Traffic signal system and method

System and method for the improved operation of a traffic signal system which reduces delays to vehicular traffic at one or more signalized intersections by detecting vehicular demand for movement through each intersection. The present invention uses detectors for detecting vehicles entering an intersection, at a distance sufficiently great from the intersection, and for detecting vehicles leaving an intersection, at a distance sufficiently near to the intersection, to permit, when possible, one or more vehicles to proceed completely through the intersection prior to coming to a near, or complete stop. The system and method further facilitates accident reduction by monitoring portions of traffic lanes entering an intersection for vehicles, determining the probability of a vehicle being able to timely stop for a halt signal, and, if it is determined that the vehicle cannot timely stop for a halt signal, attempting to clear the intersection of other vehicles by setting all signals to halt.

FIELD OF THE INVENTION 
The present invention relates to a system and method for the improved 
operation of a traffic signal system which reduces delays to vehicular 
traffic at one or more signalized intersections by detecting a vehicle's 
demand for movement through each intersection, and, when possible, 
permitting the vehicle to proceed through one or more intersections, prior 
to the vehicle coming to a near, or complete, stop and/or which 
facilitates accident reduction by monitoring portions of traffic lanes 
entering an intersection for vehicles, determining the probability of a 
vehicle being able to timely stop for a halt signal, and, if necessary, 
attempting to clear the intersection of other vehicles. 
DESCRIPTION OF THE RELATED ART 
Traffic signals are commonly used to control and facilitate the orderly 
progression of vehicles through intersections. Early traffic signals were 
operated by traffic officers using manual switches to activate signal 
displays at intersections. To automate these signals and improve 
intersection efficiency, controller units for controlling the traffic 
signals were developed and implemented. The basic function of these early 
controller units was to alternately assign the right-of-way between two or 
more traffic lanes with conflicting traffic movements. 
These early controller units were electromechanical pre-timed devices, 
using motors and gears for timing, that could change the signal displays. 
These early controllers, however, were not easily adjustable and repeated 
cycle after cycle for each lane of traffic on one or more pre-set timing 
plans, without regard to actual vehicle demand. Although the cycle timing 
could be adjusted to accommodate the needs of periodic heavy movement, 
such as morning and evening peaks, these controllers could not cope with, 
nor could they provide flexibility for, the changes in traffic flow that 
occurred throughout the day. This inflexibility often meant that a signal 
was showing stop for vehicles in a traffic lane that needed to proceed 
through the intersection, and vice versa. 
The development of traffic actuated controller units provided some 
flexibility for accommodating varied traffic flow. These actuated traffic 
controller units permitted cycle timing to be varied for some or all 
controlled conflicting traffic lanes (i.e., lanes of traffic that would 
have interfering vehicle movements through the intersection if allowed to 
operate concurrently) depending upon vehicular demand by utilizing 
detectors placed in or near the intersection and the relevant portions of 
the traffic lanes entering into the intersection. Early traffic actuated 
controller units were of a sonic type. Located at the stop lines of an 
intersection, these detectors were responsive to a motorist blowing the 
vehicle's horn. Upon actuation, these controllers permitted the motorist 
to obtain the right-of-way. These early actuated controller units were 
essentially, however, modified pre-timed controller units which provided 
only a fixed length cycle upon actuation. 
To improve intersection efficiency, control units were developed that could 
flexibly extend a cycle, within certain parameters, based upon vehicular 
demand. Detection units progressed from the early sonic type to pressure 
sensitive devices and electrical circuits that were embedded in the 
roadway and actuated by the weight of a passing vehicle or a change of 
inductance during passage of a vehicle. In addition to the above detection 
devices, current controller units utilize various detection methods, 
including sonar, radar, infrared, photo-electric cell, and magnetic 
devices for detecting vehicles in the portion of traffic lanes entering an 
intersection. 
Although intersection efficiency has increased as a result of the 
technological progression in detection devices, current traffic signal 
systems continue to operate on a cycle by cycle basis for an intersection; 
each cycle consisting of one more progression phases for certain traffic 
lanes (i.e., proceed signals for permitting movement of certain traffic 
lanes through the intersection) and non-progression phases for the 
remaining traffic lanes (i.e., caution and halt signals for halting 
movement of the remaining traffic lanes through the intersection). FIG. 1 
depicts typical traffic movements through intersection 100. 
The introduction of microprocessors to controllers brought about major 
changes in traffic signal systems, including the ability to coordinate the 
movement of traffic through two or more signalized intersections. 
Coordination of two or more signalized intersections resulted in a number 
of benefits, including the ability to change timing plans to accommodate 
predominate traffic flows; to increase capacities of major arterials to 
meet peek demands; to control speeds on major arterials; and to conserve 
energy and improve safety by reducing stops and delays. Coordination among 
two or more intersections is typically accomplished by either a 
centralized controller system (i.e., a central controller performing all 
processing functions for each of the intersections) or a distributed 
controller system (i.e., two or more controllers responsible for 
controlling the signalized intersections assigned to such controllers and 
for further communicating between one another). Communications in a 
coordinated system are typically conducted through twisted pair cable, 
coaxial cable, fiberoptic cable, or radio transmission. The operation of 
current traffic controllers and the coordination of two or more signalized 
intersections is well known in the current art and the reader is directed 
to the publication entitled "Participant Notebook" dated Jul. 1993 for 
Traffic Control Equipment & Software Demonstration Project, published by 
the United States Department of Transportation, Federal Highway 
Administration, for a more thorough discussion of current traffic 
controllers and the coordination of signalized intersections. 
As can be seen in FIGS. 2a and 2b, the current art employs zones for 
detecting vehicles 225, 230, 235, 240, 245, 250, 255, and 260 entering 
intersection 200. Detection zones 245, 250, 255, and 260 typically include 
loop detectors (i.e., a detector that senses change in inductance of its 
inductive loop sensor caused by the passage or presence of a vehicle near 
the sensor), presence detectors (i.e., a detector which is able to detect 
the presence of a vehicle and maintain the detection signal for a 
predetermined period of time that the vehicle is within its field of 
detection), or a combination of loop and presence detectors. Detection 
zones 225, 230, 235, 240, 245, 250, 255, and 260 are limited to areas 
which are in close proximity to stop lines 205, 210, 215, and 220 for each 
traffic lane. This relatively close proximity of such detection zones to 
such stop lines requires vehicles approaching such an intersection in a 
traffic lane with a caution or halt signal showing to come to a near, or 
complete, stop before each vehicle's demand for a proceed signal is 
recognized and serviced by the controller for the intersection. Thus, the 
orderly progression of vehicles through such an intersection is often 
halted or delayed because a vehicle is required to wait for a proceed 
signal, although no demand for a proceed signal exists by vehicles in one 
or more conflicting traffic lanes. Also, current traffic signal systems do 
not monitor the portions of traffic lanes leaving the intersection to 
determine if the leaving portions are not blocked (i.e., free of vehicles) 
so that complete movement of vehicles through the intersection is 
possible. 
A further limitation of the close proximity of the detection zones to the 
intersection is that current controllers and methods for operating a 
signalized intersection do not generally provide accident avoidance 
functions. That is, current controllers and methods for operating a 
signalized intersection do not monitor the portions of traffic lanes 
entering an intersection to determine the probability of a vehicle being 
able to pass through the intersection during a caution signal or to timely 
stop for a halt signal, nor do they attempt to clear the intersection, so 
as to prevent a possible accident if it is determined that a vehicle will 
not be able to pass through the intersection during a caution signal or to 
timely stop for a halt signal. 
Therefore, a system and method is desired for controlling one or more 
signalized intersections in which delays to vehicular traffic are reduced 
by detecting the presence of a vehicle, determining if the vehicle can 
proceed completely through an intersection, and when possible, providing a 
proceed signal for the vehicle before the vehicle has come to a near, or 
complete, stop. A system and method is also desired which facilitates 
accident reduction by monitoring the probability of a vehicle being able 
to pass through an intersection or to timely stop for a caution or halt 
signal, respectively, and attempts to clear the intersection when it 
becomes likely that a vehicle will not be able to pass through the 
intersection or to timely stop for a caution or a halt signal, 
respectively. 
SUMMARY OF THE INVENTION 
The present invention includes a system and method for reducing delays to 
vehicular traffic through one or more signalized intersections and for 
facilitating accident reduction at such intersections. The present 
invention comprises detectors, such as a camera system, for detecting 
vehicles entering and leaving an intersection, a plurality of signals for 
controlling the movement of vehicles through the intersection, and at 
least one controller for controlling the plurality of traffic signals at 
one or more signalized intersections. The present invention may operate in 
either a centralized system or a distributed system. The present invention 
monitors the portions of traffic lanes entering and leaving each 
intersection for vehicles and generates signals that correspond to whether 
the detected vehicles are entering or leaving each intersection. The 
controller receives these signals and, preferably using the method of the 
present invention, controls the plurality of traffic signals to permit the 
efficient movement of traffic through each intersection and to facilitate 
accident reduction at each intersection. 
The system monitors detection zones for portions of traffic lanes entering 
each intersection that comprise areas whose outer boundaries are at a 
distance from each stop line for each intersection. This permits detection 
of vehicles at a greater distance from each stop line than is conducted by 
the current art. Unlike the current art, the system also preferably 
monitors detection zones for portions of traffic lanes leaving each 
intersection to determine whether a vehicle is substantially proximate to 
the intersection, and thus, determine if there is sufficient room in the 
applicable traffic lane for another vehicle to proceed through and clear 
(i.e., not be present in) the intersection. 
Further, the detection zones for portions of traffic lanes entering each 
intersection are sufficiently distant from each stop line to permit, when 
possible, the controller to process the detection signals and cause the 
display of a halt signal on the applicable traffic signals for all 
conflicting lanes of traffic entering the intersection and a proceed 
signal for the applicable lane of traffic entering the intersection upon 
the detection of a vehicle. Thus, the detected vehicle will not be 
required to come to a near, or complete, stop before receiving a proceed 
signal. The length of the detection zones for portions of traffic lanes 
entering the intersection also permit the controller to process the 
detection signals, and when vehicles are proceeding through the 
intersection, determine if a vehicle approaching the intersection in a 
traffic lane which conflicts with the traffic lane(s) for those vehicles 
proceeding through the intersection is capable of stopping before reaching 
the intersection. If a determination is made that the vehicle cannot stop 
before reaching the intersection, the system generates halt signals to all 
signals in an effort to facilitate accident reduction by clearing the 
intersection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Turning to FIG. 3, a preferred embodiment of the invention for controlling 
movement of vehicles at an intersection includes detection zones 315, 316, 
325, 326, 335, 336, 345, and 346 for detecting vehicles entering 
intersection 300 and detection zones 310, 320, 330, and 340 for detecting 
vehicles leaving intersection 300. It should be noted that detection zones 
315, 316, 325, 326, 335, 336, 345, and 346 comprise areas whose outer 
boundaries are at a distance from stop lines 350, 355, 360, and 365, 
respectively, so as to permit detection of vehicles at a greater distance 
from stop lines 350, 355, 360, and 365, respectively, than is monitored by 
the current art (See 225, 230, 235, 240, 245, 250, 255, and 260 of FIGS. 
2a and 2b). 
The distance from stop lines 350, 355, 360, and 365 for detection zones 
315, 316, 325, 326, 335, 336, 345, and 346 is sufficiently great to 
permit, when possible, the display of a halt signal for all conflicting 
lanes of traffic and a proceed signal for the applicable lane of traffic 
upon the detection of a vehicle within applicable detection zone 315, 316, 
325, 326, 335, 336, 345, and 346 so that the detected vehicle will not be 
required to come to a near, or complete, stop before receiving a proceed 
signal. It should be noted that detection zones 316, 326, 336, and 346 
will typically be of a shorter distance from stop lines 350, 355, 360, and 
365 then detection zones 315, 325, 335, and 345 because such detection 
zones are for left turning traffic lanes. The preferred embodiment of the 
invention includes separate detectors for monitoring left turning traffic 
lanes. In other embodiments of the invention, the system does not include 
such separate detectors for monitoring left turning traffic lanes, but 
rather, includes a single detector. It should also be noted that detection 
zones 310, 320, 330, and 340 leaving intersection 300 are smaller than 
detection zones 315, 316, 325, 326, 335, 336, 345, and 346 and are 
sufficiently large to detect a vehicle that is substantially proximate to 
intersection 300 so that there is insufficient room in the applicable 
traffic lane for another vehicle to proceed through and clear (i.e., not 
be present in) intersection 300. 
As shown in FIG. 4, a preferred embodiment of system 400 for controlling 
movement of vehicles through intersection 405 having a plurality of 
traffic lanes 410, 411, 412, and 413 (wherein each of the traffic lanes 
410, 411, 412, and 413 includes a portion entering 415, 416, 417, and 418 
intersection 405, a portion leaving 420, 421, 422, and 423 intersection 
405, and a stop line 425, 426, 427, and 428 located in entering portions 
415, 416, 417, and 418) includes a plurality of traffic signals 430, 431, 
432, and 433 corresponding to each entering portion 415, 416, 417, and 418 
of traffic lanes 410, 411, 412, and 413; a plurality of detection devices 
435, 436, 437, 438, 439, 440, 441, and 442 for detecting the presence of 
vehicles in detection zones entering and leaving intersection 405 (See 
310, 315, 316, 320, 325, 326, 330, 335, 336, 340, 345, and 346 of FIG. 3) 
wherein detection devices 435, 436, 437, 438, 439, 440, 441, and 442 are 
configured to generate a demand signal or a halt signal corresponding to 
the applicable traffic lane in which the vehicle is detected for entering 
portions 415, 416, 417, and 418 or leaving portions 420, 421, 422, and 
423, respectively, of applicable traffic lane 410, 411, 412, and 413 in 
which the vehicle was detected; and controller 445 for receiving and 
processing the demand and halt signals from detection devices 435, 436, 
437, 438, 439, 440, 441, and 442 and controlling traffic signals 430, 431, 
432, and 433 to permit the movement of vehicles through intersection 405. 
It should be noted that detection devices 435, 437, 439, and 441 may 
include separate detection devices for monitoring one or more individual 
traffic lanes of entering portions 415, 416, 417, and 418 and leaving 
portions 420, 421, 422, and 423. It should be further noted that traffic 
signals 430, 431, 432, and 433 may include separate signals for 
controlling the movement of traffic in one or more individual traffic 
lanes of entering portions 415, 416, 417, and 418. One embodiment of 
detection devices 435, 436, 437, 438, 439, 440, 441, and 442 preferably 
includes a camera system as disclosed in U.S. Pat. No. 5,438,360 titled 
Machine Vision Camera and Video Preprocessing System. Other embodiments of 
detection devices 435, 436, 437, 438, 439, 440, 441, and 442 may include 
sonar, radar, infrared, and/or magnetic detection means. One embodiment of 
controller 445 preferably includes the 3000 Series Advanced Nema Traffic 
Controller manufactured by Peek Traffic-Transyt of Tallahassee, Fla. 
FIG. 5 depicts the overview of the methodology for controlling traffic 
signals 430, 431, 432, and 433 (See FIG. 4). The methodology begins 505 
with setting all traffic signals to a halt indication 510. 
If demand to enter the intersection is detected 515, a traffic lane flag is 
set which indicates the traffic lane in which the demand was detected 520. 
If demand for the traffic lane for which the traffic lane flag is set 
exists 525, a check for a conflicting demand to enter the intersection is 
conducted 540. 
Steps 540, 541, 542, 543, 544, and 545 relate to handling conflicting 
demand to enter the intersection if such demand exists and comprise the 
following. First, a determination is made as to whether the detected 
vehicle with the conflicting demand is traveling at a rate of speed that 
will permit the vehicle to stop before arriving at the applicable stop 
line for the intersection 541. If not, all signals are set to halt in an 
effort to clear the intersection 510 and, thereby, facilitate accident 
reduction. If it is determined that the vehicle can stop before arriving 
at the applicable stop line, a determination is made as to whether a wait 
flag (i.e., an indication that conflicting traffic has been waiting to 
proceed through the intersection) has been set 542. If the wait flag has 
not been set, a counter for the period of time that conflicting traffic 
must wait before being allowed to proceed through the intersection is 
initialized and the wait flag is set 543. Step 543 is avoided if the wait 
flag was previously set. Next a determination is made as to whether the 
maximum amount of time that conflicting traffic must wait before being 
allowed to proceed through the intersection has transpired 544. If the 
wait time has not transpired, the demand signal generated by the 
conflicting traffic is ignored 544. If the wait time has transpired, then 
caution and halt signals, respectively, are displayed for traffic lanes 
associated with the traffic lane flag and the wait flag for conflicting 
traffic is cleared 545. 
After determining if conflicting demand to enter the intersection exists 
525 and servicing such demand if it does exist 541, 542, 543, 544, and 
545, a determination is made as to whether non-conflicting demand to enter 
the intersection (i.e., demand from vehicles that would not interfere with 
the current demand if such vehicles were permitted to proceed through the 
intersection concurrently with the existing demand) exists 550. If such 
non-conflicting demand exists and if a predetermined amount of time 
remains before the wait time for conflicting demand (which was initialized 
in 543) transpires and if the portion of the applicable traffic lane 
leaving the intersection for the non-conflicting demand is not blocked 
(i.e., no vehicles are present in the applicable detection zone so as to 
prevent another vehicle from proceeding through and out of the 
intersection) 551, a proceed signal is displayed for those traffic lanes 
associated with the non-conflicting demand 552. 
After determining if non-conflicting demand to enter the intersection 
exists 550 and handling such demand if it does exist 551 and 552, a 
determination is made as to whether the portion of the traffic lane 
associated with the traffic lane flag leaving the intersection is blocked 
555. If the leaving portion of the traffic lane is blocked, the traffic 
lane flag is cleared 556 and a transfer to 526, which is discussed in 
detail below, is made. If the leaving portion of the traffic lane is not 
blocked, a proceed signal is displayed for the traffic lane 560 and a 
determination is once again made as to whether demand for the traffic lane 
for which the traffic lane flag is set exists 525. 
If demand for the traffic lane for which the traffic lane flag is set does 
not exist 525, service associated with the traffic lane is terminated 
(i.e., caution and halt signals are displayed), and the traffic lane flag 
is cleared 526. A determination is then made as to whether non-conflicting 
demand to enter the intersection exists 527. If non-conflicting demand to 
enter the intersection does exist, it is allowed to enter the intersection 
530 if sufficient time exists before conflicting demand must be serviced 
and if the progression lane(s) for such non-conflicting demand are not 
blocked 529. Service for non-conflicting demand to enter the intersection 
is terminated 531 when such demand no longer exists, if insufficient time 
exists before conflicting demand must be serviced, or if the progression 
lane(s) for such non-conflicting demand are blocked 529. A determination 
is then made as to whether conflicting traffic is waiting to enter the 
intersection 532, if so the wait flag is cleared, the traffic lane flag is 
set 533 and a transfer to determining whether non-conflicting demand 
exists is made 550. If there is no conflicting traffic waiting to enter 
the intersection, all signals are set to halt 510. 
While the invention has been described in detail with reference to specific 
embodiments thereof, it will be apparent to one skilled in the art the 
various changes and modifications can be made therein without departing 
from the spirit and scope thereof.