Abstract:
A lane changing assistant for motor vehicles, having a speed control system and a surroundings sensor system for recording the traffic environment including the traffic in an adjacent lane, having a decision device for deciding whether a lane changing request of the driver is to be accepted, and having a command device for issuing an acceleration command to the speed control system in the case of a lane changing request, wherein a recognition device is developed to recognize a window for swinging into the adjacent lane without danger, in the light of the data of the surroundings sensor system; and the command device is developed to compute an acceleration strategy adjusted to the window, including a point in time for the beginning of the acceleration.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a lane changing assistant for motor vehicles, having a speed control system and a surroundings sensor system for recording the traffic environment including the traffic in an adjacent lane, having a decision device for deciding whether a lane changing request of the driver is to be accepted, and having a command device for issuing an acceleration command to the speed control system.  
       BACKGROUND INFORMATION  
       [0002]     A lane changing assistant of this kind is known, for example, from German Published Patent Application No. 101 14 187, and is used in connection with a speed control system, for instance, a so-called ACC system (Adaptive Cruise Control), which is not only in a position to regulate the speed of the vehicle to a speed desired by the driver, but is also in a position to adjust the speed of one&#39;s own vehicle, if necessary, to the speed of the preceding vehicle, so that the latter is being followed at a suitable safety distance. For this purpose, a surroundings sensor system is present, for example, in the form of a radar sensor, using which the distances and also the relative speeds of preceding vehicles may be measured. Such ACC systems are particularly provided for travel on multi-lane expressways or Autobahns.  
         [0003]     The lane changing assistant is used to make it easier for the driver to change to an adjacent lane, for instance, when he wishes to start on a passing procedure.  
         [0004]     The lane changing assistant known from the abovementioned document is developed in such a way that, when a lane changing request by the driver is detected, it automatically initiates an acceleration procedure, so that pulling into the flowing traffic in the adjacent lane is made easier. The target speed or passing speed to be reached in this acceleration procedure, in this context, depends not only upon the speed of the vehicle, that is to be passed, proceeding ahead in one&#39;s own lane, but also upon the speed of one or more additional vehicles that are located in the adjacent lane, ahead of one&#39;s own vehicle. In this manner, one is able to avoid running up too closely to the vehicle in the adjacent lane.  
         [0005]     A passing intention by the driver is recognized by the known system in that, for example, the driver activates the respective direction indicator (blinker). When the passing intention is recognized, the acceleration procedure is then triggered immediately.  
         [0006]     In the same document the possibility is also mentioned that one might, for example, monitor the traffic following in the adjacent lane, using a rear-facing radar, and, when the follow-on traffic permits a passing procedure, to suggest a passing procedure to the driver automatically, by a suitable signal, which the driver may then confirm or not confirm.  
         [0007]     Furthermore, lane changing assistant systems such as blind spot warning systems are known which, with the aid of the surroundings sensor system, record the adjacent lane at the side of the vehicle and behind the vehicle, and warn the driver optically, acoustically or haptically when there is a lane change and a vehicle is located in the blind spot. However, the emission of such a warning signal may be disturbing to the driver, may be overlooked or may be wrongly interpreted by the driver.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention offers the advantage that changing lanes is made easier for the driver in that the lane changing assistant automatically recognizes a suitable window for a danger-free changing to the adjacent lane, and, in the light of this information, automatically determines the most favorable point time to initiate the acceleration procedure.  
         [0009]     Consequently, in this system the recognition of a desire to pass and the actual initiation of the corresponding acceleration procedure may occur at different times. If, for example, the driver makes it known that he wishes to pass, by setting the blinker, but a danger-free lane change is not possible at this point in time, because there is another passing vehicle in the adjacent lane that has not yet gained a sufficient safety distance from one&#39;s own vehicle, the driver&#39;s desire to pass is registered and taken into consideration by the lane changing assistant according to the present invention, but the initiation of the acceleration procedure is performed only at a later point in time, namely, when the vehicle that is passing has moved away at a sufficient safety distance. In the case where an additional vehicle is following the passing vehicle in the adjacent lane, and the gap or the “window” between these vehicles is too small, the acceleration procedure continues to be put on hold until the second vehicle has also concluded its passing procedure. In this way, the driver gets very effective support with the decision as to whether and when a danger-free lane change may be made. In this context, it is particularly advantageous that the surroundings sensor system, formed, for instance, by radar sensors, is able to measure and evaluate the relevant distances and the relative speeds substantially more accurately than would be possible to a human driver, especially an inexperienced driver. Thus, a considerable contribution to traffic safety is made by the present invention.  
         [0010]     Preferably, the initiation of the acceleration procedure by the lane changing assistant forms a signal at the same time that indicates to the driver that a lane change is possible. In addition, it may be expedient if the acceleration procedure sets in at a certain minimum acceleration, which, to be sure, is not perceived to be uncomfortable by the vehicle&#39;s passengers, but on the other hand is clearly perceptible by the driver, so that the driver obtains a recognizable feedback. In this case, one may do without the additional emitting of optical or acoustical signals.  
         [0011]     The information transmitted to the driver by the initiation of the acceleration procedure may, on the one hand, be used to accept the desire to pass, as signaled by setting the blinker. When, on the other hand, the driver operates the blinker switch and thereupon the expected acceleration thrust does not occur, this indicates to the driver that a lane change is not yet possible.  
         [0012]     As a rule, known blinker switches are developed to have two stages, and in the first stage they act as a pushbutton switch without automatic holding function, while in the second stage they lock into place in a switched in state. According to one refinement of the present invention, the blinker switch may be developed in such a way that the operation of the first stage acts to switch on the directional indicator only if a lane change is actually possible. If, on the other hand, the lane change is not possible, the blinker light is switched on effectively only if a suitable window for lane change has opened. In this manner one may avoid irritating the traffic in the passing lane by the premature setting of the blinker. The lighting up of the blinker control light on the dashboard then gives the driver an additional signal that lane change is possible. The second stage of the blinker switch gives the driver the possibility of overriding this function and switching on the blinker light at once.  
         [0013]     On the other hand, the acceleration thrust triggered by the lane changing assistant may also be used for providing the driver with a passing procedure. If a passing lane is available, and if the speed of a vehicle preceding in one&#39;s own lane is lower than the speed selected by the driver as his desired speed, one may generally impute a desire to pass to the driver. Then, if the lane changing assistant detects a window for the lane change, it is signaled to the driver, by the acceleration that is setting in, that he may now initiate the passing procedure. The driver may then, on his part, confirm this by setting the blinker. If, within a suitable time interval, the confirmation is not forthcoming, the acceleration procedure is broken off, and distance regulation with respect to the preceding vehicle is taken up again.  
         [0014]     Optionally or in supplement, other criteria for the recognition of the driver&#39;s desire to pass may also be drawn upon, such as steering motions and the like.  
         [0015]     For a reliable recognition of the window for a lane change, the surroundings sensor system should also be in a position not only to monitor only the traffic in the rear on the adjacent lane, but also to detect vehicles which are at approximately the same level with one&#39;s own vehicle. Whereas a radar sensor is generally expedient for monitoring the rear space, which also permits the measurement of relative speeds, for monitoring the space next to one&#39;s own vehicle one has the choice of a very close range sensor, such as a very close range radar, a short-range lidar system, a video system or even the use of ultrasound sensors. The speed not directly measurable by such sensors of the vehicle traveling level with one&#39;s own vehicle may, in the case of passing vehicles be estimated by extrapolation of the speed previously measured with the aid of the rear space radar.  
         [0016]     Depending on whether the vehicle will be used in a country that drives on the right or on the left, the surroundings sensor system may be developed for monitoring one of the two vehicle sides or even for monitoring both vehicle sides. In the latter case, the system may also support the driver when he returns to the slower traffic lane.  
         [0017]     The monitoring of the traffic on the adjacent lane may be performed permanently or take place only when a desire for a lane change is detected or is to be accepted.  
         [0018]     The following criteria are important for the detection of a suitable lane changing window: the distances of the vehicles from one another, the sizes of the gaps, that is, the spatial window between the vehicles on the adjacent lane, and the relative speeds of the vehicles in the adjacent lane, and thus the relative speeds of the gaps. What has also to be taken into consideration is the safety distances between the individual vehicles, the acceleration capacity of one&#39;s own vehicle and, possibly, a boundary value for a certain deceleration that may be “expected” of a following vehicle. In the case of the safety distances, one should distinguish between the regular safety distances or time gaps during longer lasting travel in column and smaller minimum clearances which may not be undershot even for a short period of time. With respect to the acceleration capacity of one&#39;s own vehicle, one should possibly also take into consideration limitations that may derive from a comfort point of view, the maximum payload and the like.  
         [0019]     Upon detection and selection of a window for the lane change, one may proceed, for example, as follows. First of all, using the very close range sensor, a check is made as to whether there is a vehicle in the adjacent lane that is approximately level with one&#39;s own vehicle. If this is not the case, it is checked whether the gap between the directly preceding vehicle in the adjacent lane and the directly following vehicle in the adjacent lane is big enough to permit swinging in, in consideration of the safety distances (minimum distances). In the case of commercial vehicles or vehicle combinations, one should also possibly take into consideration the length of one&#39;s own vehicle, in this connection. If the gap is large enough, it is checked, using the relative speed, whether it is possible to accelerate one&#39;s own vehicle to such an extent that the following vehicle in the adjacent lane, after the lane change, is able to maintain a sufficient safety distance from one&#39;s own vehicle.  
         [0020]     This condition is preferably regarded as not being satisfied if the distance of the following vehicle, at the instantaneous point in time, is already less than the minimum distance. It is theoretically conceivable, to be sure, first to accelerate one&#39;s own vehicle before the lane change, in order to produce the minimum distance, but in this context, there is the danger that the minimum distance from the vehicle to be passed is undershot, and that it is difficult for the driver to time well the right point in time for the lane change. Therefore, for safety reasons, one should not make use of this possibility.  
         [0021]     In view of these criteria, if it comes to the point that a safe lane change is not possible, in analogous fashion, one after another, advancing backwards, the gaps between the following vehicles in the adjacent lane are investigated. When a suitable gap has been found, a time window may be determined from the lengths of these gaps, the clearances and the relative speeds, within which the lane change is possible. The limits of this time window then form the basis for the determination of that point in time at which the acceleration procedure is initiated.  
         [0022]     Then, in order to calculate a suitable acceleration strategy, the following, mutually dependent, parameters may be determined: the point in time at which to begin acceleration, the target speed to be reached at the end of the acceleration procedure and the magnitude of the acceleration of one&#39;s own vehicle. This magnitude may be constant, in the simplest case, but in general it may also be time-dependent.  
         [0023]     For the establishment and calculation of these parameters, various algorithms are conceivable. One approach is, while taking into consideration the acceleration capacity of one&#39;s own vehicle and points of view concerning comfort, first to establish a suitable value for the acceleration as well as a suitable value for the target speed, which should be reached at the latest possible lane changing point in time, and then, in the light of these values, to calculate the point in time to begin the acceleration. In this context, depending on the situation, the target speed may be the speed of the preceding vehicle in the adjacent lane, the speed of the following vehicle in the adjacent lane or a suitable compromise between these two. In order to shorten the passing procedure, in this context, exceeding of the desired speed selected by the driver, that is moderate and limited in time, may also be permissible. An increase in comfort may be achieved by only providing a gradual increase and decrease in the acceleration, using a limited rate of change, when establishing the acceleration, apart from the abovementioned acceleration thrust by which the passing possibility is signaled to the driver.  
         [0024]     One alternative approach is that, for the distance of the vehicle that is marked by the front end of the swing-in gap, and/or for the distance of the vehicle that is marked by the rear end of the swing-in gap, an optimal path/time curve, that is characterized by accelerations that are as small as possible, is established, and then the acceleration curve is derived from this curve.  
         [0025]     Finally, under consideration of the distance and the relative speed of the vehicle to be passed, one may establish a suitable breaking-off criterion for the case in which the driver, after the acceleration procedure has been initiated, does not confirm the lane changing desire by setting the blinker, by corresponding steering actions, or the like, and/or does not initiate the lane changing procedure by an appropriate steering action. In this case, the acceleration procedure should be broken off in time in such a way that the normal clearance control may be broken off again without undershooting of the minimum distance and without uncomfortably large vehicle decelerations. Expediently, the determination of this break-off criterion is made by calculating a break-off point in time up to which the driver must have confirmed or executed the lane change. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  shows a block diagram of a speed control system having a lane changing assistant according to the present invention.  
         [0027]      FIG. 2  shows a sketch of a traffic situation to explain the manner of functioning of the lane changing assistant.  
         [0028]      FIG. 3  shows a path/time diagram on what is shown in  FIG. 2 .  
         [0029]      FIG. 4  shows a sketch of another traffic situation.  
         [0030]      FIG. 5  shows a path/time diagram on what is shown in  FIG. 4 .  
         [0031]      FIG. 6  shows a sketch of an additional traffic situation.  
         [0032]     FIGS.  7  to  9  show path/time diagrams on what is shown in  FIG. 6 . 
     
    
     DETAILED DESCRIPTION  
       [0033]     As a simplified block diagram,  FIG. 1  shows a speed control system  10  for a motor vehicle, and an appertaining lane changing assistant  12 . Speed control system  10  receives signals from a front radar  14  and intervenes in drive system  16 , as well as in the braking system of the vehicle, if necessary, that is not shown. The functions of speed control system  10  correspond to those of a known ACC system, and are therefore not explained here in greater detail.  
         [0034]     Front radar  14  is a part of a surroundings sensor system  18 , to which there also belongs a rear-facing radar  20  and a very close range sensor  22 .  
         [0035]     Lane changing assistant  12  is represented here by three function blocks, namely, one decision device  24  to decide whether a lane changing request should be accepted, a recognition device  26  to recognize a suitable lane changing window in flowing traffic in the adjacent lane, and a computing device  28  to compute an acceleration strategy, and to output a corresponding acceleration command to speed control system  10 . In practice, the function blocks named of lane changing assistant  12  and of speed control system  10  may be formed by program modules which run on a microcomputer or a network of microcomputers.  
         [0036]     In the example shown, let it be assumed that decision device  24  accepts a lane changing request of the driver if a passing lane is available, and if the absolute speed of the directly preceding vehicle in the driver&#39;s own lane, located by front radar  14 , is less than the desired speed selected by the driver for the speed control. These data are made available by speed control system  10  to decision device  24 . The availability of a passing lane may be recognized, for example, from the fact that angular resolution front radar  14  occasionally locates vehicles in the left adjacent lane (in the case of right-hand traffic). Furthermore, decision device  24  receives a signal from a blinker switch  30  that indicates the setting of the left blinker. In the example examined here, this signal is used to confirm the lane changing intention of the driver.  
         [0037]     If decision device  24  decides that a lane changing request of the driver is to be accepted, it induces recognition device  26 , with the aid of rear-facing radar  20  and very close range sensor  22  to monitor the traffic in the adjacent lane, behind and next to the driver&#39;s own vehicle, and to search for suitable swing-in gaps. When a swing-in gap has been found, the corresponding distance data and speed data are transmitted to command device  28 , which establishes an acceleration strategy for pulling into the flowing traffic on the adjacent lane, and issues corresponding acceleration commands to speed control system  10 .  
         [0038]      FIG. 2  shows, in a top view, a two-lane roadway having a right lane  32  and a left adjacent lane  34  (passing lane). In right lane  32  there travels a vehicle  36  that is equipped with the system according to  FIG. 1 . The hatched areas show the locating range  38  of front radar  14 , locating range  40  of rear-facing radar  20  and locating range  32  of very close range sensor  22 . The front radar monitors the traffic in both lanes  32 ,  34  in front of vehicle  36 , while the rear-facing radar monitors the traffic in adjacent lane  34 , behind vehicle  36 . It is true that locating range  40  of the rear-facing radar also records right lane  32 , but, based on its angle-resolving capacity, the rear-facing radar is in a position to distinguish between the two lanes, and vehicles in right lane  32  are ignored. Locating range  42  of the very close range sensor closes the gap between locating ranges  38  and  40 , so that the traffic in adjacent lane  34  is able to be monitored in a gap-free manner.  
         [0039]     In the situation shown in  FIG. 2 , adjacent lane  34  is free, and ahead of vehicle  36  a slower vehicle  44  is traveling.  
         [0040]     In  FIG. 3 , vehicle distance d is plotted against time t. Curve d 44  gives the curve in time of the distance of vehicle  44  measured using front radar  14 . Curve s 44  in  FIG. 3  differs from curve d 44  by a minimum safety distance dmin which vehicle  36  is supposed to maintain with respect to vehicle  44 .  
         [0041]     First of all, up to point in time t 1 , both vehicles  36 ,  44  travel at constant speed, and, accordingly, curve d 44  drops off linearly. Vehicle  36  travels at the intended speed of the driver. Since vehicle  44  is slower, decision device  24  decides that a passing intention, and thus a lane changing intention on the part of the driver should be accepted. Recognition device  26  thereupon looks for a swing-in window in adjacent lane  34 , and, since this lane is free, swing-in window extends indefinitely. Therefore, the driver of vehicle  36  has great leeway in time, for carrying out the lane change.  
         [0042]     If the driver does not make a lane change, at a point in time ta 1  the ACC system would become active, and would cause a deceleration of vehicle  36 , so that vehicle  44  is followed at an appropriately large time gap. In this situation, command device  28  at point in time t 1 , that lies briefly before point in time ta, determines the output of an acceleration command. Vehicle  36  is accelerated, while vehicle  44  continues at constant speed. This manifests itself in  FIG. 3  in that curves d 44  and s 44  bend downwards from t 1  on. The acceleration setting in at t 1  is perceptible to the driver of vehicle  36 , and represents a signal for him or a request to initiate a passing procedure, and to change lanes.  
         [0043]     In the time interval between t 1  and ta 1 , the driver confirms this request by activating blinker switch  30 . If this confirmation does not take place at the latest at point ta 1 , command device  28  would break off the acceleration procedure, and the speed control system would return to normal clearance control.  
         [0044]     The steering maneuver for the actual lane change is left to the driver. If the driver does not undertake the lane change, at point tmax the minimum distance dmin to vehicle  44  would be undershot. In order to avoid this, the acceleration procedure is broken off already at a suitably calculated, earlier point ta 2 , if the driver has not carried out the lane change until that time.  
         [0045]     In the example shown, the passing request is confirmed and implemented, so that the acceleration procedure is continued. At point in time t 0 , vehicle  44  is passed at a speed which, because of the acceleration procedure, is slightly above the desired speed originally selected by the driver. Thereafter, command device  28  induces a deceleration of the vehicle, until it has returned again to the desired speed.  
         [0046]     The acceleration procedure induced by command device  28  would, in this situation, not be necessary for passing, since vehicle  36  is going faster than vehicle  44  in any case. However, this acceleration procedure fulfills a twofold function within the scope of the present invention. On the one hand, it is used as a lane changing request for the driver, and, on the other hand, it accelerates and abbreviates the passing procedure. This is of advantage, for example, in cases in which a vehicle, that has not yet been located by the rear-facing radar, approaches rapidly in the passing lane, or in cases in which, ahead of vehicle  44 , in lane  32 , there is an even slower vehicle which forces the driver of vehicle  44  either to brake, or to initiate, on its part, a passing procedure.  
         [0047]     The determination that the acceleration command is output at point t 1 , that is, shortly before the point in time at which the ACC clearance control would set in, has the advantage that a consistent, quiet driving behavior is achieved, and vehicle  36  is not first decelerated and then still accelerated again for the acceleration procedure.  
         [0048]      FIGS. 4 and 5  illustrate a modified traffic situation, in which, ahead of vehicle  36  in adjacent lane  34 , there is traveling an additional vehicle  46 , that is located by front radar  14 . In  FIG. 5 , the distance of vehicle  46  is represented by the curve d 46 , and curve s 46  represents the corresponding minimum distance.  
         [0049]     In the time period preceding the situation shown in  FIG. 4 , vehicle  46  was first located by rear-facing radar  20  and then by very close range sensor  22 . Recognition device  26  has recognized that adjacent lane  34  is not free, and that, therefore, no lane changing request was output. Instead, speed control system  10  has reduced the speed of vehicle  36  to the speed of vehicle  44 . This may be seen in  FIG. 5  by the fact that curve d 44  first runs horizontally. Vehicle  44  is followed by vehicle  36  at a constant distance dt, which corresponds to the setpoint time gap set in the ACC system. This distance is greater than dmin.  
         [0050]     At point t 1  in  FIG. 5 , the distance of vehicle  46  becomes greater than minimum distance dmin. At the earliest at this point t 1 , vehicle  36  may change to adjacent lane  34 , without the minimum distance from vehicle  46  being undershot. In the example shown, command device  28  is therefore designed in such a way that, in this situation, only at point t 1  the acceleration command is output, and therewith the lane changing request. Thus the driver is not tempted to change to the adjacent lane too early.  
         [0051]     Vehicle  36  is now accelerated, either to the desired speed selected by the driver or if, as in the example shown, the speed of vehicle  46  is less, to the speed of vehicle  46 , which then, after the driver has undertaken the lane change, is followed in adjacent lane  34  at a distance dt. The time window available to the driver for the lane change is, in this case, limited by points t 1  and ta 2 . At t 0  vehicle  44  is finally passed.  
         [0052]     As in the case described above, a break-off point ta 1  is established by the lane changing assistant, up to which the driver must have accepted the lane changing request.  
         [0053]      FIGS. 6 and 7  show an additional traffic situation which clearly differs from the situation in  FIGS. 4 and 5  by the fact that, in adjacent lane  34 , a still further vehicle  48  follows behind vehicle  46 , which is located by rear-facing radar  20  of vehicle  36 . The distance of vehicle  48  measured by the rear-facing radar is shown in  FIG. 7  by curve d 48 . Curve s 48  represents the appertaining minimum distance between vehicles  48  and  36 . Vehicles  46  and  48  travel at the same speed, and their mutual distance is less than the sum of the minimum distances dmin, so that, in  FIG. 7 , the “forbidden zones”, which are bordered by curves d 46  and s 46  on the one side and curves d 48  and s 48  on the other side, overlap with each other. Recognition device  26  thus determines that, between vehicles  46  and  48  no window is open for a safe lane change. Accordingly, the acceleration command does not happen, and all vehicles continue their travel at constant speed. At the earliest at point t 2  could an acceleration command be given out, if vehicle  48  is not followed by an additional vehicle.  
         [0054]     When vehicle  48  leaves locating range  40  of the rear-facing radar, it enters locating range  42  of the very close range sensor, and in this case, too, recognition device  26  recognizes that the lane changing window continues to be closed. At point t 2 , vehicle  48  is already located within locating range  38  of the front radar, so that its relative speed may be measured.  
         [0055]      FIG. 8  illustrates a similar situation to that in  FIG. 7 , with only the difference that now the distance between vehicles  46  and  48  is greater, so that, between curves s 46  and s 48  a window opens up for a lane change without danger. Accordingly, at t 1  the acceleration command is given by command device  28 . The target speed to which vehicle  36  is accelerated is, in this case, equal to the agreeing speed of vehicles  46  and  48 . The magnitude of the acceleration is selected by command device  28 , within firmly specified comfort limits, at a magnitude at which the distance between vehicles  36  and  48  is at no point in time less than dmin, i.e. curve s  48  does not reach the t axis. It is thereby ensured that a sufficient minimum distance from vehicle  48  is maintained, without vehicle  48  having to reduce its speed, independent of at which point in time, within the window, the driver undertakes the lane change.  
         [0056]      FIG. 9  illustrates a situation which differs from the situation according to  FIG. 8  in that vehicle  46  has a higher speed, which is greater than the desired speed for vehicle  36 . The speed of vehicle  48  is somewhat less than the desired speed, so that the distance between vehicles  48  and  36  (curve d 48 ) increases again at the end of the acceleration procedure. However, at point t 1 , at which the acceleration command is given, vehicle  48  has already approached vehicle  36  so closely that the distance between vehicles  48  and  36  undershoots the minimum distance, even at maximum possible or permissible acceleration of vehicle  36 . In  FIG. 9  this may be recognized by the fact that curve s 48  temporarily rises above the t axis. This means that, if the driver of vehicle  48  continues to travel at undiminished speed, the minimum distance is temporarily undershot. Consequently, the driver of vehicle  48  is forced, on his part, to decelerate the vehicle, in order to maintain the minimum speed. This is shown symbolically in  FIG. 9  by curves d 48 ′ and s 48 ′.  
         [0057]     This situation is permitted, in the exemplary embodiment described, under the condition that the forced deceleration of vehicle  48  remains below a certain acceleration boundary value or deceleration boundary value. This boundary value is selected in such a way that the driver of vehicle  48  is not excessively impeded and in any case not endangered, and should, in fact, be selected so that the required deceleration of vehicle  48  is less than the typical engine braking effect of a vehicle, so that the driver of vehicle  48  is not forced actively to put on the brakes.  
         [0058]     To the extent that the acceleration capacity of vehicle  36  permits it, the slight hindrance of vehicle  48  may also be avoided in that the comfort limit for the acceleration of vehicle  36  is increased.  
         [0059]     A comparable situation, in which an undershooting of the minimum distance may occur, even if the acceleration capacity of vehicle  36  is sufficient, is derived if the desired speed for vehicle  36  is less than the speed of vehicle  48 . Even in this case, the driver of vehicle  48  is finally forced to reduce his speed. A possible countermeasure is temporarily to raise the target speed for vehicle  36  above the desired speed, in order to give more time for speed adjustment to the driver of vehicle  48 .  
         [0060]     In general, for the acceleration of vehicle  36 , in the acceleration procedure triggered by command device  38 , one would select a boundary value, from a comfort point of view, which does not exhaust the actual acceleration capacity of the vehicle. However, this boundary value may vary as a function of the situation. For example, one may admit greater accelerations if the discrepancy between the speed of preceding vehicle  44  and the desired speed is very large. It is also conceivable that, at a high traffic density, the acceleration boundary is increased, so that the probability is enhanced of finding a suitable window for a lane change without danger.  
         [0061]     Whereas in the situations shown in  FIGS. 8 and 9  point in time t 1 , at which the acceleration command is issued, is in each case picked in such a way that it occurs at the same point at which front vehicle  46  has reached the minimum distance dmin on the adjacent lane (the intersection of curve s 46  with the t axis), in a modified specific embodiment, the acceleration command may also be issued already at an earlier point in time. Thereby, too, the chance improves of finding a suitable lane changing window. It is true that, in this context, one should take into consideration that, in that case, point in time tmax is also reached earlier, and that, in the case of a break-off of the acceleration procedure, a sufficient deceleration path must remain without letting vehicle  36  drive up too close to front vehicle  44 . When there is great traffic density, therefore, it may be expedient, within the scope of normal ACC control, to increase the time gap that determines distance dt, so that a greater “start-up” path is available.