Abstract:
A travel following system includes a lower limit vehicle speed setting means and an upper limit vehicle speed setting means for previously setting a lower limit vehicle speed and an upper limit vehicle speed to control the vehicle speed by a vehicle speed control means, and a control start commanding means for instructing the vehicle speed control means to start the vehicle speed control upon operation by the driver. The vehicle speed control means is adapted to discontinue a vehicle speed control at a vehicle speed between the lower limit vehicle speed set by the lower limit vehicle speed setting means and a vehicle speed in a stopped state, thereby permitting a vehicle speed control conducted by the driver, and to carry out the vehicle speed control at a vehicle speed exceeding the lower limit vehicle speed in a state in which an accelerating or decelerating operation is not conducted by the driver. The vehicle speed control means is also adapted to ensure that the vehicle speed control is discontinued when the vehicle speed becomes a level equal to higher than the upper limit vehicle speed, and even if the vehicle speed becomes again lower than the upper limit vehicle speed, the vehicle speed control is suspended until the operation of the control start commanding means is conducted. Thus, a moderate tension is provided to the driver during operation of the travel following system so that braking operation can be easily conducted and starting of the vehicle based on the driver&#39;s will can be carried out without switching operation by the driver.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a travel following system for a vehicle, which is designed to allow the subject vehicle to travel following a preceding vehicle in a relatively low speed range.  
           [0003]    2. Detailed Description of the Related Art  
           [0004]    A Stop &amp; Go system having the following functions is known as a conventional travel following system operated in a lower speed range:  
           [0005]    (1) when a preceding vehicle has been detected in a speed range from the stopped state to a predetermined vehicle speed, the subject vehicle is allowed to travel following the preceding vehicle;  
           [0006]    (2) when the subject vehicle is stopped automatically following the stopping of the preceding vehicle during operation of the system, the stopped state of the subject vehicle is maintained;  
           [0007]    (3) the driver is informed of the starting of the preceding vehicle;  
           [0008]    (4) the starting of the operation of the system is carried out by a switching operation by the driver rather than automatically: and  
           [0009]    (5) a condition for canceling the system is any of the cases where the driver has carried out the braking operation,  
           [0010]    the driver has carried out an accelerating operation,  
           [0011]    sight of the preceding vehicle has been missed, and  
           [0012]    another vehicle has intruded into between a vehicle provided with the system (which will be referred to as a subject vehicle) and a preceding vehicle.  
           [0013]    One example of a Stop &amp; Go system is known from Japanese Patent Application Laid-open No. 11-48826.  
           [0014]    In the above-described conventional Stop &amp; Go system, when a preceding vehicle is stopped, the subject vehicle is stopped automatically without a braking operation by a driver. For this reason, if the driver excessively relies on the system to be neglectful of the braking operation, there is a possibility that the braking operation by the driver is retarded when the automatic braking cannot cope with a case where another vehicle or a pedestrian dashes out from the side or a case where a preceding vehicle is suddenly stopped.  
           [0015]    In the case where the system is operated from a state in which the subject vehicle has been once stopped, if the driver tries to start the subject vehicle automatically in response to the starting of a preceding vehicle, it matters whether or not the driver is ready to drive the vehicle. In the conventional system, in order to confirm a driver&#39;s will, the driver is informed of that a preceding vehicle has been started, and the system is operated when a switch is turned on by the driver. However, if the system is designed as described above, there is a possibility that a switching operation is troublesome and the convenience of the system is detracted. If the switching operation by the driver who has been informed of the starting of the preceding vehicle is retarded, the subject vehicle may be left behind the preceding vehicle, and cannot travel following the preceding vehicle in some cases. For this reason, the driver is forced to be tense so as not to retard the switching operation. Also in this case, the convenience is detracted.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention has been accomplished with the above-described circumstance in view, and it is an object of the present invention to ensure that a moderate tension feeling is provided to a driver during operation of a travel following system so as to easily conduct a braking operation, and the starting of a vehicle based on a driver&#39;s will can be carried out without a switching operation by the driver.  
           [0017]    According to the present invention, the above object is achieved by an arrangement shown in a diagram in FIG. 11 corresponding to claims.  
           [0018]    According to a first feature of the present invention, there is proposed A travel following system comprising: an object detecting device for detecting an object in a direction of travel of a subject vehicle, a motional state detecting means for detecting a motional state of the subject vehicle, a preceding vehicle judging means for judging a preceding vehicle which the subject vehicle is to follow, based on outputs from the object detecting device and the motional state detecting means, a vehicle speed detecting means for detecting a vehicle speed of the subject vehicle, and a vehicle speed control means for controlling the vehicle speed of the subject vehicle so that the subject vehicle follows the preceding vehicle judged by the preceding vehicle judging means, wherein said system further includes a lower limit vehicle speed setting means for previously setting a lower limit vehicle speed to control the vehicle speed by the vehicle speed control means, said vehicle speed control means being adapted to ensure that a vehicle speed control discontinued at a vehicle speed between the lower limit vehicle speed set by the lower limit vehicle speed setting means and a vehicle speed in a stopped state, thereby permitting a vehicle speed control conducted by a driver, and the vehicle speed control is carried out at a vehicle speed exceeding the lower limit vehicle speed in a state in which an accelerating or decelerating operation is not conducted by the driver.  
           [0019]    With the above arrangement, the vehicle speed control is discontinued at the vehicle speed between the lower limit vehicle speed set by the lower limit vehicle speed setting means and the vehicle speed in the stopped state, thereby permitting a vehicle speed control conducted by the driver, and the vehicle speed control is carried out at the vehicle speed exceeding the lower limit vehicle speed in the state in which the accelerating or decelerating operation is not conducted by the driver. Therefore, when the vehicle speed of the subject vehicle is reduced to a level equal to or lower than the lower limit vehicle speed with the deceleration of the preceding vehicle, the vehicle is stopped by a spontaneous braking operation by the driver. As a result, it is prevented that the driver excessively relies on the system to be neglectful of the braking operation, and a prompt braking operation by the driver is possible in case of emergency.  
           [0020]    According to a second feature of the present invention, in addition to the first feature, the system further includes an upper limit vehicle speed setting means for previously setting an upper limit vehicle speed to control the vehicle speed by the vehicle speed control means, and a control start commanding means for instructing the vehicle speed control means to start the vehicle speed control upon operation by the driver, said vehicle speed control means being adapted to ensure that the vehicle speed control is discontinued when the vehicle speed becomes a level equal to higher than the upper limit vehicle speed, and even if the vehicle speed becomes again lower than the upper limit vehicle speed, the vehicle speed control is suspended until the operation of the control start commanding means is conducted.  
           [0021]    With the above arrangement, the vehicle speed control is started by the driver operating the control start commanding means, and when the vehicle speed becomes the level equal to or higher than the upper limit vehicle speed previously set, the vehicle speed control is discontinued. Even if the vehicle speed is again lower than the upper limit vehicle speed, the vehicle speed control is suspended until the operation of the control start commanding means is conducted. Therefore, it is possible to prevent an unexpected vehicle speed control conducted by the driver from being started to provide an uncomfortable feeling, when the vehicle becomes again lower than the upper limit vehicle speed after the vehicle speed becomes a level equal to or higher than the upper limit vehicle speed to discontinue the vehicle speed control. In addition, after the driver operates the control start commanding means to start the vehicle speed control, the vehicle speed control is continued as long as the vehicle speed is the upper limit vehicle speed or more. Therefore, it is unnecessary to frequently operate the control start commanding means, leading to an enhancement in convenience.  
           [0022]    According to a third feature of the present invention, in addition to the first or second feature, the system further includes an informing means for informing the driver of that the vehicle speed is immediately before becoming a level equal to or lower than the lower limit vehicle speed, or that the vehicle speed has reached the level equal to or lower than the lower limit vehicle speed.  
           [0023]    With the above arrangement, the driver is informed of that the vehicle speed is immediately before becoming a level equal to or lower than the lower limit vehicle speed, or that the vehicle speed has reached the level equal to or lower than the lower limit vehicle speed. Therefore, the driver can recognize that the spontaneous vehicle speed control is possible.  
           [0024]    According to a fourth feature of the present invention, in addition to the third feature, the informing means informs the driver of contents urging the driver for a decelerating operation.  
           [0025]    With the above arrangement, when the vehicle speed is immediately before becoming a level equal to or lower than the lower limit vehicle speed, or when the vehicle speed has reached the level equal to or lower than the lower limit vehicle speed, the driver is informed of contents urging the driver for a decelerating operation. Therefore, the driver can carry out the decelerating operation for immediately stopping the vehicle.  
           [0026]    A set switch  33  in an embodiment corresponds to the control start commanding means of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    [0027]FIG. 1 is a block diagram of an object detecting device.  
         [0028]    [0028]FIG. 2 is a perspective view of the object detecting device.  
         [0029]    [0029]FIG. 3 is a diagram showing a main switch for a travel following system.  
         [0030]    [0030]FIG. 4 is a diagram showing a set switch and a cancel switch for the travel following system.  
         [0031]    [0031]FIG. 5 is a diagram showing a main switch turned-on state indicating lamp and a warning lamp.  
         [0032]    [0032]FIG. 6 is a diagram showing a display of the travel following system.  
         [0033]    [0033]FIG. 7 is a diagram for explaining conditions for switching over various modes of the travel following system.  
         [0034]    [0034]FIG. 8 is a diagram showing turned-on states of the indicating lamp and the display in various situations.  
         [0035]    [0035]FIG. 9 is a diagram for explaining the operation when a vehicle is changed from a stopped state to a travel following controlled state.  
         [0036]    [0036]FIG. 10 is a diagram for explaining the operation when a preceding vehicle has been stopped during a travel following control.  
         [0037]    [0037]FIG. 11 is a block diagram (a diagram corresponding to claims) of a control system for the travel following system. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0038]    A preferred embodiment of the present invention will now be described with reference to the accompanying drawings.  
         [0039]    As shown in FIGS. 1 and 2, an object-detecting system R for detecting the distance and direction to an object ahead of a subject vehicle includes a laser radar device, and comprises a light transmitting section  1 , a transmitted-light scanning section  2 , a light-receiving section  3 , a received-light scanning section  4  and a distance-measuring section  5 . The light-transmitting section  1  includes a laser diode  11  integrally provided with a light-transmitting lens, and a laser diode drive circuit  12  for driving the laser diode  11 . The transmitted-light scanning section  2  includes a light-transmitting mirror  13  for reflecting the laser output by the laser diode  11 , a motor  15  for reciprocally turning the light-transmitting mirror  13  about a vertical axis  14 , and a motor drive circuit  16  for controlling the driving of the motor  15 . A light beam emitted from the light-transmitting mirror  13  has a vertically elongated pattern having a limited lateral width, and is moved reciprocally in a lateral direction at a predetermined cycle to scan an object.  
         [0040]    The light-receiving section  3  includes a light-receiving lens  17 , a photodiode  18  adapted to receive a reflected wave converged by the light-receiving lens  17  to convert the reflected wave into an electric signal, and a received-light amplifying circuit  19  for amplifying a signal output from the photodiode  18 . The received-light scanning section  4  includes a received-light mirror  20  adapted to reflect a reflected wave from the object to introduce it to the photodiode  18 , a motor  22  for reciprocally turning the received-light mirror  20  about a lateral axis  21 , and a motor drive circuit  23  for controlling the driving of the motor  22 . A light-receiving area having a laterally elongated pattern having a limited vertical width is moved reciprocally in a vertical direction at a predetermined cycle by the received-light mirror  20  to scan the object.  
         [0041]    The distance measuring section  5  includes a control circuit  24  for controlling the laser diode drive circuit  12  and the motor drive circuits  16  and  23 , a communication circuit  26  for communicating with an electronic control unit  25  controlling a travel following system (which will be hereinafter referred to as “the following system”), a counting circuit for counting a time from the transmission of the laser to the reception of the laser, and a central calculation device  28  for calculating a distance from the subject vehicle to an object and a direction to the object.  
         [0042]    Thus, an area where the vertically elongated transmitted-light beam and the laterally elongated transmitted-light beam cross each other, is an instantaneous detection area. The detection area has a lateral width equal to a lateral scanning width of the transmitted-light beam, and is moved in a zigzag manner over the entire detection region having a vertical width equal to the vertical scanning width of the light-receiving area, to thereby scan the object. A distance to the object is detected based on time from the transmission of the light beam to the reception of the reflected wave resulting from the reflection of the light beam from the object, and a direction to the object is detected based on a direction of an instantaneous detection area at that time.  
         [0043]    A driver interface of the following system according to the present embodiment will be described below.  
         [0044]    As shown in FIG. 3, a main switch  31  for the following system is mounted at an upper portion of an instrument panel. As shown in FIG. 4, a set switch  33  and a cancel switch  34  are mounted on a steering wheel  32 .  
         [0045]    As shown in FIG. 5, a main switch turned-on state indicating lamp  35  and a warning lamp  36  are mounted on a meter panel. The main switch turned-on state indicating lamp  35  is turned on when the main switch  31  is turned on. The warning lamp  36  is turned on when an abnormality is generated in the following system. As shown in FIG. 6, a system operation indication  38  for indicating that the following system is in operation, a preceding vehicle presence indication (a travel following control indication)  39 , a brake warning indication  40  and a fail indication  41  are provided on a display  37  mounted in the meter panel, so that they can be turned on. In the embodiment, a preset vehicle-vehicle distance for travel following control is capable of being switched over three stages. When the preset vehicle-vehicle distance is large, all three blocks of the system operation indication  38  are turned on. When the present vehicle-vehicle distance is medium, two of the three blocks are turned on. When the present vehicle-vehicle distance is small, one of the three blocks is turned on.  
         [0046]    The outline of the control system for the following system will be described below with reference to FIG. 11.  
         [0047]    The following signals are input to the electronic control unit  25 : a signal from a motional state-detecting means M 1  for detecting a motional state such as a yaw rate, a steering angle and a vehicle speed of the subject vehicle; a signal from a vehicle speed detecting means M 3  for detecting the vehicle speed of the subject vehicle; a signal from an object detecting device R and a signal from a control start commanding means  33  (the set switch  33 ). Based on these signals, the electronic control unit  25  controls the operations of a throttle actuator  42  for opening and closing a throttle valve, a brake actuator  43  for operating a brake device and a buzzer  44  for generating a warning sound.  
         [0048]    The electronic control unit  25  includes a preceding vehicle judging means M 2 , a vehicle speed control means M 4 , a lower-limit vehicle speed setting means M 5 , an upper-limit vehicle speed setting means M 6  and an informing means M 7 .  
         [0049]    The preceding vehicle judging means M 2  judges, as a preceding vehicle, a moving object on a travelling locus of the subject vehicle presumed based on the motional state detected by the motional state-detecting means M 1 . The lower-limit vehicle speed setting means M 5  sets a lower-limit vehicle speed (5 km/hr in the embodiment) on the order of a vehicle speed immediately before stopping or immediately after starting, at which the travel following control is carried out. The upper-limit vehicle speed setting means M 6  sets an upper-limit vehicle speed (40 km/hr in the embodiment) at which the travel following control is carried out. In a state in which the control start commanding means  33 , that is, the set switch  33  is turned on, the vehicle speed control means M 4  controls the vehicle speed by operating the throttle actuator  42  and the brake actuator  43  so that the subject vehicle travels following the preceding vehicle detected by the preceding vehicle judging means M 2 , when a vehicle speed of the subject vehicle detected by the vehicle speed detecting means M 3  is between the lower-limit vehicle speed and the upper-limit vehicle speed. The informing means M 7  informs a driver by operating the buzzer  44 , when the vehicle speed of the subject vehicle detected by the vehicle speed detecting means M 3  is equal to or lower than the lower-limit vehicle speed and when the preceding vehicle in a stopped state is started.  
         [0050]    As shown in FIG. 7, the following system has “system turned-off mode”, “system turned-on mode”, “standby mode” and “travel following control mode” as main modes. When the main switch  31  is turned on from “system turned-off mode”, the mode is changed to “system turned-on mode”. When the main switch  31  is turned off from any of “system turned-on mode”, the “standby mode” and “travel following control mode”, the mode is changed to “system turned-off mode”. When the set switch  33  is turned on from “system turned-on mode”, the mode is changed to “standby mode”. When the cancel switch  34  is turned on from “standby mode” or “travel following control mode”, the mode is changed to “system turned-on mode”. The following system is adapted to be operated during traveling of the vehicle at a relatively low speed lower than 40 km/hr, and when the vehicle speed is equal to or higher than 40 km/hr in “standby mode” or “travel following control mode”, the mode is automatically changed to “system turned-on mode”, even if the cancel switch  34  is not turned on.  
         [0051]    If all the following conditions are established in “standby mode”: (1) the vehicle speed is in a range of 5 km/hr to 40 km/hr; (2) a brake pedal is not operated; (3) an accelerator pedal is not operated; and (4) a preceding vehicle has been detected, the mode is changed to “travel following control mode”. A condition that an engine rotational speed is an idling rotational speed may be used in place of the condition (3) that the brake pedal is not operated. If any of the following conditions is established in “travel following control mode”: (1) the brake pedal has been operated; (2) the accelerator pedal has been operated; (3) a preceding vehicle is not detected; and (4) the vehicle speed is equal to or lower than 5 km/hr, the mode is changed to “the standby mode”. A condition that the engine rotational speed is higher than the idling rotational speed may be used in place of the condition (2) that the accelerator pedal has been operated.  
         [0052]    [0052]FIG. 8 shows examples of indications on the main switch turned-on state indicating lamp  35 , the warning lamp  36  and the display  37 .  
         [0053]    When the following system is in the system turned-off mode, namely, when the main switch  31  is in a turned-off state, all the main switch turned-on state indicating lamp  35 , the warning lamp  36  and the display  37  are turned off.  
         [0054]    When the mode is the system turned-on mode, namely, when the main switch  31  is in a turned-on state, the main switch turned-on state indicating lamp  35  is turned on, and the warning lamp  36  and the display  37  are turned off.  
         [0055]    When the following system is in the standby mode, namely, when the main switch  31  is in the turned-on state; the set switch  33  is in a turned-on state, a preceding vehicle is not detected and the vehicle speed is in a range of 0 km/hr to 40 km/hr, the warning lamp  36  is turned off; the main switch turned-on state indicating lamp  35  is turned on; and the system operation indication  38  on the display  37  is turned on.  
         [0056]    When the following system is in the travel following control mode, namely, when the main switch  31  is in the turned-on state, namely, the main switch  33  is in the turned-on state; the preceding vehicle is not detected; the vehicle speed is in a range of 5 km/hr to 40 km/hr; and neither the accelerator pedal nor the brake pedal is not operated, the warning lamp  36  is turned off; the main switch turned-on state indicating lamp  35  is turned on, and the system operation indication  38  and the preceding vehicle presence indication (the travel following control indication)  39  on the display  37  are turned on.  
         [0057]    When the following system is in the brake warning mode, namely, when the preceding vehicle is decelerated during the travel following control, and hence the further depression of the brake pedal of the subject vehicle is required, the warning lamp  36  is turned off; the main switch turned-on state indicating lamp  35  and the system operation indication  38 , the preceding vehicle presence indication (the travel following control indication)  39  and the brake warning indication  40  on the display  37  are turned on.  
         [0058]    When the following system is in the trouble warning mode, namely, when a trouble occurs in the following system, the warning lamp  36  is turned on; the main switch turned-on state indicating lamp  35  is turned on, and the fail indication  41  on the display  37  is turned on.  
         [0059]    The operation when the following system is changed from the stopped state to the travel following control state will be described below with reference to FIG. 9.  
         [0060]    As shown in FIG. 9A, the preceding vehicle is in stoppage; the subject vehicle is in stoppage in the rear of the preceding vehicle in a state in which the main switch  31  is in the turned-on state, the set switch  33  is in the turned-on state and the driver has depressed the brake pedal. At this time, the following system is in the standby state, and the system operation indication  38  on the display  37  is in the turned-on state. As shown in FIG. 9B, when the preceding vehicle is started, the buzzer  44  is operated to urge the driver for the starting of the subject vehicle, and the subject vehicle is started in a creeping manner by releasing the brake pedal by the driver who has heard the buzzer. When the vehicle speed reaches 5 km/hr, the following system is changed from the standby mode to the travel following control mode, and the preceding vehicle presence indication (the travel following control indication)  39  in addition to the system operation indication  38  on the display is turned on.  
         [0061]    As shown in FIG. 9C, the travel following control to the preceding vehicle is started, and as long as the vehicle speed is in the range of 5 km/hr to 40 km/hr, the travel following control is continued. As shown in FIG. 9D, when the driver has depressed the accelerator pedal to increase the vehicle speed of the subject vehicle to a speed equal to or higher than 40 km/hr, the following system is changed to the system turned-on mode, and the display  37  is turned off.  
         [0062]    The operation when the preceding vehicle has been stopped during the travel following control will be described below with reference to FIG. 10.  
         [0063]    As shown in FIG. 10A, it is supposed that the subject vehicle traveling at 60 km/hr has approached a preceding vehicle traveling at a speed lower than 40 km/hr. In this case, the main switch  31  is turned on to provide the system turned-on mode, and the display  37  is in the turned-off state. As shown in FIG. 10B, when the set switch  33  is turned on at the time when the subject vehicle is decelerated to a speed lower than 40 km/hr, the following system is changed from the system turned-on mode to the standby mode, and the system operation indication  38  on the display  37  is turned on. As shown in FIG. 10C, when the driver releases the brake pedal or the accelerator pedal, the following system is changed from the standby mode to the travel following control mode, and the travel following control is started by the following system, whereby the system operation indication  38  and the preceding vehicle presence indication (the travel following control indication)  39  are turned on.  
         [0064]    As shown in FIG. 10D, when the preceding vehicle is decelerated and as a result, the further depression of the brake pedal is required, the mode of the following system is changed to the brake warning mode, and the brake warning indication  40  is turned on. As shown in FIG. 10E, when the driver depresses the brake pedal, or the vehicle speed reaches to a level equal to lower than 5 km/hr, the following system is changed from the travel following control mode to the standby mode, and the travel following control by the following system is discontinued, whereby the preceding vehicle presence indication (the travel following control indication)  39  is turned off. When the subject vehicle is then stopped, the system is restored to the above-described state shown in FIG. 9A.  
         [0065]    When the vehicle speed of the subject vehicle reaches to a speed equal to or lower than 5 km/hr which is the lower limit vehicle speed due to the deceleration of the preceding vehicle during the travel following control, the driver is informed of this fact by the buzzer  44 , whereby the driver is urged to conduct the braking operation. Therefore, the driver can immediately carry out the braking operation for stopping the vehicle. By stopping the vehicle by the spontaneous braking operation by the driver in the above-described manner, it is prevented that the driver excessively relies on the following system to be neglectful of the braking operation. Thus, the prompt braking operation by the driver is enabled in case of emergency.  
         [0066]    The spontaneous acceleration or deceleration by the driver is enabled at a vehicle speed equal to or lower than 5 km/hr which is the lower limit vehicle speed. Therefore, if the set switch  33  is once operated, it is not required that the set switch  33  is operated every time the vehicle in the stopped state is started, unless the vehicle reaches a level equal to or higher than 40 km/hr which is the upper limit vehicle speed, leading to an enhancement in convenience. Moreover, when the vehicle speed reaches a level equal to or higher than 40 km/hr which is the upper limit vehicle speed, whereby the vehicle speed control is discontinued, the vehicle speed control is suspended until the set switch  33  is subsequently turned on. Therefore, when the vehicle speed is again reduced to a speed lower than the upper limit vehicle speed after the vehicle speed has been increased to a speed equal to or higher than the upper limit vehicle speed to discontinue the vehicle speed control, the unexpected vehicle speed control by the driver is prevented from being started to provide an uncomfortable feeling to the driver.  
         [0067]    Although the embodiment of the present invention has been described in detail, it will be understood that various modifications in design may be made within a scope which does not depart from the subject matter of the present invention.  
         [0068]    For example, the laser radar device has been employed as the object detecting system R in the embodiment, but a milliwave radar device or a television camera may be employed.  
         [0069]    In addition, the driver is informed by another means such as a chime, a voice and a lamp in place of the buzzer  44  employed in the embodiment.  
         [0070]    Further, the values of the upper and lower limit vehicle speeds are not limited to those in the embodiment and may be appropriately changed.  
         [0071]    Yet further, the driver is informed by the buzzer  44  when the vehicle speed reaches to the level equal to or lower than the lower limit vehicle speed in the embodiment, but the driver may be informed when the vehicle speed nears the lower limit vehicle speed.