Apparatus for monitoring road traffic to control an associated signaling system

An apparatus designed to monitor traffic on a section of road of given length L for modifying the operation of a traffic light at an intersection approached by that road comprises speed-sensing circuits at opposite ends of the surveyed road section, the entrance-side circuit also emitting pulse trains reflecting the lengths Li of passing vehicles. A calculator determines from the measured entrance and exit speeds a mean overall speed VM which is inversely proportional to the mean transit time L/VM and enables the computation of an occupancy density DE(t)=.SIGMA.Li/L from which in turn an encumbrance P(t)=DE(t)/VM is derived. The traffic light can be controlled directly by a signal which is proportional to this encumbrance P(t), or which represents a related function F(t). An additional modification of the operating cycle of such traffic light can be brought about by a signal indicating the approach of a vehicle of unusual length on the surveyed road section.

FIELD OF THE INVENTION 
My present invention relates to an apparatus for monitoring road traffic in 
order to control an associated signaling system usually comprising one or 
more traffic lights. 
BACKGROUND OF THE INVENTION 
In certain parts of the road network, and particularly in cities, a 
signaling system is necessary for regulating the traffic flow. For this 
purpose, for example, traffic lights can be controlled to give priority to 
traffic flow on one road section rather than on another, especially at 
intersections between minor roads and major highways or between roads 
having widely differing traffic patterns. 
Normally, traffic-regulating systems have fixed repetition cycles. The 
resulting predetermined passage time for vehicles coming from a given road 
section can lead to congestion at the crossroads in question. Thus, 
traffic-flow conditions evolve differently over a 24-hour period on road 
sections leading to one and the same crossroads. It is therefore necessary 
to take account of these traffic variations on intersecting roads. 
Moreover, the increasing number of long and heavy vehicles of the bus and 
truck-and-trailer types make it necessary to modify the operating cycle of 
the traffic lights in such a way that when such a vehicle arrives at the 
intersection it can easily pass across it. 
Devices known in the prior art make it possible to define certain 
characteristic parameters of the traffic flow on a given road section. In 
view of their measuring simplicity, the most frequently used means of this 
sort respond to the flow rate of vehicles per unit of time, the average 
speed of the vehicles passing a certain location at a given time, the 
concentration of the vehicles on a given road section and the extent of 
occupancy of vehicles traversing this section. The direct control of 
traffic lights, for example at a road intersection, as a function of the 
values obtained from measuring the aforementioned parameters leads to 
numerous disadvantages. Measurements have been taken in urban networks on 
road sections having widely differing traffic patterns. With comparisons 
based on measurements of speed, flow rate and concentration or occupancy 
level, for example, the mean value of the variations obtained for the 
traffic-signal control at the road sections involved was low. This shows 
that ambiguities exist, which can be of a highly prejudicial nature, if 
consideration is given to only one of the aforementioned parameters. 
OBJECT OF THE INVENTION 
The object of my present invention is to obviate these disadvantages by 
defining a new parameter, referred to hereinafter as encumbrance P(t), to 
be used for the control of traffic signals. 
SUMMARY OF THE INVENTION 
A traffic-monitoring apparatus embodying my present invention comprises 
speed-sensing means disposed at a road section to be surveyed, arithmetic 
means connected to the speed-sensing means for determining a mean overall 
speed VM of vehicles passing over the surveyed road section, 
pulse-generating means disposed at an entrance end of that road section 
for emitting trains of measuring pulses representing by their number the 
lengths Li of vehicles entering same, processing means connected to the 
arithmetic means and to the pulse-generating means for obtaining from the 
pulse trains and from the means speed VM a count of measuring pulses 
representing the combined length .SIGMA.Li of vehicles simultaneously 
present on the surveyed road section and for deriving from that count an 
occupancy density DE(t)=.SIGMA.Li/L, and computer means connected to the 
processing means and to the arithmetic means for generating an output 
signal proportional to an encumbrance P(t)=DE(t)/VM which is indicative of 
the degree of loading of the road section by vehicular traffic and which 
can be used, directly or through the intermediary of a function generator 
emitting a related signal, for modifying the operation of 
traffic-regulation means (such as a traffic light) at a location 
approached by the vehicles. 
Pursuant to a more particular feature of my invention, the speed-sensing 
means may comprise first and second measuring circuits respectively 
disposed at the entrance end and at an exit end of the surveyed road 
section, these circuits feeding respective speed signals to a first and a 
second averaging circuit forming part of the aforementioned arithmetic 
means; a third averaging circuit, with inputs connected to the first and 
second averaging circuits, emits a signal representative of the mean speed 
VM.

DETAILED DESCRIPTION 
As stated above, the apparatus according to my invention utilizes a 
variable parameter, termed encumbrance P(t), which is a function of time t 
and is defined by: 
EQU P(t)=DE(t)/VM (1) 
where DE(t) corresponds to the measurement of the occupancy density of a 
given road section of length L as a function of time t and VM is the 
average speed of the vehicles traveling along that road section. 
The apparatus according to my invention therefore has at least two 
calculating circuits, the first determining the occupancy density DE(t) 
and the average speed VM of the vehicles traveling along this road section 
and the second computing the encumbrance P(t) as a function of these two 
variable parameters. 
FIG. 1 shows an embodiment of the apparatus according to my invention for a 
single-lane road section 11 having a length L between an entrance end E 
and an exit or departure end D. The apparatus comprises a circuit 
arrangement 60 including a first local measuring circuit 1 at end E, 
serving to determine the speed and length of vehicles 2 entering the 
selected road section 11, and a second local measuring circuit 3 located 
at end D which determines only the speed of the vehicles leaving the road 
section. These two local measuring circuits 1 and 3, containing 
conventional speed sensors, are respectively connected by leads 1a and 3a 
to two circuits 4 and 8 which determine the average entry speed Vem and 
the average departure speed Vdm of vehicles 2 on road section 11. 
Arithmetic units 4 and 8 are connected on the one hand to a processing 
memory unit 10, comprising three storage circuits 5, 6, 7 of the 
shift-register type, and on the other hand to another arithmetic unit 9 
determining the average overall speed VM of vehicles 2 on the surveyed 
road section 11. The memory unit 10 is connected to a circuit 12 for 
determining the occupancy density De(t). The outputs of circuits 12 and 9 
are connected to respective inputs of a computer stage 20 for calculating 
the encumbrance P(t), circuit 20 being connected to an output terminal 21 
by way of a comparator 47 and having another input connected to a memory 
61. 
The entrance-end measuring circuit 1 detects the passage of each of the 
vehicles 2 entering the road section 11. For each entering vehicle this 
circuit transmits on the lead 1a a signal representing the speed Vei of 
such vehicle at the measuring point and on another lead 1b an indication 
of the length Li of the vehicle in the form of an uninterrupted succession 
of binary pulses whose number is directly proportional to the length of 
the vehicle in question. This succession of unity-amplitude measuring 
pulses is introduced into the first storage circuit 5 of memory unit 10 
which is stepped by a clock circuit 25 whose frequency is directly 
proportional to the average speed Vem of the vehicles entering the road 
section 11 as determined by circuit 4. The value of the average departure 
speed Vdm of the same vehicles, leaving the road section 11, is determined 
by circuit 8 and transmitted to circuit 9 for determining the average 
overall speed VM of the vehicles traveling over road section 11. This 
value VM is the mean of the entrance and departure speeds, i.e. 
VM=(Vem+Vdm)/2. The second storage circuit 6, receiving from circuit 5 the 
binary pulse train representing the lengths of vehicles entering the road 
section 11, is stepped by a clock 26 whose frequency is directly 
proportional to the mean vehicular speed VM and thus inversely 
proportional to the mean transit time. In the same way the third storage 
circuit 7, connected to the output of the second storage circuit 6, is 
stepped by a clock 27 whose frequency is directly proportional to the 
average speed at which the vehicles leave the road section 11 at its end 
D. 
The occupancy-determining circuit 12 continuously divides the sum of the 
lengths Li of the vehicles on road section 11 by the length L thereof. For 
this purpose, circuit 12 continuously receives the contents of the several 
shift registers of memory unit 10 in the form of parallel output pulses. 
The combined length of the vehicles occupying this road section is then 
determined by simply counting the measuring pulses contained in circuits 
5-7 of memory unit 10 and available at their outputs at any time. To this 
end the occupancy-determination circuit 12 can incorporate a 
microprocessor-type calculator establishing the ratio between the sum of 
the lengths Li of the vehicles and the length L of the road section 11. 
The storage capacity of circuits 5, 6, 7 depends on the number of unity 
pulses envisaged for representing a given length. It is obvious that the 
accuracy of the measurement of the lengths of vehicles entering the road 
section 11 is directly dependent on the length of a pulse cycle of the 
local measuring circuit 1 and consequently on the sampling frequency of 
that circuit. Any increase in the accuracy of the vehicular-length 
measurement and therefore of the determination of the occupancy density 
DE(t) also entails an increase in the storage capacity and consequently in 
the overall dimensions and costs of the components of memory unit 10. The 
choice of this storage capacity and therefore the measuring accuracy is 
determined by the road section to which the apparatus is to be applied. 
As the pulse train on lead 1b representing the length Li of a given vehicle 
passes through the cascaded storage circuits 5, 6 and 7 with delays 
dependent on the instantaneous operating frequencies of clocks 25, 26 and 
27, proper correlation of these operating frequencies with the mean speed 
values Vem, VM and Vdm will indeed let the contents of memory unit 10 
reflect at any time the distribution of vehicles 2 on road section 11. 
It is clear from the preceding description that the monitoring circuitry 60 
of FIG. 1 is limited to the surveillance of a single lane. Thus, the shift 
registers 5, 6, 7 can only represent the images of vehicles traveling one 
behind the other. The occupancy-density signal DE(t) from circuit 12 is 
transmitted in the form of binary words to the circuit 20 calculating the 
encumbrance value P(t). The computer stage 20 thus supplies a value P(t), 
equal to the occupancy density DE(t) divided by the average speed VM of 
the vehicles on road section 11, which recurs at the cadence of the binary 
pulses emitted on lead 1b by the local measuring circuit 1. This 
encumbrance value P(t) is multiplied by a constant .alpha., contained in a 
memory 61, designed to provide more easily manipulatable values. A 
preferred value for this multiplication factor is .alpha.=10. The 
encumbrance signal P(t) controls a traffic light 21 at a crossroads 
approached by road section 11. This control could be such, for example, 
that the changeover time of traffic light 21 varies progressively as a 
function of the values of the encumbrance signal P(t) indicative of the 
degree of loading of that road section. 
According to the preferred mode of operation illustrated in FIG. 1, 
however, traffic light 21 is indirectly controlled from computer stage 20 
by way of comparator 47 which continuously compares the encumbrance values 
P(t) from calculating circuit 20 with predetermined constant thresholds 
P.sub.0, P'.sub.0 contained in a memory 62 illustrated in FIG. 2. The 
comparator 47 then supplies a control signal to the load represented by 
traffic light 21 if P(t) is equal to or greater than threshold P.sub.0 or 
is equal to or less than threshold P'.sub.0, i.e. if the output signal of 
stage 20 deviates from a predetermined operating range. This latter 
control mode for the signaling system represented by traffic light 21 is 
simpler and can be more easily adapted to existing installations. 
FIG. 2 shows a second embodiment of the apparatus according to my invention 
which is more particularly usable on a road section 11' of length L having 
several lanes. 
This apparatus comprises a circuit arrangement 60', including the 
aforedescribed measuring circuits 1 and 3 at opposite ends E, D of road 
section 11' which determine the length and the entrance and exit speeds of 
vehicles 2 to enable a determination of the magnitudes of the occupancy 
density DE(t) and of the average overall speed VM. This circuit 
arrangement also includes the computer stage 20, calculating the 
encumbrance P(t), shown connected to memory 61 for supplying an amplified 
encumbrance signal 
EQU .alpha..multidot.P(t)=.alpha..multidot.DE(t)/VM=.alpha..multidot..SIGMA.Li/ 
L.multidot.VM (2) 
This signal is transmitted to a comparator 47, identical with that of FIG. 
1, and in parallel therewith to a calculating circuit 48 which supplies at 
a load terminal 50 an output signal corresponding to a function F(t) more 
fully described hereinafter. The aforementioned memory 62, in which are 
stored the predetermined thersholds P.sub.0 and P'.sub.0, is connected to 
one of the inputs of comparator 47. 
Monitoring circuitry 60' comprises arithmetic means including two circuits 
32 and 33 with inputs connected to leads 1a and 3a for determining the 
average speed at which the vehicles enter and leave the road section 11', 
these circuits being controlled by a clock 31 determining the time T 
during which the average speeds are calculated. The outputs of averaging 
circuits 32 and 33 are connected to a circuit 34 which calculates the 
average overall speed VM defined by the half-sum of the mean entry and 
exit speeds Vem and Vdm as described with reference to circuit 9 of FIG. 
1. The output of circuit 34 is connected to a comparator 36 which is also 
connected to a memory 37 in order to compare the value of average speed VM 
with a predetermined threshold V.sub.0 stored therein. 
The output of comparator 36 is connected on the one hand to the 
encumbrance-calculating circuit 20 and on the other to a circuit 38 
dividing the length L of road section 11', contained in a memory 39, by 
the average overall speed VM. The resulting quotient T.sub.0 is fed to 
processing means including a controlled group of switches 40. The latter 
also receive from lead 1b of measuring circuit 1 the signals corresponding 
to the length Li of the vehicles 2 entering road section 11'. This group 
of switches 40 are connected to a multiplexer 45 via a set of identical 
adders 41-44. A clock circuit 35 is connected on the one hand to the 
control inputs of switches 40 and on the other hand to the control input 
of multiplexer 45. The output of multiplexer 45 is connected to a circuit 
70 for calculating the occupancy density of road section 11', the computer 
stage 70 being also connected to memory 39 containing the value L of the 
length of the road section 11'. The output of circuit 70 is connected to 
the circuit 20 for calculating the encumbrance P(t). 
A first test is performed on the output signal of circuit 34 by the 
comparator 36 designed to set off an alarm if the average speed VM is 
below the predetermined threshold V.sub.0, thereby indicating a 
congestion. This alarm signal is available at a terminal 80. The quotient 
calculated by circuit 38 corresponds to the mean transit time T.sub.0 
needed by a vehicle 2 to trasverse the road section 11'. Each adder 41-44 
receives the vehicular-length signals coming from measuring circuit 1 
during such transit time T.sub.0. The transit times T.sub.0 during which 
the adders sum the lengths Li of the vehicles entering road section 11' 
are relatively staggered by equal delays T.sub.1. The period T.sub.0 
during which one of these adders sums up the vehicular-length signals Li 
is accordingly offset by the delay T.sub.1 from the period during which 
the following adder performs a similar summation. 
Thus, after a delay T.sub.1 from the instant at which the final adder 44 
starts the summation of the vehicular-length signal Li, the first adder 41 
recommences the summation of these signals. 
Adders 41-44, accordingly, operate in cyclic succession during time 
intervals of constant duration staggered relatively to one another by the 
invariable delay T.sub.1, dependent on the number N of those adders. This 
number N (here equal to four) consequently determines the repetition 
frequency of the information concerning the measurement of the occupancy 
density DE(t) available at the output of multiplexer 45. Thus, multiplexer 
45 supplies at the cadence of signals from clock circuit 35 the results of 
different summations, performed during respective time intervals, of the 
lengths of vehicles entering road section 11'. The value of the occupancy 
density DE(t) is determined by circuit 70 feeding that value to computer 
stage 20 for the calculation of the encumbrance P(t) which is performed in 
the same way as in the embodiment of FIG. 1. The use of that parameter in 
the control of a signaling system for traffic regulation can be 
implemented either directly by comparator 47, via an output terminal 49 
thereof, or by means of calculating circuit 48 transforming the 
time-varying encumbrance P(t) into a function F(t) selected to vary almost 
linearly with the traffic conditions on road section 11'. 
Experiments have shown that it is advantageous to select a logarithmic 
function although a function F(t)=.alpha..sqroot.P(t) can also be used. I 
therefore prefer to design calculating circuit 48 in such a way that the 
signals available at its output terminal 50 correspond to a function 
F(t)=.beta..multidot.log [1+P(t)] where .beta. is another predetermined 
constant. The signals from circuit 48 can then control the associated 
traffic-regulating system either directly or via a nonillustrated 
comparator similar to component 47. 
It will be noted that in the monitoring circuitry 60' of FIG. 2 the 
division by the length L of road section 11', according to equation (2), 
is carried out by computer stage 70 rather than in stage 20 as in FIG. 1. 
In the embodiment of FIG. 1 the occupancy-determination circuit 12, 
especially when it incorporates a microprocessor-type calculator with 
associated peripheral units, can directly supply a control signal to the 
traffic-regulating system (e.g. to traffic light 21) in case the length of 
an entering vehicle, represented by a series of unity pulses on lead 1b, 
equals or exceeds a predetermined value permanently stored in a memory 
incorporated in the occupancy-determination circuit 12. This can be of 
particular interest in the case of a road section carrying numerous heavy 
vehicles such as buses or trucks. Such a signaling mode would supplement 
the control of the traffic-regulating system by the evaluation of the 
encumbrance function as described above.