U.S. patent number 3,626,413 [Application Number 05/007,586] was granted by the patent office on 1971-12-07 for traffic surveillance and control system.
Invention is credited to Howard C. Zachmann.
United States Patent |
3,626,413 |
Zachmann |
December 7, 1971 |
TRAFFIC SURVEILLANCE AND CONTROL SYSTEM
Abstract
An all-weather system utilizing doppler radar devices for
monitoring traffic on highways to provide warnings and diversionary
information to motorists. Radar equipment mounted on gantries at
selected locations along the highways provides traffic information
on the number, type and velocity of vehicles and together with
complementary equipment such as a data-linked computer develops
information on traffic density and required clear distance for
trailing vehicles, continuously taking into account local
environmental conditions. The latter "tailgate" information as well
as velocity and diversionary information is provided at the
gantries as a visual display. Counting of vehicles by class is
performed both by doppler pulse amplitude and interval at a
monitored speed techniques and two alternate configurations for
providing the tailgate function are disclosed, one being completely
self-contained at the gantry.
Inventors: |
Zachmann; Howard C. (Ellicott
City, MD) |
Family
ID: |
21727039 |
Appl.
No.: |
05/007,586 |
Filed: |
February 2, 1970 |
Current U.S.
Class: |
342/104; 340/934;
342/58; 701/118; 340/933; 340/936 |
Current CPC
Class: |
G08G
1/0104 (20130101) |
Current International
Class: |
G08G
1/01 (20060101); G01s 009/44 (); G06f 015/48 () |
Field of
Search: |
;343/6A,8,9
;235/150.24 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hubler; Malcolm F.
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. A traffic control system for developing information of existing
traffic conditions comprising means responsive to the movement of
vehicles along a roadway for producing a series of signals
indicative of such traffic, one of said signals being
representative of the velocity of a vehicle, means responsive to
said one signal for generating a time interval having a duration
related to said one signal, means for sensing the condition of the
production of a second one of said signals within such time
interval, and means responsive to said sensing means for providing
an indication of such condition.
2. The control system as set forth in claim 1 wherein said
indication means comprises a device for presenting information in
visual form, said device being disposed near the roadway being
monitored for viewing from the vehicles.
3. The control system as set forth in claim 2 wherein said time
interval generating means comprises a computer programmed to
develop an output as a function of the velocity of the vehicle,
said output being operative to condition said sensing means for
receipt of said second signal.
4. The control system as set forth in claim 3 further including
means responsive to said signal producing means for discriminating
among signals representative of the type of vehicles, said
discriminating means being operative to modify the output of said
computer.
5. The control system as set forth in claim 3 further including
means for registering environmental conditions in the area of the
traffic, said registering means being operative to modify the
output of the computer.
6. A system for monitoring the movement of vehicles along a roadway
comprising means for directing a train of signals at a
predetermined location of the highway which signals are intercepted
and reflected by moving vehicles, means for receiving the reflected
signals from the vehicles, doppler frequency sensing means coupled
to said receiving means for providing an indication of the velocity
of said vehicles, discriminating means coupled to said sensing
means for categorizing the types of reflected signals into various
classes of vehicles, means responsive to said doppler means and
said discriminating means for determining a time interval having a
duration related to the velocity and class of vehicles, means
operative within such time interval for sensing the receipt of a
reflected signal at said receiving means, and indicator means
responsive to said sensing means for registering such
condition.
7. The system as set forth in claim 6 wherein said indicator means
comprises a display proximate to the roadway in the line of sight
of operators of the vehicles.
8. The system as set forth in claim 7 wherein said discriminating
means comprises a first detector coupled to said receiving means
for monitoring the amplitude of the reflected signals and for
providing output signals representative thereof.
9. The system as set forth in claim 8 wherein said discriminating
means further comprises a second detector coupled to said receiving
means for monitoring the interval of the reflected signals, said
second detector being responsive to said doppler means for
providing output signals representative of classes of vehicles, and
means for combining the outputs of said first and second detector
means to provide a further classification of the vehicles.
10. The system as set forth in claim 9 further including means for
monitoring environmental conditions in the area of the roadway and
for providing a signals representative thereof, said time interval
determining means being further dependent upon said monitoring
means.
11. The system as set forth in claim 10 wherein said time interval
determining means comprises a computer programmed to combine the
indications of velocity, vehicle class and environmental conditions
to provide an indication of a time interval related thereto, said
sensing means being operative in response to said computer
indication.
12. The system as set forth in claim 11 further including a second
computer operatively connected to receive the indications of
velocity and vehicle class from said doppler means and said
discriminating means respectively, said second computer being
programmed to accumulate such indications and compute density of
vehicles and flow rates for the roadway.
13. A system for monitoring vehicular traffic comprising a radar
system for producing sequential echo signals from the moving
vehicles, means responsive to the echo signals for indicating the
velocity of the vehicles, means responsive to the echo signals for
indicating the class of vehicles, means operatively connected with
said class means and said velocity means for detecting sequential
indications within a time interval determined by such indications,
and means operatively connected with said detecting means for
providing a visual display representing such sequential
indications.
14. The system as set forth in claim 13 wherein said detecting
means comprises a variable time delay circuit, said circuit being
triggered into operation by receipt of a first indication, a gate
responsive to operation of said circuit and a second indication for
providing a signal to said display means, and means for varying the
delay of said circuit as a function of the indications.
15. The system as set forth in claim 14 wherein said delay circuit
is a one-shot circuit and said varying means comprises a variable
impedance element operatively connected to control the discharge
time of said one-shot circuit.
16. The system as set forth in claim 14 wherein said delay circuit
is a digital counter and said varying means comprises a computer
operatively connected to control the maximum count of said counter,
said computer being operative in the interval between sequential
indications.
17. The system as set fourth in claim 13 wherein said detecting
means comprises an information register for storing the indications
of said class means and said velocity means, a sequence circuit for
interrogating said register, a computer connected to said sequence
circuit for developing an interval instruction as a function of the
information in said register, a circuit responsive to said interval
instruction and a sequential indication to provide an output signal
to said display means, and means for synchronizing said computer
with the receipt of the sequential indications.
18. The system as set forth in claim 17 wherein said synchronizing
means comprises a clock source operatively connected with said
information register and said computer and operative to determine
the magnitude of the interval instruction.
19. A system for classification of vehicles comprising means for
developing a pulse group of echo signals from a moving vehicle,
said signals having amplitude related to frontal reflective surface
of the vehicle, doppler frequency related to velocity and duration
related to length and velocity, means for sensing amplitude of the
signals and for providing output signals representative of the
frontal reflective surface of the vehicles, doppler frequency
detector means for determining the velocity of the vehicles, means
for sensing the duration of the signals to provide indications
proportional thereto, means for combining the indications of said
duration sensing means and said doppler detector means to provide
output signals representative of the length of the vehicles, and
means for combining the output signals to provide a combined
classification of the frontal surface and length of the
vehicles.
20. The system as set forth in claim 19 wherein said first named
means comprises a radar unit located above a lane of traffic to be
monitored and directed at a small angle below a horizontal line
toward oncoming traffic to provide both frontal and longitudinal
reflection from the vehicles.
Description
This invention relates to traffic monitoring and motorist assisting
systems and more particularly to a system operating on the doppler
principle which provides a segregation of trucks and pleasure cars
and performs a computation upon the data received to provide useful
traffic information in a visual display for immediate aid to the
motorist as well as a tabulation of such traffic data at a central
control station for statistical surveys, and traffic control in
real time.
While much consideration has been given to the surveillance of
traffic flow on the principle of apprehending traffic offenders as
well as obtaining knowledge of highway use and efficiency, little
consideration has been given to supplying meaningful information to
the vehicle operator so that he can make his own decision as to
routes of preference and the like as well as to provide a
continuous monitor of his vehicle operability characteristics.
It is well known in many of the present day highway systems that
saturation conditions often exist and that motorists endure such
situations even though alternate routes exist. While little
effective results can be obtained with variations in the interstate
traffic patterns, most of the traffic around urban areas of the
country comprises operators who are generally familiar with
alternate routes of travel and who would avail themselves of same
if sufficient information were provided of congested areas and
traffic flow patterns which are outside their range of
recognition.
It is also well known that the high velocities of vehicle operation
on today's highways are a major factor in causing accidents and
traffic congestion, not only from a failure of mechanical equipment
but more so from a lack of education and familiarity of the greatly
changed conditions occurring not only at the different rates of
speed but also under the varying environmental conditions. A recent
survey has indicated that a six fold increase in traffic mishaps
accompanies each 10 mile per hour increment of parallel vehicles
velocities and it is well known that accident rates are accentuated
under poor environmental conditions such as the change from day to
night and vice versa and the sudden encroachment of fog or other
adverse weather conditions.
While it is not suggested that supplying certain information to the
motorist will obviate all traffic mishaps nor that it is practical
to supply all possible information to the operator, it is clear
that certain fundamental data would be extremely invaluable as a
check upon the operating characteristics of the individual vehicle
as well as an instruction as to its proper operation, and would
provide the advantages of supplying information on environmental
conditions and traffic flow patterns beyond the operator's area of
immediate concern. The typical original equipment speedometer in
most vehicles is an instrument highly subject to error both from a
mechanical design standpoint and the influence of tire wear and
environmental factors so that an overall accuracy of approximately
10 percent is not considered unusual. It would be beneficial to
advise the operator from time to time of an extremely accurate
appraisal of his operating velocity and instrument error both for
his own personal benefit and for his influence upon other traffic
in the vicinity.
The tailgating information has been conspicuously absent in the
past requiring a complicated evaluation not only of a particular
vehicle's velocity and type, but also of the characteristics of the
environment at a particular location. While vehicle braking figures
have been published in the past and guides have been given as to
the proper operation of vehicles at various highway speeds, such as
for example recommending a certain multiple of car lengths
separation for different speeds of operation, such information is
easily forgotten or if retained is difficult to apply realistically
under driving conditions. Further, many people choose to ignore
such guidelines preferring to depend instead upon their own
evaluation of the driving conditions and their enhanced estimate of
speed of reaction and vehicle capabilities. The rapid calculation
of an almost instantaneous display of such encroachment upon the
minimum safe distance behind a moving vehicle would provide an
essential piece of information for the concerned driver and a
signal of warning to those in the immediate vicinity. Further, such
information can be invaluable as an aid in signalling an
unrecognized approach into a dangerous condition causing an
immediate response in the operator of the vehicle similar to the
sudden illumination of a stop light in the vicinity.
Still further, while all drivers realize the convenience and
desirability of having a well marked highway wherein alternate
routes and approaching conditions are displayed, little attention
has been given in the past to providing a highly versatile and
refined system which can supply any form of desired information to
the operator from instructions developed at a central control
station.
Therefore it is one object of this invention to provide an improved
traffic control system which provides general information to all
traffic in a particular vicinity and specific information to
individual vehicles in a lane of traffic.
It is another object of this invention to provide an improved
traffic surveillance and control system which operates on the
doppler principle for measuring vehicle velocity and utilizes the
same received signals to discriminate among the various classes of
vehicles.
It is yet another object of this invention to provide an improved
traffic control system which senses definite characteristics of
vehicle movement and operates in conjunction with a computer to
provide information to the operators, calculated as a function of
such data and the environmental conditions existing at that
instant.
It is still another object of this invention to provide an improved
traffic control system which provides a visual indication of the
presence of a vehicle within a calculated unsafe distance behind a
preceding vehicle.
It is a still further object of this invention to provide an
improved traffic control system which operates to gather
information on a per lane basis as to vehicle class, velocity and
tailgate encroachment and accumulates this data on a lane, highway
or traffic system basis to provide statistical data as to density
of vehicles, flow rates of movement, and efficient use of highway
systems.
Other objects and advantages of the present invention will become
apparent as the following description proceeds.
To the accomplishment of the foregoing and related ends, the
invention, then, comprises the features hereinafter fully
described, the following description and the annexed drawings
setting forth in detail certain illustrative embodiments of the
invention, these being indicative, however, of but one of the
various ways in which the principles of the invention may be
employed.
In said annexed drawings:
FIG. 1 is an environmental view of a portion of a highway system
showing a single gantry for monitoring traffic on a dual lane
highway and for providing visual information;
FIG. 2 is a block diagram of the logic of the system for surveying
and controlling traffic in two adjacent lanes;
FIG. 3 is a block diagram of the preferred embodiment of the
tailgate computer depicted in FIG. 2;
FIG. 4A is a curve showing a typical response characteristic from
moving vehicles as received at the radar detector;
FIG. 4B is a timing graph showing the relation of pulse signals
from trailing vehicles with the determination of the tailgate
interval;
FIG. 5 is a first alternative embodiment for determining the
tailgate function; and
FIG. 6 is a block diagram of a second alternative embodiment for
determining the tailgate function wherein all circuit components
are contained at each gantry.
Referring now to FIG. 1 there is shown an environmental view of a
portion of a highway system which in this instance is a dual lane
highway 10 having vehicles 11 moving therealong at varying
velocities. Spanning the highway is a gantry 12 for support of a
portion of the apparatus of the invention, the latter comprising a
generally rectangular enclosure 14 centrally located above the
lanes of the highway and comprising in part on the forward portion
thereof a display board 15 for presentation of visual information
to the motorists in the vehicles.
The display board 15 may be any one of many commercially available
units but preferably is of the type which provides an illuminated
display. The board 15 contains an area 16 to provide a variety of
printed information at the upper portion thereof common to both
lanes, and for each lane an illuminable tailgate indication 18 and
digit indication 19 for providing a readout of monitored velocity
of passing vehicles. Preferably there is sufficient separation
between the displays for the individual lanes so that there will be
no confusion of interpretation of data represented on the display
board and as pointed out in greater detail hereinafter illumination
of the signals will be sufficiently synchronized with movement of
the vehicles beneath the display board so as to avoid
misinterpretation of the information.
Also associated with each lane of the highway 10 and mounted above
the housing 14 are the transmitting and receiving units 20, 21 for
the radar signals which provide surveillance of the traffic on the
highway. The radar system is of the conventional type for detecting
the movement of traffic preferably operating in the range of from 8
to 12 gHz to provide the typical doppler spread of received
frequencies which are proportional to vehicle speeds.
The radar transmitting and receiving units 20, 21 are preferably of
the highly directional type and are aimed at a location down the
highway 10 from which the vehicles 11 are moving at an angle from
the horizontal sufficient to provide a suitable received signal
from the vehicle while not creating too great a masking effect
between trailing vehicles. Since it is desired to obtain two types
of signals along with the doppler signal of frequency variation
some compromise must be made between elevation of the transmitting
and receiving units 20, 21 above the roadway surface and angular
direction of same along the lane of traffic. The additional signals
desired are a frontal surface reflection from the vehicles 11
providing a variation in amplitude of the received signal and an
interval signal indicative of the length of time the vehicles 11
are exposed to the radar signals, the latter being related to the
length of the vehicle.
A typical example of such received energy is depicted in FIG. 4A
wherein a curve 22 includes a first burst 23 of reflected energy of
relatively large amplitude and a second burst 25 of received energy
at a later time of relatively small amplitude. Assuming some
correspondence between the capability of an exposed area of a
vehicle to reflect a quantity of energy proportional to such area,
such difference in reflected amplitudes of received signals may be
detected by a simple level detector circuit to provide
discrimination among the various types of vehicles, providing
information, for example, of the presence of passenger cars versus
trucks.
It is significant to note also in FIG. 4A that the bursts of energy
23, 25 occur over different intervals, the interval 26 of the first
burst 23 being substantially larger than the second interval 28 and
being indicative of the time of receipt of reflected energy.
Assuming the two vehicles which produce the bursts 23, 25 of energy
were moving at the same velocity when monitored, then such bursts
would be related to the times that the vehicles were in the beam
area of the radar system to thus provide a measure of the overall
lengths of the vehicles.
As is well known to those skilled in the art, the frequency of the
received bursts 23, 25 is modified from the frequency of the
transmitted signal as a function of the velocity of the vehicle
and, by suitable detection circuitry, can provide a direct
indication of such velocity. Thus upon receipt of an isolated burst
of reflected energy, an immediate measurement of velocity of the
vehicle can be performed and this information can be utilized in
conjunction with the duration of receipt of the reflected signal to
provide a discrimination between the various types of vehicles, in
this instance providing a relative measure of the length of same
and an indication, for example, of a truck or passenger car.
While no definite limits have been placed on the elevation of the
radar devices 20, 21 above the roadway surface, it has been
determined that an elevation of from 20 to 60 feet will provide the
useful data in most environments and a figure of 30 feet of
elevation may be adopted as nominal. Similarly the most useful
angle of direction of the radar devices 20, 21 with respect to the
horizontal appears to lie in the range of from 9.degree. to
21.degree., and with an angle of 10.degree. adopted as nominal
produces approximately a 35 foot length of roadway sensitive to the
radar signals at minus 3 db. beam levels, assuming a 2.degree. beam
width. With such nominal factors, approximately a 1/2 -second echo
signal is received from a vehicle passing through the sensing area
at approximately 50 m.p.h.
Referring now to FIG. 2, there is shown in block diagram form the
interconnection of components for a single gantry 12 capable of
monitoring traffic flow in a pair of adjacent lanes of traffic as
depicted in FIG. 1. Only the components for lane 1 are shown in
full together with those components common to both lanes and it
will be understood that such system is duplicated in part for a
processing of information of the lane 2 traffic and may be expanded
for any number of lanes of monitoring.
The components mounted at the gantry 12 include the radar generator
30 which is a conventional microwave generator operating in the
range of 10 gHz and adapted to energize the radar transmitters 31,
32 for both lanes 1 and 2. Power for the radar generator 30 and the
remainder of the components on the gantry 12 is obtained in any of
the conventional manners preferably being energized from a nearby
transmission line. The radar generator 30 is adapted to supply
continuous signals to the transmitters 31, 32 so as to provide a
continuous surveillance of the traffic conditions in both lanes and
the echo or reflected signals, indicated by the arrows 33, 34 are
recognized at the receiver units 35, 36 as bursts of high-frequency
energy having the doppler frequency variation characteristic of the
rates of movement of the reflecting vehicles 37, 38 along the
highway, as graphically depicted in FIG. 4A.
The receiver units 35, 36 comprise minimally antennas sensitive to
the microwave energy, being highly directional to receive only the
desired signals from one lane and further may comprise amplifiers
or demodulating units for combining the reflected signals with the
signal of the radar generator 30 so as to produce sum or difference
frequencies which contain the information characteristic of the
vehicle movement. As indicated in FIG. 2, the output of the radar
receiver 35 is applied to a plurality of components as indicated by
the distribution line 39 and similarly the output of the receiver
36 for lane 2 is indicated as being applied to similar circuitry
(not shown) by the dashed line 40.
The information on line 39 is applied to a speed module 41 wherein
a computation is performed to provide an output on line 42
indicative of the instantaneous velocity of the vehicle 37. This
computation can be performed directly from the information
available in the received pulse, consisting generally of a
frequency to voltage conversion. Preferably the output of the speed
module 41 and other components are digitized into binary signals
for information handling purposes and for compatibility with the
computers and display board to be described, however, it is clear
that the selection of the format of the signals within the system
is primarily a matter of choice taking into account factors such as
accuracy, economy and efficiency of operation.
The output of the speed module 41 is directed by way of line 44 to
a gate module 45 and eventually to the speed indicating portion 46
of the display board 47 to provide a visual indication to the
motorist while he is still in the range to view the information
provided. It will be clear that additional holding and
synchronizing circuitry may be incorporated in a system of this
type for retaining the desired indications on the display board 47
for predetermined intervals so as to be available to the motorist
and to avoid confusion of interpretation of the data by preceding
or following motorists.
It is also desired to identify the class or type of vehicle 37
producing the reflected modulations, which information is useful
not only for statistical surveys of the use of the highways, but
also is significant in producing the tailgate display 48 as
consideration is given to the different driving conditions effected
with various types of vehicles.
The information on line 39 is applied to a circuit 49 identified as
a class by amplitude discriminator which circuit 49 is effective to
sense the various amplitudes of the received signal and categorize
the same as indicative of the types of vehicles, ranging from the
large signal received from a tandem trailer, for example, to the
small amplitude signal received from a compact automobile. The
discriminator circuit 49 may comprise simply a voltage level
detector circuit providing a digitized output signal indicated at
line 50 for application to further circuitry.
A second identification of vehicle classification is made on the
basis of width of reflected signal, such circuitry comprising an
interval sensor circuit 51 adapted to receive the output of the
receiver unit 35 and functionally operative to convert the received
signal to an indication in time. Such signal by itself is useful
only at a known velocity of movement of the vehicles and therefore
further circuitry identified as a class by interval discriminator
52 is utilized to convert the signal from the interval circuit 51
to a digitized output on line 54 as a function of the speed signal
received from line 42. The output of the interval discriminator 52
is then in substantially the same format as the output of the
amplitude discriminator 49 and both are applied to the class
identification circuit 55 to provide an output, as at 56,
identified as to class of vehicle, for each vehicle which is
sensed. While the discrimination of vehicles by two separate
systems of classification may be redundant in part, the additional
sensitivity in definition provides a more accurate identification
than is possible with either singular system. The class
identification circuit 55 may simply comprise an AND gate combining
circuitry for providing additional classifications of vehicles, or
the input information may be combined in other manners to provide a
partially redundant measurement for a smaller classification of
vehicles, the only requirement for such circuit 55 being that of
providing a single output in one of the specified classes which
output can be accumulated as a count of the traffic occurring on
the highway.
The components on the gantry 12 further include a data link unit 57
comprising a transmitter 58 and receiver 59 for the transmission
and receipt of information both to a tailgate computer 60 which is
operative to perform the tailgate function for a plurality of
gantries located in a common area and to a central computer 61 for
tabulation, further analysis and command information. The central
computer 61 is remotely located and adapted to receive information
from all gantry sensing stations in an area wide traffic control
system, as for example, the complete traffic system associated with
a particular city or area of the country. The transmitter 58 and
receiver 59 of the gantry system depicted in FIG. 2 preferably
comprise a microwave data link and are operative to act upon the
information received both from the lane 1 and lane 2 components,
preferably operating on a time sharing basis. As indicated, the
output of the class identification circuit 55 and the output of the
speed module 41 are applied to the transmitter 58 on lines 56, 42,
together with auxiliary available information such as, for example,
an excessive speed signal derived from module 62 adapted as a level
detector to provide a pulse output whenever a nominal predetermined
operating speed is exceeded.
The output of the receiver 59 of the data link is connected via
line 64 through the gate 45 to the tailgate portion 48 of the
display board 47 to provide, upon command from the tailgate
computer 60, the visual indication to the motorists while still
within the viewing angle of the board. Another output 65 of the
receiver 59 is connected to the information portion 66 of the
display board 47 providing the instructional and informative
information for motorists assistance, such information primarily
being received from the central computer 61, being manually entered
by a traffic controller who can monitor a widespread area of the
system and provide suggestions as to alternative routes or
indications of traffic tie-ups and the like.
A further component of the gantry control system includes a low
speed lockout circuit 68 which is effective to control the
indications of speed and tailgating on the display board 47 by
means of opening or closing the gate 45. The lockout circuit 68 is
utilized under high density traffic conditions wherein very slow
movement of vehicles is encountered due to an accident or traffic
backup, the circuit comprising essentially a detector for sensing
closely spaced pulse groupings on line 39 and operative to provide
an on-off signal on output line 69. Under such conditions, the
information on the display board 47, other than the general
instructional information, is essentially useless and confusing to
the motorists and calls for rapid response of the mechanism for
changing the indications on the board.
The tailgate computer 60 is a general purpose digital computer
programmed to determine an unsafe following distance behind any
specific class of vehicle travelling at any velocity as a function
of the local environmental conditions including weather conditions,
the amount of daylight available, road surface conditions and the
like. A preferred embodiment of the tailgate computer 60 depicted
in FIG. 2 is shown in greater detail in FIG. 3 comprising
components having sufficient capacity to handle the information
supplied from a plurality of gantries in a localized area and
preferably operative on a time sharing basis.
The function of the tailgate computer 60 is depicted in the graph
in FIG. 4B wherein a timing diagram is shown of the receipt of
information from a single line of traffic, a pulse 70 occurring at
the initial portion of the graph indicating the presence of a
specific form of vehicle moving at a known velocity. It is desired
to calculate instantaneously a distance related to such vehicle
which would be dangerous for a trailing vehicle to encroach, based
on knowledge of good driving practices, vehicle mechanical
capabilities and speed of human response. It is also desired to
modify such distance calculation as a function of the environmental
conditions existing at the time, including the weather, available
light and road surface conditions. For ease of information handling
this distance can be treated as a time interval indicated by the
line 71 in FIG. 4B and the recognition of the presence of any type
of trailing vehicle within such interval as indicated by the dashed
pulse 72 will suffice to satisfy the conditions of the tailgate
computer 60. The end of the interval 71 also signifies a cutoff
condition whereupon the receipt of a later pulse 74 as indicated in
dashed lines is ignored for satisfying the tailgate function. Of
course, the pulse 74 of the trailing vehicle is treated in an
identical manner as the first received pulse, and the system
contains sufficient capacity to create a new time interval and
provide the desired visual presentations for such succeeding
vehicles.
As indicated also in FIG. 4B, the tailgate computer 60 operates on
a time sharing basis in servicing a plurality of gantries and it is
to be noted that the information can be received from the specific
gantry and relayed back as indicated by the dashed vertical lines
75, in sufficient time to prepare the components of the system to
perform the tailgate function prior to receipt of the pulses from
the trailing vehicles.
Essentially then it is only necessary to calculate a definite time
interval for a received set of input data, synchronize such time
interval with the receipt of the first pulse and sense whether a
second pulse of information is received within that time interval
in order to actuate the tailgate display.
In the preferred embodiment of the tailgate computer depicted in
FIG. 3, a data link receiver 76 and transmitter 78 is provided for
communications with a plurality of gantry locations, discrimination
between signals being effected by frequency separation, coded
signals or the like. For purposes of economy, only a single
computer 79 is utilized for a plurality of gantries and
conveniently, other register and storage circuitry 80 can be
located therewith for ready access to the computer. The block
diagram showing in FIG. 3 includes sufficient capacity to handle
the information from four lanes of traffic and it will be
understood that such circuitry may be expanded to accommodate any
number of lanes of traffic having consideration for the capacity of
the computer and the ability to rapidly treat the information.
A sequencer unit 81 is shown associated with the computer 79 for
cyclically interrogating the storage registers 80 for each
individual lane of traffic and for storing the computed interval
for control of the display function. The registers 80 include
information registers 82 connected to the receiver 76 of the data
link to randomly receive and store the class identification and
velocity information as received from each lane of traffic and
simultaneously to record the time of receipt of such information,
such latter data being provided by a clock source 84 connected to
all of the registers 80 and to the computer 79. Upon interrogation
of the data in the information register 82 and application of same
to the computer 79 for calculation purposes, the information
register 82 will be reset to a condition in preparation for receipt
of additional information from the lane of traffic. Such
information register 82 thus may comprise a conventional binary
register of the integrated circuit type, for example, having
sufficient capacity to contain the desired information.
The computer 79 acts upon such information along with information
of the existing local conditions, provided at 85, to calculate the
proper time interval, synchronize the time interval with the time
of receipt of the information by means of the clock 84, and set the
remaining interval into interval registers 86 for control of the
tailgate function. In this embodiment of the invention, the
interval registers 86 thus may comprise a binary counter which are
presettable to any desired count within their capacity, which count
is continuously diminished by input pulses received from the clock
source 84.
Logic elements 88 comprising AND gates are connected to sense the
presence of a count in the interval registers 86 and receive at a
second input pulse information from the receiver 76 so that when a
succeeding pulse is received within the desired interval, an output
pulse on one of the lines 89 is developed for application to the
transmitter 78. The signal is then transmitted to the appropriate
gantry receiver 59 in order to activate the tailgate display 48 for
that lane of traffic. When the interval register 86 is counted down
to zero count, indicating the completion of the interval, the
AND-gate 88 will be in an off condition to prevent the transmission
of succeeding pulses. It will be evident then in this embodiment of
the invention that input information is entered randomly into the
information registers 82 as received, is interrogated cyclically by
the sequencer 81, operated upon and synchronized by the computer 79
and stored in the interval register 86 for receipt of a further
pulse of random information.
The environmental factor upon the calculation if indicated as the
entry into the computer 79 of information received from a source 85
labeled local conditions. This source 85 may provide data from
manually adjustable devices relating to the type of roadway upon
which the traffic is being monitored; continuously variable
information from humidity, fog, rain and ice detectors; as well as
data from a clock or photoelectric light sensor for introducing the
light factor into the computation.
While the exact programming of the computer 79 is not described in
detail, it will be apparent to those skilled in the art that such
factors can be readily accommodated to have the effect of either
lengthening or shortening the time or distance interval of unsafe
conditions. The weight given to each of such individual factors may
be determined from engineering and/or statistical studies and may
be varied to some extend to suit known results from an operating
system. However, it will be generally appreciated that the computer
79 may be programmed along the following outline: Faster speeds
call for a greater unsafe distance interval which is converted to
an interval of time at any particular speed. Also, a relatively
longer time interval indicating a larger unsafe following distance
is desired when the class of vehicle is large, the road surface is
of less than the best grade, the weather conditions are poor, and
the available light is dim. Shorter intervals are indicated when
the converse conditions prevail.
Referring now to FIGS. 5 and 6, two alternative embodiments for the
tailgate computer 60 are depicted generally being connected into
the system shown in FIG. 2 as the dashed block 90 designated
alternate tailgate by the dashed line interconnections. These
alternative embodiments have the advantage of locating much of the
logic circuitry at the particular gantry thereby reducing the
complexity of a remotely located computer and the embodiment
depicted in FIG. 6 obviates completely the necessity for a remote
computer as at 60 providing the complete computation at the gantry
location. Further, such alternative embodiments alleviate to a
great extent the problem of synchronizing the receipt of pulse
information with the calculation of the desired interval of unsafe
following distance.
Referring now to FIG. 5 there is shown a binary counter 92 having
sufficient capacity to count throughout the longest time interval
which will be calculated, such counter 92 receiving an energizing
pulse from the class identification circuit 55 to initiate an
automatic counting mode. A binary register 94 of similar capacity
is depicted, such register 94 being presettable to a specific
number as determined from the remote computer 60 and adapted to
retain such number until reset. A comparator circuit 95 monitors
the outputs of the counter 92 and register 94 on a continuing basis
for control of an AND-gate 96, such arrangement being effective to
hold the gate 96 in the open condition so long as the count in the
counter 92 is below that indicated in the register 94. When a
succeeding pulse is received from the class identification circuit
55 while the gate 96 is in the open condition, such pulse will be
transmitted to the tailgate portion 48 of the display board 47 for
illumination of the display for a predetermined interval as
explained with reference to the preferred embodiment of the
invention.
In the mode of operation of this system, the count in the counter
92 is always synchronized with the receipt of the first pulse and
the only requirement is that the information entered into the
register 94 be provided prior to the completion of the interval,
within a sufficient time so as to allow for receipt of a trailing
pulse. The remote computer 60 again can operate on a time shared
basis, sequentially interrogating the data available at each
gantry, and with the present day third generation type of
computers, operating times are sufficiently fast to allow such
calculations to be performed in relatively small time interval in
relation to the unsafe following distance time interval calculation
desired.
Yet another form of apparatus for providing the tailgate interval
is depicted in the FIG. 6 embodiment of the invention wherein a
one-shot timing component 97 is shown adapted to be triggered from
a pulse received from the class identification circuit on line 98.
The one-shot 97 may be any form of monostable device having the
characteristic of changing state for a predetermined time interval
upon energization. An AND-gate 99 is connected to sense the change
of state of the one-shot 97 being conditioned for receipt of a
succeeding pulse from line 98, to transmit the pulse to the display
board 48. Typically, the one-shot 97 operates on the principle of
capacitor discharge, the rate of discharge determining the desired
time interval and such rate is modified by a variable impedance
element 100 connected in the capacitor circuit. The variable
impedance element 100 may comprise a form of ladder impedance
network receiving as inputs the class identification signal, the
speed signal, and the data representing environmental conditions,
the latter now comprising the appropriate sensors located in situ
at the gantry itself and exactly coordinated with the environmental
conditions at that particular location. Again, in this embodiment
of the invention, automatic synchronization between the unsafe
following interval and the receipt of the pulse from the class
identification circuit 55 automatically occurs so that no clock or
timing source is required. It is still desired, however, to relay
the information to the central computer 61 associated with the
highway system and the display of such retransmitted information is
fully compatible in this embodiment of the invention.
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