to 5, are shown. 
In appearance the units resemble conventional "cat's eyes". However, 
instead of having passive light reflectors, the units 1 to 5 are 
electrically powered devices, each housing a bank of lights and active 
electronic circuitry. Each unit performs a number of functions, including 
controlling its own lights, detecting and timing the passage of vehicles 
and signalling to neighbouring units, the principal goal being to indicate 
to a vehicle driver that he is too close, in relation to his speed, to the 
vehicle ahead. Light signals from the units flow along with the vehicle 
for which they are generated so as to be continuously visible to the 
driver as he advances along the road. Although the signals provided by the 
system will be useful to drivers in normal road conditions and especially 
at night, their greatest value will be in conditions of poor visibility, 
such as fog or smog, when judgement of distance and speed becomes more 
difficult. 
It will be appreciated that other facilities may be added to the basic 
system for the performance of subsidiary functions. For example, there may 
be provision for: 
(i) indicating to a driver that there is slow-moving traffic ahead in his 
lane; 
(ii) indicating to a driver that he is exceeding the local speed limit; 
(iii) detecting and indicating the presence of ice, fog or surface water; 
(iv) adjusting the light signals to take account of these conditions. 
The construction of the signalling units 1 to 5 will be described in detail 
later, but, for now, the basic principles of operation of the system will 
be considered. The signalling units are positioned preferably centrally 
within each traffic lane and spaced a suitable distance apart (say, 5 
meters). They are powered by a common electric cable buried in a slot 
along the road. Signalling between neighbouring units is achieved by 
individually interconnecting adjacent units. The interconnecting cabling 
may be electrical or fibre optic. Fibre optic cabling 42 between units 1 
to 4 is shown in FIG. 5. Signals which it is required to transmit to a 
number of units are relayed from unit to unit by means of the cabling. In 
an alternative arrangement, however, each signalling unit may be powered 
by a rechargeable battery linked to a solar cell exposed to daylight and 
vehicle headlight. Signalling between such independently-powered units may 
be achieved by means of low-power ultrasonic or electromagnetic radiation, 
each unit having an individual transmitter/receiver module. No cabling is 
then required between the units and they may be easily removed, replaced 
or fitted temporarily in the road surface. 
Each signalling unit comprises: 
(i) a light emitting means in the form of a bank of three high-intensity 
coloured lights (red, amber and green), the beams from which are directed 
through an optical element so as to be visible to passing drivers; 
(ii) a vehicle detector to detect the presence of a vehicle adjacent the 
unit; and 
(iii) electronic circuitry including a timing means, such as a clock, for 
providing vehicle timing (speed) data, a coding means adapted to transmit 
back a coded signal indicative of a vehicle detected at a unit ahead, and 
a processor for performing data processing and light control functions. 
Returning to FIG. 1, vehicle B is shown passing unit 5 and a following 
vehicle A (in full lines) is adjacent unit 2. When a vehicle passes a 
signalling unit the following things happen: 
(a) the vehicle passage is detected; 
(b) a signal is sent to the next unit ahead instructing that unit to start 
its clock; 
(c) the clock in the unit being passed is stopped, giving the time interval 
in which the vehicle has travelled from the previous unit, which, with 
known distance between adjacent units, provides a measure of vehicle 
speed; 
(d) a coded signal is initiated by the coding means of the unit being 
passed, and this signal is sent in a rearward direction, being relayed 
from unit to unit, until the signal reaches a unit where a vehicle (the 
following vehicle) has been detected; at this unit the coded signal is 
supplied to the local processor and a new coded signal is generated by the 
local coding means for transmission further back along the traffic lane; 
(e) the processor determines from the coded signal the number of units (and 
therefore the distance) to the vehicle ahead; 
(f) at each unit where a vehicle has been detected, the processor is 
supplied with the distance and time data of steps (c) and (e) and 
determines according to pre-programmed rules, whether or not the passing 
vehicle is too close, in relation to its speed, to the vehicle ahead; 
(g) if it is determined that the vehicle is at least a minimum safe 
distance from the vehicle ahead, a signal is sent ahead to illuminate the 
green lights on, say, the next 6 units forward; 
(h) if it is determined that the distance between the vehicles is not safe, 
a signal is sent to, say, the next 6 units forward of the following 
vehicle to switch on either their amber lights or red lights, according to 
the degree of danger which the distance and speed of the vehicles 
represents; 
(i) if, in the case of (g) or (h) above, there are fewer than 6 units 
between the two vehicles, only the light emitting means in the units 
between the two vehicles will be activated so that light signals intended 
for the driver of a following vehicle will not become visible to the 
driver of the vehicle ahead; 
(j) lights in the unit being passed are extinguished, although they may 
subsequently be turned on again after the vehicle has passed the unit in 
response to the presence of another following vehicle; 
(k) if a vehicle slows down to a speed below a set limit, then, as an 
optional subsidiary function, a separate rearward signal may be generated 
instructing the 40 (say) units to the rear to show a flashing red signal 
and the 40 (say) units beyond those to show a flashing amber signal. This 
rearwardly directed signal is arranged to over-ride the normal signalling. 
It should be noted that in steps (g) and (h) above, the light control 
signal is transmitted initially only to the next adjacent unit ahead. The 
signal is successively transmitted forward by each unit in the chain in 
"bucket-brigade" fashion. 
Referring again to FIG. 1, the processor in unit 2 is supplied with two 
pieces of information, the coded (distance) signal originating from unit 
5, indicating the presence and distance of the vehicle B ahead, and a time 
(speed) value representing the time interval in which vehicle A has 
travelled from unit 1 (where vehicle A is shown dotted) to unit 2. This 
data may be used to predict the time lapse before vehicle A will reach the 
position at that moment of vehicle B (i.e. unit 5) if it continues at the 
same speed. Thus, the safety determination made in step (f) above may be 
achieved by a simple multiplication of the number of units between the 
vehicles A and B (provided by the coded signal in step (e) above) and the 
time interval indicative of vehicle A's speed (determined by step (c) 
above). Such a calculation of the predicted time lapse effectively gives 
the time delay between two successive vehicles passing a given point on 
the road. It may be used to determine whether the vehicles are safely 
spaced by comparison with a predetermined minimum safe value or range of 
values, as explained later. The result of the comparison determines which 
signals are presented to the driver of vehicle A. 
FIG. 2 is a block diagram of a basic scheme for the system showing features 
of each signalling unit and the interconnection of two such units. Each 
signalling unit is capable of performing the steps (a) to (j) above. To 
achieve the function described in (k) above, i.e. to indicate slow-moving 
traffic, requires some modification of the scheme shown in FIG. 2, 
including the provision of a facility for timing the period for which a 
vehicle remains inside the sensitive range of a signalling unit's vehicle 
detector and for transmitting a light control signal in the rearward 
direction. This function is described more fully later with reference to 
FIG. 4. In FIG. 2, for simplicity, only the basic system requirements, 
that is to switch the lights on to indicate a dangerous traffic condition 
and off in all other circumstances, will be considered. It will be 
appreciated that in a simple system of this nature, each signalling unit 
may comprise a pair of lights, one permanently powered at night and in 
conditions of poor visibility for road guidance purposes and the other 
controlled by the system units to provide traffic signalling. 
FIG. 2 shows two signalling units, unit (N) and unit (N-1) and their 
interconnection. In a complete system, the two units would, of course, 
form part of a long chain of such units, adjacent units being 
interconnected in the same manner. With reference to unit (N), each 
signalling unit comprises essentially a vehicle detector 19' for detecting 
a vehicle adjacent the unit, a coding means in the form of a counter 9' 
for receiving, generating and transmitting vehicle distance information by 
means of a coded signal, a clock 15' for vehicle speed measurement, a 
light emitting means 14' for providing traffic signals and an electronic 
processor 11' for controlling the operation of the unit. 
The two units, (N) and (N-1), are shown interconnected by three separate 
signal lines 8,10 and 12, which carry essential system signalling 
information between the two units. As will be explained later, all three 
signals may be carried on a single data link by the use of multiplexing 
techniques. However, for the purpose of clarity of description, three 
separate signal paths will be considered. The direction of traffic flow is 
indicated by the arrow 6. Thus, the line 8 carries a `clock-start` signal 
in the forward direction from unit (N-1) to unit (N). The line 10 carries 
a `count` (or distance) signal in a rearward direction from unit (N) to 
unit (N-1). This count signal may be relayed by unit (N-1) to the next 
unit back (not shown) by means of line 10'. The line 12 carries a `light 
control` signal in the forward direction from unit (N-1) to unit (N) to 
control the light emitting means 14' in unit (N) via the processor 11'. 
Considering first the count (distance) signal carried by line 10, counter 
9' in unit (N) is started when a vehicle is detected at the unit by a 
vehicle detector 19'. The counter 9' generates a count signal which is 
sent back to the unit (N-1), where counter 9" increments the count and 
relays the count signal back to the next unit (not shown) by means of line 
10'. This count signal continues to be sent back and the count incremented 
by each unit in the chain until it reaches the next unit at which a 
vehicle has been detected. Assume, for example, that a vehicle has been 
detected at unit (N-1). The count at that unit is supplied to local 
processor 11" on line 18". The counter 9" is reset by reset circuit 16" 
and a fresh count signal is generated and transmitted via line 10'. Thus 
processor 11" of unit (N-1) at which the vehicle has been detected has a 
count of the number of units to the next vehicle ahead, which count 
represents the distance between the vehicles. In conditions when traffic 
is light and there is considerable separation between vehicles it is not 
necessary for the count signal to be sent back further than some 
predetermined number of units. Thus, each counter 9 is provided with an 
overflow monitor 13 so that if the count reaches a preset maximum, the 
counter is stopped and the maximum count is held. When a vehicle is next 
detected at that unit, the counter is reset and re-started by means of the 
reset circuit 16. If a following vehicle is so far behind the vehicle in 
front that the counter in the unit at which it is detected holds the 
maximum count, the processor will interpret the maximum count signal as 
meaning that the vehicle ahead is a safe distance away, regardless of the 
speed of the local (following) vehicle. 
The clock-start signal carried by line 8 is used for time (speed) 
measurement. Each signalling unit has its own timing clock 15 which 
receives a start signal from the unit behind generated by a circuit 17. 
The circuit 17 is triggered by the detection of a vehicle at that unit. 
Thus, when a vehicle is detected at a unit, a signal is transmitted in a 
forward direction to start the clock in the next unit ahead. For example, 
referring to FIG. 2, when a vehicle is detected by detector 19" of unit 
(N-1), a clock-start signal is generated by circuit 17" to start the clock 
15' in unit (N). When the vehicle reaches unit (N) the clock 15' is 
stopped and reset by circuit 21'. At the same time the clock-start signal 
is generated by circuit 17' and transmitted on line 8" to the clock in the 
next unit ahead (not shown). The time interval for the vehicle to travel 
from unit (N-1) to unit (N) is provided by clock 15' to the processor 11' 
in unit (N) on line 20'. Since the spacing of the units is a known 
distance, this time interval is representative of vehicle speed. 
Thus, the processor 11 in each unit has available the following 
information: 
(i) whether there is an adjacent vehicle (from the output on line 22 of 
detector 19), 
(ii) the distance to the next vehicle ahead (from the output on line 18 of 
counter 9), and 
(iii) the speed of an adjacent vehicle (from the output on line 20 of clock 
15). 
Further, the processor in each unit receives a light control signal on line 
12 for controlling its light emitting means 14. If there is no vehicle 
present at a unit, the processor may also pass this signal to the next 
unit ahead so that a series of lights can be illuminated ahead of a unit 
at which a vehicle has been detected. For example, a signal received on 
line 12 by processor 11' in unit (N) will be transmitted to the next unit 
ahead via line 12", provided no vehicle has been detected by detector 19'. 
If a vehicle is detected at unit (N), the associated light 14' is 
extinguished and control for the lights of the units ahead originates from 
the local processor 11'. 
This mode of operation ensures that signals to control the lights ahead of 
a unit are not transmitted beyond the unit adjacent the vehicle ahead, 
where they could present misleading information to other drivers. Control 
of the lights in each unit is achieved by means of the local processor and 
is based on the information listed in (i) to (iii) above, plus the light 
control information provided on line 12 by the processor in the unit 
behind. It is seen, therefore, that there are three basic signals which it 
is required to exchange between adjacent signalling units; two 
forward-going signals, the clock-start and light control signals, and one 
rearward-going signal bearing the count (distance) information. The three 
signals may be carried by a single electric cable or optical fibre by the 
use of frequency division multiplex (FDM) or time division multiplex (TDM) 
techniques or by using respective cables or fibres. Signals travelling in 
the same direction may be combined and coded in the form of a multi-digit 
number in which specific digits, or groups of digits, represent different 
signal information. If this mode of signalling is adopted the multi-digit 
number signal is supplied directly to the processor for decoding to 
extract the required control information. 
The specific methods described above for measuring the speed, time and 
separation of vehicles and for the communication of this information 
between adjacent signalling units are given by way of example only. Other 
methods will be apparent to those skilled in the art. For instance, the 
vehicle speed assessment may be made by measuring the period for which a 
vehicle remains inside the sensitive range of the detector of a local 
unit. It will also be appreciated that the essential requirement of the 
count (distance) signal is that it provides at any unit an indication of 
the distance to the originating unit. This may be achieved, as described, 
by a simple counting process initiated by the detection of a vehicle at 
the originating unit. Alternatively, the counter 9 in each unit may be 
replaced by a signal generator. If each unit is allocated an 
identification code, which can be carried by a coded signal generated by 
the signal generator on detection of a vehicle, then a simple comparison 
of the coded signal received by a given unit with its own identification 
code will enable the required distance data to be obtained. The comparison 
function may be conveniently performed by the unit processor. 
A brief description will now be given, by way of example, of one 
construction of the signalling unit. FIG. 3 is a part-sectioned schematic 
illustration of one unit 27. A housing 40 accommodating the unit 27 is 
permanently embedded in the road surface 35 and connected by cable 31 to 
the two neighbouring units. The unit 27 comprises a cylindrical body 37 
having a domed upper surface 39 carrying an optical element 29 for 
emitting light. The cylindical body 37 carries a flange fitting into the 
housing 40 and having a hermetic seal 24. Electrical connections to the 
unit are made via a plug 23 at the base of body 37 which mates with a 
socket 28 in the housing 40. A solenoid 26 mounted on a ferromagnetic core 
constitutes a vehicle detector for the unit. 
In a system providing three light signals--red, amber and green--there are 
respective light sources 30, of which one only is shown in FIG. 3. A bank 
of optical filters 32 (one only shown) comprises one filter each for 
green, amber and red light. The light sources 30 are aligned with their 
associated filters 32 so that, by selecting the appropriate source, the 
light emitted through the element 29 is either red, amber or green. The 
element 29 is designed to reflect light out of the unit by total internal 
reflection at the surface 38. 
An electronic circuit 25 comprising the clock, counter and processor of the 
unit is sealed within the body 37 of the unit, with external connections 
(not shown) to the plug 23, the detector solenoid 26 and the light sources 
30. 
The vehicle detector need not necessarily be a magnetic type. However, this 
type of detector produces a particularly suitable pulse type output (FIG. 
4(a)) when a vehicle passes the unit. Other vehicle sensing techniques are 
well known in the art. Some of these are reviewed in Vehicle 
Detection--Taylor, Bell and Thancanamootoo (Highways & Transportation, 
June 1987). An advantage of the pulse output (FIG. 4(a)) is that it can be 
used to detect slow-moving traffic. The separation between adjacent 
signalling units along the road should be such that a vehicle is always 
within the sensitive detection range of one of the signalling units, i.e. 
adjacent units must be sufficiently close that a vehicle does not become 
"lost" between them. By setting a threshold A.sub.T at an appropriate 
proportion of the peak output amplitude A.sub.M, a square wave pulse (FIG. 
4(b)) can be generated, the duration of which gives the interval t.sub.v 
in which a vehicle remains within the sensitive range of the detector. 
This interval can be used to detect slow-moving traffic for the purposes 
of signalling back to following vehicles, as already mentioned. To provide 
this function a further clock is needed, being controlled in response to 
the detector output crossing the threshold amplitude A.sub.T. The interval 
t.sub.v for which the detector output exceeds the threshold A.sub.T is 
supplied to the local processor for comparison with a fixed reference to 
determine whether the vehicle detected is slow-moving. When a slow-moving 
vehicle is so detected a light control signal is transmitted back to a 
number of units to the rear to warn traffic of the presence of the 
slow-moving vehicle. This warning signal may be arranged to over-ride the 
normal `safe distance` signals and may be such that it causes the amber 
lights (say) to flash to avoid the possibility of confusion with the other 
signals. The rearward-going light control signal is not constrained from 
being relayed back beyond the next vehicle, so the warning lights will be 
visible to a number of following vehicles. 
The threshold for defining detection of a vehicle is set at a level above 
the amplitude of the detector output when the vehicle is mid-way between 
two adjacent units. In this way, for the purpose of vehicle detection a 
vehicle is unambiguously detected by a single signalling unit at any given 
position on the road. 
As already mentioned, a major factor in road safety, particularly on 
motorways and main highways, is the distance between vehicles in relation 
to their speed. This is equivalent to the interval t.sub.D between 
successive vehicles in a traffic lane passing a fixed point on the road. 
The generally accepted safe interval for good road conditions is not less 
than 2 seconds. In the system described, the interval t.sub.D is 
calculated in the processor of each unit at which a vehicle has been 
detected to give a predicted time for the vehicle at that unit to reach 
the current position of the next vehicle ahead. The calculation is a 
simple multiplication of the time the vehicle concerned has taken to 
travel from the previous unit to the current unit, and a number which is 
(strictly) one more than the number of units between the two vehicles. 
Each signalling unit can show to approaching drivers either a red, amber or 
green light. At each unit, the interval t.sub.D is calculated as a vehicle 
passes and is then compared with one or other of a set of number-pairs. 
Acceptable values of t.sub.D for different road conditions are given, by 
way of example, in Table 1 below. 
TABLE 1 
______________________________________ 
t.sub.D ROAD 
GREATER THAN LESS THAN CONDITIONS 
______________________________________ 
1.5 2 GOOD 
1.75 2.25 WET SURFACE 
2 3 ICE 
2 4 FOG 
______________________________________ 
In a simple system there may be a fixed minimum acceptable value of t.sub.D 
pre-programmed in the processor circuit of each signalling unit. 
Otherwise, the appropriate number-pair may be selected automatically by 
the processor in response to signals from road condition sensors 
(described below). Alternatively, the number-pair may be determined at 
roadside stations positioned at intervals of, say, 1 km along the road, 
each station controlling a local group of, say, 200 signalling units. Such 
roadside stations may also provide the power for each group of units, as 
well as collecting traffic count and other statistical information 
relating to road usage for transmission to a central control system. 
Signal information may be transferred between the signalling units and 
their associated roadside station by means of a common power cable for 
each group of units. 
The outcome of the comparison between the calculated value of t.sub.D and 
the appropriate pair of values is used by the processor in each unit to 
determine which colour lights are to be presented to a driver. 
Potential icing conditions occur when the road surface temperature falls to 
0.degree. C. or less. Thus, to detect road ice, a temperature sensitive 
element or probe 34 may be built into the exposed surface 39 of the unit 
27 (FIG. 3). The output of the temperature probe 34 is supplied to the 
processor in each unit, so that a different pair of t.sub.D values is 
selected when the temperature is such that ice is likely to be present on 
the road. The detection of surface water on the road is a similarly useful 
facility to incorporate into the signalling unit. The presence of water 
may be detected by measuring the conductivity between two mutually 
insulated electrodes (not shown) mounted flush with the exposed surface 39 
of the unit 27. 
Fog can be detected in several ways, one of the more sensitive methods 
being to measure the light back-scattered from fog particles using a 
detector co-located with a light source. Typically, the light source is a 
modulated beam of infra-red radiation and the detector has a narrowband 
response centred on the modulation frequency. Although it is not practical 
to incorporate a fog sensor into the signalling units, signals from 
roadside fog sensors could be fed to local units to control the range of 
the safe time interval t.sub.D selected by the processors. Signals from 
such fog sensors may be supplied to the signalling units via the nearest 
roadside station. 
It will be appreciated that although the signalling unit described has an 
individual processor, some of the processing electronics may be more 
conveniently housed in the roadside station associated with each local 
group of units. 
In an alternative embodiment of the invention, illustrated in FIG. 6 the 
signal for controlling the light emitting means of each unit may be 
supplied to a transmitter 41', 41" adapted to generate a local 
electromagnetic field capable of being detected by a receiver on the 
passing vehicle. In this way, instead of signalling directly to the driver 
by way of lights on the road, the same information may be received by an 
audible or visual signal generated inside the vehicle. Transfer of the 
local signal information from the signalling unit to the vehicle-borne 
receiver may be by inductive coupling or by way of an RF carrier signal. 
One advantage of such an arrangement may be a reduction in the power 
consumption of the signalling units resulting from removal of the light 
emitting means.