US3384869A - System for freeway access ramp traffic control - Google Patents

Description

May 21, 1968 L. F. WALDRON SYSTEM FOR FREEWAY ACCESS RAMP TRAFFIC CONTROL 3 Sheets-Sheet 1 Filed Jan.

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INVENTOR. L. F WALDRON HIS ATTORNEY ay 21, 1968 L. F. WALDRON SYSTEM FOR FREEWAY ACCESS RAMP TRAFFIC CONTROL 3 Sheets$heet 2 Filed Jan.

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590 A G E 6 29555 m 50 0 o 566 Gamma. m @E HIS ATTORNEY Unite States Patent 3,384,869 SYSTEM FOR FREEWAY ACCESS RAMP TRAFFIC (IONTROL Leslie F. Waldron, Rochester, N.Y., assignor to General Signal Corporation, Rochester, N.Y., a corporation of New York Continuation-impart of application Ser. No. 346,177, Feb. Zil, 1964. This application Jan. 17, 1%6, Ser. No. 521,184

Claims. (Cl. 34036) ABSTRAQT OF THE DISCLOSURE A system for controlling a signal governing ramp traffic into a controlled access highway having detectors in the highway and metering timer means controlled by the detectors for governing the permissible proceed time for the signal in accordance with trafiic conditions. A detector in the ramp between the signal and the highway is used at times to restore the signal to stop.

This application is a continuation-in-part of my application Ser. No. 346,177, filed Feb. 20, 1964, now abandoned.

This invention relates to traffic control systems, and it more particularly pertains to a system for regulating entry of trafiic onto a freeway from an access ramp in accordance with freeway traffic characteristics.

Modern high-speed freeways require free-flow of traffic in order to avoid congestion resulting ultimately in traffic stoppages. Under heavy freeway traffic conditions, entry of vehicles onto a freeway without limitation may ultimately result in freeway traffic stoppages. Under light freeway traffic conditions, however, it is desirable to permit vehicles to enter onto the freeway with a minimum amount of limitation. Under freeway traffic conditions ranging between light and heavy, it is desirable to limit access to the freeway from a ramp in a graduated manner; that is, as freeway traffic increases from light to heavy, for exam-ple, an increasing limitation on vehicles entering the freeway from a ramp is desirable. By so doing, traffic conditions on the freeway may be maintained within predetermined maximum limits, so as to avoid a high degree of congestion, resulting in stoppages.

The system according to the present invention provides for the control of a ramp signal selectively according to two modes. One of these modes is a normally clear mode wherein substantially unlimited trafiic is permitted to enter the freeway from the ramp when there is no congestion of traffic on the freeway, and the second mode is a normally stop mode wherein a traffic signal governing the entry into the freeway from the ramp is normally at stop, and is permitted to indicate proceed to start vehicles into the freeway at metered intervals at a rate selected in accordance with the extent of congestion of traffic on the freeway. In the normally stop mode, the signal is permitted to indicate proceed only for the interval required for the passage of a vehicle past the signal because means is provided for detecting the vehicle having passed the signal, and such means causes the signal to be put to stop. A signal thus put to stop is maintained in a stop indicating position for an interval dependent upon the degree of congestion of traflic on the freeway. In one form of the invention, it is further required that a vehicle be in the immediate approach of the ramp traffic signal before it is given a proceed indication by the signal.

In accordance with the operation of the ramp traffic signal selectively in a normally clear or a normally stop mode, a special warning can be provided in accordance with the display of a distinctive signal aspect, for exam- Patented May 21, 1968 "ice ple, when the system changes from a normally clear mode to a normally stop mode.

An object of the present invention is to provide means for automatically selecting a timing cycle for a ramp traffic signal in accordance with measured trafiic characteristics such as characteristics related to traflic congestion on a freeway.

Another object of the present invention is to provide means for controlling a ramp traffic signal to admit traftic onto a freeway from a ramp at a metered rate selected in accordance with trafiic characteristics on the freeway.

Another object of the present invention is to control a ramp traffic signal selectively in normally clear and normally stop modes, dependent upon the traflic characteristics on the freeway.

Another object of the present invention is to control the ramp tratlic signal, under conditions where traffic on the freeway is congested, in a manner to require the presence of a vehicle substantially immediately in approach of the traffic signal before the signal is permitted to display a proceed indication.

Another object of the present invention is to selectively determine minimum durations between proceed intervals of the ramp trafiic signal in accordance with the congestion of traffic, and to limit the proceed intervals in accordance with the sensing of traffic having passed the ramp signal.

Other objects, purposes and characteristic features of the present invention will be in part obvious from the accompanying drawings and in part pointed out as the description of the invention progresses.

In describing the invention in detail, reference is made to the accompanying drawings, wherein similar reference characters are used throughout the several figures to designate corresponding parts and wherein:

FIG. 1 illustrates a system for the control of a ramp signal according to one embodiment of the present invention;

FIG. 2 illustrates a second embodiment of the present invention wherein the ramp signal can be selectively controlled either manually or automatically to provide the desired mode and metering selection; and

FIG. 3 illustrates another embodiment of the present invention employing additional trafiic sensing means in the access ramp for use in controlling the ramp traffic signal during transition from a normally clear to a normally stop mode.

With reference to FIG. 1, an access ramp is shown leading onto a freeway. A plurality of vehicle detectors are situated so as to sense traffic characteristics which can be considered as generally relating to traffic congestion on the respective lanes of the freeway upstream from the location at which the access ramp empties into the freeway, although it is to be understood that these detectors can be located at other points in the freeway in accordance with the requirements of practice. One example of a detector which may be used with this system is disclosed in the Kendall et al. Patent No. 3,042,303, issued July 3, 1962. Although three detectors A, B and C are illustrated, a single detector may also be utilized, especially if the assumption is made that freeway traffic tends to distribute itself substantially equally over all lanes. On the other hand, a single detector may be used if preferred to monitor traliic only in the lane into which the access ramp trafiic empties directly.

Outputs of dectors A, B and C may be registered in any suitable manner, such as in the manner illustrated wherein the relays 11, 12 and 13 are controlled by the detectors respectively and have respective front contacts 14, 15 and .16 which provide separate inputs to a trafiic congestion computer '17. If the degree of congestion is to be determined in accordance with lane occupancy, the

computer 17 can be provided in a manner described in the application of J. H. Auer, Jr., ct al., Ser. No. 292,584,

led July 3, 1963. If this type of a computer is used, the voltage amplitude produced by the computer is proportional to the average of lane occupancy data supplied to its inputs. On the other hand, if detectors A, B and C are to sense tratfic congestion by measuring the volume of tratfic, computer 17 may comprise a volume computer such as that shown in the J. H. Auer, Jr., et al application Ser. No. 342,467, filed Feb. 4, 1964. in such case, output voltage amplitude produced by the computer 17 is proportional to the average of volume data supplied to its inputs.

Output voltage produced by traffic congestion computer 17 is applied to a level monitor 18. In FIG. 1, the level monitor .18 utilizes a plurality of output contacts 19, 20, 21 and 22. The heel of contact 19 is coupled to a source of energy, while back contacts 19, 20, 21 and 22, as well as front contact 22 are respectively coupled to key operated contacts 34, 35, 36, 37 and 38 of a dial unit 23. The dial unit 23, which is preferably driven by a IUD-segment dial geared for sixty-second operation, permits preselection of a wide variety of closed contact intervals, dependent upon key selection and placement. Front contact 19 is coupled to the heel of contact 20, front contact 20 is coupled to the heel of contact 21, and front contact 21 is coupled to the heel of contact 22.

Dial unit output energy is coupled to a relay 24 having a pair of contacts 25 and 26. Contact 26 supplies energy to the indicator lamps of a traflic signal 27 regulating entry of trafiic from the access ramp onto the freeway. Back contact 26 provides energy to the red lamp, while front contact 26 provides energy to the green lamp. Front contact 25 provides a stick circuit for relay 24 through a back contact 28 of a relay 29. The system as illustrated in FIG. 1 is shown as being in a normally clear mode wherein no traffic congestion is detected on the freeway, the green lamp of signal 27 being energized to provide a proceed indication for the ramp signal in accordance with the closure of front contact 26 of relay 24.

A suitable vehicle detector D is situated just beyond traffic signal 27 for the purpose of sensing vehicles which have passed signal 27. Output energy from detector D is coupled to a relay 30 having a contact 31. The heel of contact 31 is coupled to a capacitor 32. Front contact 31 receives energy through a current limiting resistor 33, while back contact 31 is coupled to relay 29.

In operation, detectors A, B and C sense traffic conditions on the freeway and actuate relays 11, 12 and 13 accordingly. Computer 17 then receives energy in accordance with the operations of relays 11, 12 and 13, and provides an output proportional to tratfic characteristics such as trafiic congestion on the freeway. The computer output voltage, which may represent an analog of freeway lane occupancy or freeway volume, dependent upon whether a lane occupancy computer or volume computer is used, or some other congestion parameter, is supplied to a level monitor 18 which closes its front contacts in stepwise fashion in accordance with changing amplitude of input voltage supplied thereto. Hence, when output voltage supplied by computer 17 to monitor 18 is at a minimum level, back contacts 19, 20, 21 and 22 are all closed. Under these conditions, back contact 19 supplies energy to dial unit 23. However, as the amplitude of computer 17 output voltage increases, front contact 19 closes, thereby providing output energy from level monitor 18 at back contact 20. As computer 17 output voltage increases further, front contact 20 closes, while front contact 19 remains closed. Hence, output voltage is provided at back contact 21. If voltage supplied to level monitor 18 increases still further, front contact 21 closes, while front contacts 19 and 20 remain closed, so that output voltage is supplied at back contact 22. Ultimately, at maximum levels of output voltage produced by computer 17, front contacts 19, 20, 21 and 22 of level monitor 18 all close, so that output voltage is provided at front contact 22 of the level monitor. In the foregoing fashion, level monitor 18 classifies amplitude levels provided by traffic congestion computer 17, supplying, in effect, analog to digital conversion.

As output voltage amplitude produced by traffic congestion computer 17 increases, energy is transferred from one dial unit 23 contact to another. It can be seen that only One dial unit 23 contact may be energized at any given time. Moreover, each dial unit 23 contact remains closed for a different length interval during each timing cycle. in this fashion, therefore, relay 24 receives energy from dial unit 23 during different length intervals, dependent upon the amplitude of output voltage produced by tratfic congestion computer 17.

Normally, with no traffic present on either the ramp or the freeway as is illustrated in FIG. 1, the relay 24 is maintained in its picked up position by its circuit through back contact 28 of relay 29 and its own front contact 25, and in accordance therewith, the green lamp G of signal 27 is energized to display 2. proceed indication through front contact 26. The energization of relay 24 by this circuit maintains the relay 24 steadily energized as long as no traffic is sensed having passed the signal 27, irrespective of the pick up circuit for the relay 24 being intermittently opened by the operation of the dial unit 23. On the other hand, while relay 24 is energized from the dial unit 23, any number of vehicles may pass detector D without causing a change in indication from,

green to red. Assuming now that energy is removed from relay 24 by the dial unit 23, as long as no vehicle is sensed by detector D, relay 24 remains energized by energy through its front contact 25. However, when a vehicle is sensed by detector D during the time in which dial unit 23 provides no energy to relay 24, the relay 24 becomes dropped away in accordance with the momentary operation of relay 29. Relay 30 is momentarily energized by the detector D, and this relay actuates its contact 31 to first charge the capacitor 32, and then upon the dropping away of relay 30, to momentarily pick up relay 29 to open the stick circuit for relay 24 at back contact 28. The dropping away of relay 24 shifts its contact 26 from a front to a back position, thus causing the deenergization of the green lamp G of signal 27 and the energization of the red lamp R of that signal. Thus the ramp signal 27 governing vehicle access to the freeway from the ramp is changed from a proceed indication to a stop indication.

It will be noted that under conditions in which heavy traffic enters the freeway from the access ramp, and depending upon the amplitude of output voltage provided by computer 17, the traffic signal 27 regulating entry of vehicles onto the freeway is controlled by the dial unit 23 so as to limit passage of vehicles on the freeway only during those predetermined intervals established in the dial unit 23 and selected by the level monitor 18 in accordance with amplitude of the computer 17 output voltage. However, under light access ramp trafiic conditions, the traflic signal 27, when in the proceed condition, may remain in the proceed condition until detector D senses passage of a vehicle during a portion of the cycle of the dial unit 23 in which the dial unit is not supplying energy to relay 24. In a sense, therefore, the end of the proceed interval, under light access ramp traffic conditions, is variable, and may actually extend into the next timing cycle to provide a substantially continuous proceed indication. It should also be noted that detector D may be of the same type as detectors A, B and C, or of any other well-known type.

With reference to FIG. 2, another embodiment of the invention is disclosed wherein the system more definitely operates either in a normally proceed mode or in a normally stop mode as selected in accordance with the detection of congestion of trafiic on the freeway. If there is no traffic congestion on the freeway, the access ramp signal 50 steadily displays a proceed indication by energization of the green lamp G of that signal through front contact 51 of relay SC2 and back contact 52 of relay SCI.

The sensing of trafiic congestion on the freeway for the system according to FIG. 2, can be by a traffic congestion computer 17 the same as has been described with reference to FIG. 1. The output of the traffic computer 17 of FIG. 2. is applied to a level monitor 53, which is similar to the level monitor 18 of FIG. 1. The level monitor 53 has output contacts 54, 55 and 56 which are progressively actuated to their picked up positions in accordance with the increase in traffic congestion along the freeway. The output of the level monitor 53 is applied through a suitable multiple position switch 57 which can be operated manually to select either the automatic operation of the system in accordance with freeway congestion as detected by the detectors A, B and C, or the different degrees of congestion can be manually designated by the switch 57 for use in controlling the ramp trafiic signal 50. A code characteristic of the degree of traffic congestion is thus applied over code wires 58 and 59 of relays C1 and C2. It is to be understood that such a control switch can also be used with other embodiments of the present invention.

A suitable indicator 60 is provided for indicating to an operator the different levels of congestion that are detected on the freeway, or to indicate the ditferent levels of congestion that are designated manually by the switch 57, if the switch is operated to positions other than the automatic position in which it is shown. With the switch 57 in the automatic position, and with no congestion detected on the freeway, the lamp of indicator 60 is energized through back contact 54 of level monitor 53 and contact 61 of switch 57 in its automatic position. There is a lamp illustrated in the indicator 60 for registering levels 1, 2 and 3 of traffic congestion respectively. If congestion is light so as to close only front contact 54 of the level monitor 53, the relay C1 becomes picked up by energization through front contact 54 and back contact 55 of level monitor 53, contact 62 of switch 57, diode 63 and wire 58. The indicator lamp 0 is extinguished because of the opening of back contact 54, and the lamp 1 is energized in an obvious manner because of its connection to the circuit just described. If traffic congestion is medium, both contacts 54 and 55 of level monitor 53 are closed and the relay C2 is energized over wire 59 through front contacts 54 and 55 and back contact 56 of level monitor 53, contact 64 of switch 57 and diode 65. The lamp 2 of indicator 60 is energized in accordance with its connection to this same circuit. If heavy congestion on the freeway is detected, front contacts 54, 55 and 56 of the level monitor are all closed to provide for energization of both relays C1 and C2. Relay C1 is energized through contact 62 of switch 57 and diode 67, and relay C2 is energized through diode 68. Thus the registration of light traffic congestion causes the picking up of relay C1 with the relay C2 in its dropped way position, the registration of medium congestion results in the picking up of relay C2 but not relay C1, and the registration of heavy congestion results in the picking up of both relays C1 and C2.

Signal control relays C1 and SC2 are provided for the control of signal 50. Relay 8C1 controls the aspects of signal 50 only during the transition period from a normally proceed m de to a normally stop mode.

A metering timer 69 is provided for timing the duration between proceed intervals of the signal 50' when traffic congestion on the freeway has been registered. A start timer 70 is provided for timing a yellow aspect of the signal 50 which is displayed for a time interval when the system is shifted from a normally proceed mode to a normally stop mode. Vehicle detectors 1 and 2 are provided in the access ramp at positions to respectively sense the presence of vehicles in approach of the signal 50 and after having passed the signal 59. Detector relays D1 and D2 are associated with the detectors 1 and 2, and these relays become actuated in response to the sensing of the presence of vehicles by their associated detectors.

To consider the specific mode of operation of the system as illustrated in FIG. 2, it will first be assumed that the system is in a normally proceed mode with no traflic congestion having been registered as is indicated by the condition of the apparatus as illustrated. Under these conditions the control relay SC2 is maintained steadily energized by a circuit including back contact 71 of relay C1, back contact 72 of relay C2 and diode 73. With relay SC2 steadily energized, the green lamp G of signal 50 is maintained steadily energized to permit steady flow of traffic from the access ramp to the freeway.

When traffic congestion is registered by the picking up of either one or both of the relays C1 and C2, the circuit for relay SC2 becomes opened, and energy is applied to one of the inputs to the metering timer 67 and to the input of the start metering time-r 70. If congestion is light and the relay C1 is picked up and relay C2 is in its dropped away position, for example, energy is applied to input wire 74 of the metering timer 69 through front contact '71 of relay C1 and back contact 75 of relay C2. Energy from this circuit is also applied as an input to the start metering timer 70 through diode 76. Energy from this circuit also is applied to the signal control relay SCl through back contact 77 of start metering timer 70. Relay SCI remains picked up until the end of an interval which has been started by the energization of the input wire of the start metering timer 70 and which is terminated when the start interval timing is completed upon the opening of back contact 77.

Upon the picking up of relay SCI, the opening of back contact 52 causes the green lamp G of signal 50 to be extinguished, and the clOsure of front contact 52 causes the energization of the yellow lamp Y of signal 50 to indicate to approaching traffic that the control of the signal is being shifted from the normally proceed mode to the normally stOp mode. The energization of the yellow lamp Y is maintained effective until the start metering timer has completed the timing of its interval, at which time the opening of back contact 77 causes the dropping away of relay SCI to open front contact 52 in the circuit for the yellow lamp Y of signal 50.

Because of the picking up of the relay C1, the circuit by which the relay SC2 has been energized is opened at back contact 71 of relay C1, and thus the relay SC2 is dropped away to close its back contact and thus condition a circuit for the energization of the red lamp R of signal 50 when the relay SCI becomes dropped away. Such energization of the red lamp R is in accordance with putting the system into a normally stop mode, and the energization of this lamp is maintained effective for a duration timed by metering timer 69. At the end of this interval, which is selected by the particular input wire that has been energized, the front contact 78 of metering timer 69 becomes closed, and the closure of this c ntact permits the relay SC2 to become picked up when the detector 1 senses the presence of a vehicle in the immediate approach of the signal 50. At this time the relay D1 becomes picked up, and with the relay D2 in its dropped away position, the relay SC2 becomes energized through back contact '79 of relay D2, front contact 80 of relay D1 and front contact '78 of metering timer 69.

The shifting of contact 51 of relay SC2 from its back to its front position extinguishes the red lamp R and energizes the green lamp G of signal 50 to start a proceed interval wherein the vehicle in approach of signal 50 is permitted to proceed. When the detector 2 senses that the vehicle has passed signal 50, the relay D2 becomes momentarily picked up, and the picking up of this relay charges a capacitor 81 through front contact 82, so that the metering timer 69 becomes reset by the discharge of capacitor 81 through back contact 82 when the relay D2 becomes dropped away. Relay SC2 is deenergized upon the picking up of relay D2 by the opening of its circuit at back contact 79, and the resetting of the metering timer 69 as has been described prevents the re-energization of the relay SC2 through front contact 78 until the metering timer has completed another cycle of timing. The start metering timer 70 becomes reset only when the system shifts from a normally stop mode to a normally proceed mode in accordance with there being no registration of congestion of traflic on the freeway. Under these conditions, the resetting of the timer 70 is rendered effective through back contacts 71 and 72 of relays C1 and C2 respectively.

Having thus described specifically the mode of operation of the system according to FIG. 2 for timing one particular selected interval in accordance with one degree of traffic congestion, it should be understood that a similar mode of operation is effective when other degrees of congestion are registered, the metering timer 69 having its timing adjusted in accordance with such degrees of congesti n in accordance with energy applied selectively to the plurality of input control wires for the metering timer 69.

With reference to FIG. 3, another embodiment of the present invention is illustrated wherein the general mode of operation is maintained that has been described for the system according to FIG. 2, except that the ramp control signal 85 of FIG. 3 is a two indication signal as compared to the three indication signal 50 of FIG. 2. In the system according to FIG. 3, rather than providing the special yellow indication when the system shifts from a normally proceed mode to a normally stop mode, a 1 teams is provided for shifting at a time when it is sensed that no trafiic is in the immediate approach of the signal 85, and in this manner there is no shift from steady green to red of the signal 85 when a vehicle is in immediate approach of the signal.

In FIG. 3 the start metering timer 70 is controlled through a back contact 86 of a detector relay D3, which registers the sensing of vehicle traific by a detector 3 disposed in the access ramp at a point upstream from the signal 85. The purpose of this detector is to delay the change in mode in the control of the signal 85 until there is no vehicle in the immediate approach of the signal 85. This mode of operation is accomplished by permitting the start metering timer 70 to operate only provided that the back contact 86 of detector relay D3 is closed. Thus the start metering timer 70 of FIG. 3 becomes stopped whenever a vehicle is detected by the detector D3 as being in approach of the signal 85. The start metering timer 70 is maintained in its operated position once it has completed its timing operation, as long as energy is applied from a relay C1 or C2 in acordance with congestion on the freeway. This is provided by the connection of front contact 77 of the timer 70 in multiple with back contact 86 in the input circuit for start metering timer 70. Until such time as the start metering timer 70 completes its timing operation, the relay SC3 is maintained energized in accordance with energy feeding from relays C1 and C2 through back contact of the start metering timer 7%. Thus the relay SC3 maintains energy on the green lamp G of signal 85 through front contact 51 until an interval is completed by the start metering timer 70.

The mode of operation during the normally stop mode for the apparatus according to FIG. 3 is the same as has been described relative to FIG. 2, the metering timer 69 being reset upon sensing the passage of a vehicle past the signal 85, and the start metering timer 70 being reset when it is registered that there is no trafiic congestion on the freeway in accordance with the relays C1 and C2 both being operated to their dropped away positions.

Having thus described several embodiments of the present invention as applied to the control of trafiic en.- tering a freeway from an access ramp, it is to be understood that these forms have been selected for the purpose of facilitating disclosure of the present. invention rather than to limit the number of forms the present invention may assume. While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes wi.hin the purview of the appending claims may be made without departing from the true scope and spirit of the inven;ion in its broader aspects.

What I claim is:

1. In a system for controlling the flow of traffic onto a controlled access highway from an entrance ramp, :1 trafiic signal for selectively displaying proceed and stop indications for controlling the passage of vehicles from the ramp onto the highway, first vehicle detectin" means for providing an output characteristic of traffic congestion on the highway, second vehicle detector means for providing an output in response to a vehicle on the ramp having passed the trafiic signal, and control means governed by the output of the first detector means for selecting different times of initiation of a proceed indication of the signal in accordance with respectively different degrees of congestion of tratiic on the highway, the control means being operatively governed by the output of the second detector for initiating a stop indication of the trafiic signal in response to the detection of a vehicle having passed the trafiic signal.

2. The invention according to claim 1 wherein the signal is governed by the control means to substantially continuously provide :1 proceed indication except when traffic congestion is detected on the highway by the first detector means.

3. The invention according to claim 1 wherein the signal is governed by the control means so as to be put to stop each time a vehicle is detected by the second detector, provided that tratfic congestion on the highway is de tected by the first detector.

4. A system for regulating entry of vehicles from an access ramp onto a roadway in accordance with trafiic congestion on the roadway comprising, first vehicle detecting means sensing traffic on the roadway for providing an output characteristic of trafiic congestion on the roadway, a traffic signal for designating intervals during which vehicles may move from the ramp onto the roadway, second vehicle detecting means for generating an ou-put in response to sensing the presence of a vehicle which passes the signal to enter the roadway, first control means governed by the output of the first vehicle detecting means for controlling the duration between intervals of the traffic signal to different values selected in accordance with different degrees of congestion of traffic on the roadway, and means controlled by the second sensing means for at times limiting the duration of the intervals.

5. A system for controlling a tratfic signal governing ramp traffic entering a roadway selectively in normally stop and normally proceed modes comprising, first vehicle detecting means sensing trafiic on the roadway for reg istering whether or not there is trafiic congestion on the roadway, second vehicle detecting means for registering the presence of traffic in approach of the trafiic signal, control means for operating the trafi'ic signal in a normally stop .mode to initiate a proceed indication only after a vehicle has registered its presence by actuation of the second detecting means when trafiic congestion is registered by the first detecting means, said control means being effective to operate the traffic signal in a normally proceed mode for permitting continuous flow of traffic from the ramp to the roadway if no traffic congestion is registered by the first vehicle detecting means, and third vehicle detecting means including a vehicle detector along the ramp between the signal and the roadway for resetting the signal to stop in response to passage of a vehicle past the signal when the control means for the traffic signal is in a normally stop mode.

6. The system according to claim 5 wherein timing means is rendered effective upon changing from a normally proceed to a normally stop mode for governing the control of the signal to time an internal between a time at which the normally proceed mode is terminated and a time at which the normally stop mode can be initiated.

7. The system according to claim 6 wherein the signal provides a distinctive indication during the interval timed by the timing means.

8. The system according to claim 7 wherein an aspect displayed by the signal for the distinctive indication is of a color different than aspects used for proceed and stop indications.

9. A system for regulating entry of vehicles from an access ramp onto a roadway in accordance Wi.h trafiic congestion on the roadway comprising, first Vehicle detecting means providing an output proportional to traffic congestion on the roadway, a trafiic signal designating intervals during which vehicles may move from the ramp onto the roadway, second vehicle detecting means situated to sense vehicles passing the trafiic signal, means coupling the first vehicle detecting means to the trafiic signal for defining minimum length of the intervals of the traffic signal in accordance with roadway trafiic congestion, and switching means coupling the second vehicle detecting means to the trafiic signal initiating prohibitory indications in response to vehicles passing the traffic signal.

10. The system according to claim 9-wherein the switching means is rendered operable to control the trafiic signal only after completion of the minimum length of each of the intervals.

References Cited UNITED STATES PATENTS 3,304,539 2/1967 Auer 34036 THOMAS B HABECKER, Primary Examiner.

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