System for determining passability of vehicle

[0002] 1. Field of the Invention

[0003] The present invention relates to a system for determining the passability of a vehicle, which involves perceiving a shape of a road based on map data comprised of an aggregation of a plurality of nodes and determining whether the vehicle can pass through the nodes.

[0004] 2. Description of the Related Art

[0005] Such a system for determining the passability of the vehicle is known from Japanese Patent Application Laid-open No. 5-141979, wherein a radius of curvature of a curve of a road is presumed by calculating a radius of an arc passing through three nodes, and a passable speed of the vehicle calculated based on the radius of curvature of this curve is compared with a passage-predicting speed of the vehicle calculated based on a current vehicle speed. If the passage-predicting speed is equal. to or lower than the passable speed, it is determined that the vehicle can pass through the curve.

[0006] In the above conventional system, if three nodes N₁, N₂and N₃exist on the same curve, as shown in FIG. 12A, a radius R of curvature of the curve can be correctly presumed. However, the above conventional system suffers from the following problem: If central one of the three nodes N₁, N₂and N₃exists on a curve and the nodes N₁and N₃in front and in rear of the central node are out of the curve, as shown in FIG. 12B, a radius R′ of curvature of the curve presumed from the three nodes N₁, N₂and N₃is larger than an actual radius R of curvature of the curve and hence, the shape of the curve cannot be correctly perceived.

[0007] Accordingly, it is an object of the present invention to ensure that even when three or more nodes do not exist on the same curve, it can be correctly determined whether the vehicle can pass through the nodes.

[0008] To achieve the above object, according to the present invention, there is provided a system for determining the passability of a vehicle, comprising map information outputting means for outputting a map information as an aggregation of a plurality of nodes constituting a road section, subject-vehicle position detecting means for detecting a position of the subject vehicle on a map, curve section determining means for determining whether a node ahead of the subject vehicle position exists on a curve section, passing-state determination amount calculating means for calculating a passing-state determination amount of the node existing on the curve section as a value resulting from division of an amount of variation in azimuth angle of the vehicle by a distance of movement of the vehicle, and passability determining means for determining whether the vehicle can pass through the node existing on the curve section, based on the passing-state determination amount.

[0009] With the above arrangement, if it is determined whether the node ahead of the subject vehicle position exists on the curve section, the system determines whether the vehicle can pass through the node existing on the curve section. The need for determining whether the vehicle can pass through a node existing on a straight road section is thereby eliminated, resulting in few calculations being performed. In addition, the passing-state determination amount is calculated from dividing the amount of variation in azimuth angle of the vehicle by the distance of movement of the vehicle. The passing-state determination amount thus serves to accurately represent a degree of difficulty in the passage of the vehicle through the node. Even when only one or two nodes exist on a curve, it is possible to accurately determine whether the vehicle can pass through the curve.

[0010] In addition to the above feature, the amount of variation in azimuth angle of the vehicle is calculated as an angle formed by a line segment connecting the adjacent nodes and the distance of movement of the vehicle is calculated based on a distance between the adjacent nodes. The amount of variation in azimuth angle and the distance of movement can be easily and accurately calculated.

[0011] In addition to the above feature, the system further includes passable speed calculating means for calculating a passable speed enabling the vehicle to pass the node existing on a curved road section, based on the passing-state determination amount, and a preset lateral acceleration limit in order for the vehicle to pass through the node. The passing-state determination amount may be defined according to the following equation:

[0012] passable speed=(preset limit lateral acceleration/passing-state determination amount)^½

[0013] Thus, it is possible to accurately calculate a passable speed which does not exceed the preset lateral acceleration limit.

[0014] In addition to the above feature, the system further includes passable speed calculating means for calculating a passable speed enabling the vehicle to pass the node existing on a curved road section, based on the passing-state determination amount. The passable speed calculating means calculates the passable speed based on a preset yaw rate limit that is permissible in the passage of the vehicle through the node, and the passing-state determination amount according to the following equation:

[0015] passable speed=preset limit yaw rate/passing-state determination amount

[0016] Thus, it is possible to accurately calculate a passable speed which does not exceed the preset yaw rate limit.

[0017] The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the accompanying drawings.

[0018] FIGS. 1 to 12B illustrate an embodiment of the present invention, wherein

[0019] FIG. 1 is a block diagram illustrating the present system according to the embodiment of the present invention;

[0020] FIG. 2 is an illustration for explaining a function of road shape determining means;

[0021] FIG. 3 is a diagram for explaining a pre-reading section and a searching section utilized by the present system;

[0022] FIG. 4 is a first portion of a flow chart illustrating the operational steps of the present system;

[0023] FIG. 5 is a second portion of the flow chart;

[0024] FIG. 6 is a third portion of the flow chart;

[0025] FIG. 7 is an illustration for explaining a link length I_Nand a crossed axes angle θ_N;

[0026] FIGS. 8A and 8B are illustrations for explaining step S5 in the flow chart;

[0027] FIGS. 9A, 9B and 9C are illustrations for explaining the definition of a passing-state determination amount θ_N/L_N;

[0028] FIGS. 10A and 10B are illustrations for explaining step S13 in the flow chart;

[0029] FIGS. 11A and 11B are illustrations of maps for searching correcting factors K_Cand K_S;

[0030] FIGS. 12A and 12B illustrate shortcomings with the prior art;

[0031] FIGS. 13 to 17 illustrate a first modification of the present invention, wherein

[0032] FIG. 13 is a block diagram of the entire arrangement of the system;

[0033] FIG. 14 is a diagram for explaining a pre-reading section and a searching section;

[0034] FIG. 15 is a third portion of a flow chart corresponding to FIG. 6;

[0035] FIGS. 16 and 17 illustrate shortcomings with the prior art;

[0036] FIGS. 18 to 20 illustrate a second modification of the present invention, wherein

[0037] FIG. 18 is a block diagram of the entire arrangement of a system;

[0038] FIG. 19 is a third portion of a flow chart corresponding to FIG. 6; and

[0039] FIG. 20 is an illustration for explaining the operation of the present system.

[0040] The present invention will now be described by way of a preferred embodiment with reference to FIGS. 1 to 11B.

[0041] Referring to FIG. 1, a system for determining whether a vehicle can pass according to the present invention includes map information outputting means M1, subject-vehicle position detecting means M2, curve-section determining means M3, passing-state determination-amount calculating means M4, passable-speed calculating means M5, vehicle speed detecting means M6, passage-predicting speed calculating means M7, passability determining means M8, vehicle control warning means M9, and vehicle speed regulating means M10. The curve-section determining means M3 and the passing-state determination-amount calculating means M4 constitute road-shape determining means M11.

[0042] The map information outputting means M1 and the subject-vehicle position detecting means M2 are mounted in a known navigation system for a motor vehicle. The map information outputting means M1 reads and outputs road data in a predetermined range previously stored on an IC card, a CD-ROM, a photo-magnetic disk or other device on which data can be rewritten. The subject-vehicle position detecting means M2 detects the position of the subject vehicle on a map by superposing subject-vehicle data received from a GPS transmission. The road data is comprised of a large number of nodes N_Nestablished at predetermined distances along a road.

[0043] As can be seen from FIG. 1 in combination with FIG. 2, the curve-section determining means M3 includes first determining means for determining whether a node N_Nahead of the position P of the subject vehicle exists on a curve or on a straight road section, based on the road data and the position P of the subject vehicle. The passing-state determination-amount calculating means M4 includes second determining means M4₁whose contents will be described hereinafter, third determining means M4₂, fourth determining means M4₃, first correcting means M4₄and second correcting means M4₅. The passing-state determining-amount calculating means M4 calculates a passing-state determination amount θ_N/L_Nwhich is used in determining whether the vehicle can pass through a curve.

[0044] The passable-speed determining means M5 calculates a passable speed V_maxNwhich is a maximum vehicle speed at which the vehicle can pass safely through the Node N_N, based on the passing-state determination amount θ_N/L_Nand a preset lateral acceleration limit G (or preset yaw rate limit YR) previously set at a value at which a driver can drive the vehicle to safely pass through a curve.

[0045] The vehicle speed detecting means M6 detects a current vehicle speed V of the subject vehicle based on output generated by wheel speed sensors mounted on the vehicle wheels. The passage-predicting speed calculating means M7 calculates a passage-predicting speed V_Nat which the vehicle will pass through the node N_N, based on the vehicle speed V, the position P of the subject vehicle and a reference deceleration rate or value β previously set for the vehicle. The passability determining means M8 compares the passage-predicting speed V_Nwith the passable speed V_maxN. If V_N≦V_maxN, then it is determined that the vehicle can pass through the node N_N, and if V_N>V_maxN, then it is determined that it is difficult for the vehicle to pass through the node N_N, at which point the warning means M9 comprising a buzzer or a lamp is activated to alert the driver to decelerate the vehicle, and the vehicle speed regulating means M10 comprising automatic brake means or engine output reducing means is operated to automatically decelerate the vehicle.

[0046] As shown in FIG. 3, a pre-reading section and a searching section are established on a road ahead of the position P of the subject vehicle. The pre-reading section is established between the position P of the subject vehicle and the node N_Nthrough which it is determined that the vehicle can pass, in order to ensure a predetermined time t exists until the vehicle passes through the pre-reading section and reaches the node N_N. The pre-reading section thereby allows for a sufficient time to determine the passibility of the vehicle and to operate the warning means M9 and the vehicle speed regulating means M10. The searching section is established for determining whether the vehicle can pass through the node N_Nexisting within the searching section. Thus, determinations of vehicle passability through distant nodes N_Nfar ahead of the searching section are avoided.

[0047] The pre-reading section is determined by a distance Vt−(βt²/2) through which the vehicle travels within the predetermined time t, wherein β represents a previously set reference deceleration rate or value which is presumed to be generated by braking action spontaneously initiated by the driver at the position P of the subject vehicle in order to allow the vehicle to pass through a curve ahead of the subject vehicle. A starting point of the searching section is established at a terminal end of the pre-reading section, and a terminal end of the searching section is set at a location at which the vehicle, which decelerated at the reference deceleration rate β, is stopped, i.e., at a location spaced apart from the subject vehicle position P by a distance V²/2β.

[0048] The operation of the embodiment of the present invention will be described below with reference to flow charts in FIGS. 4 to 6.

[0049] First, at step S1, coordinate points of a plurality of nodes N_N(N_N=N₁, N₂, N₃. . . ) existing in the searching section are read, and at step S2, a link length 1_Nand a crossed axes angle θ_Nat each of the nodes N_Nare read. As shown in FIG. 7, the link length 1_Nis defined as a distance between adjacent nodes N_Nand N_N+1, and the crossed axes angle θ_Nis defined as an angle formed between a link N_N−1N_Nand a link N_NN_N+1located ahead of the link N_N−1N_N. The link length 1_Nand the crossed axes angle θ_Ncan be geometrically calculated from the fact that the position of each of the nodes N_Nis provided by the coordinates.

[0050] Next, the first determining means of curve section determining means M3 determines whether the node N_Nexists on a curve or on a straight road, as shown in FIG. 2. This determination is performed by calculating θ_N/1_Nat each of the nodes N_Nat step S3 and comparing the θ_N/1_Nwith a previously set first determining reference value φ_REF. If θ_N/1_N≦φ_REF, then it is determined that the node N_Nexists on a curve, thereby proceeding to step S5. The θ_N/1_Ncorresponds to an amount of variation in azimuth angle of the vehicle relative to the distance of movement of the vehicle. A larger value of θ_N/1_Nindicates that the road is curved, and a smaller value of θ_N/1_Nindicates that the road is straight.

[0051] Even if θ_N/1_N<φ_REFat step S3, the crossed axes angle θ_Nis compared with a second determining reference value θ_REF. If θ_N≧θ_REF, i.e., if the crossed axes angle θ_Nitself is equal to or larger than the second determining reference value θ_REF, it is determined that the road is curved, and the system proceeds to step S5. On the other hand, if θ_N/1_N<φ_REFat step S3 and θ_N<θ_REFat step S4, it is determined at step S15 that the road is straight. When the road is straight, a passing-state determination amount θ_N/L_N, which will be described hereinafter, is set at zero.

[0052] If it is determined by the curve section determining means (the first determining means) M3 that the node N_Nexists on the curve, it is determined by the second determining means M4₁whether two continuous nodes N_Nand N_N+1exist on the same curve. The determination by the second determining means M4₁is carried out by comparing the link length I_Nbetween the nodes N_Nand N_N+1with a third determining reference value I_maxNat step S5. If I_N≦I_maxN, it is determined that the nodes N_Nand N_N+1exist on the same curve, proceeding to step S6. If I_N>I_maxN, it is determined that the nodes N_Nand N_N+1do not exist on the same curve, and the system proceeds to step S10.

[0053] Step S5 will be described below with reference to FIGS. 8A and 8B. FIG. 8A shows a state in which three nodes N_N−1N_Nand N_N+1are arranged along the same arcuate path about a center O, as a representative example of a common curve. The crossed axes angle θ_Nat the node N_Nis equal to a center angle ∠N_NON_N+1of the arc and hence, the link length I_Nbetween the node N and N_N+1is given according to

[0054] I_N=2d/tan (θ_N/4)  (1)

[0055] wherein d is a sag between the nodes N_Nand N_N+1.

[0056] The sag d is a value which is a criterion for determining the distance (i.e., the link length I_N) between the adjacent nodes N_Nwhen data for a road map are made. By determining the position of each node N_Nso that the sag d is equal to or smaller than a predetermined value (such as between approximately 3.5 m to 6 m), the shape of a road is expressed by a minimum number of nodes N_N. As a result, nodes N_Nare densely disposed such that the link length I_Nbetween adjacent nodes N_Nis smaller in a sharp curve. Nodes N_Nare disposed roughly along a less sharp curve such that the link length I_Nbetween the adjacent nodes N_Nis larger.

[0057] Thus, the third determining reference value I_Nmaxis defined as a maximum link length according to

[0058] I_maxN=2d/tan (θ_N/4)  (2)

[0059] and if I_N≦I_maxNis established at step S5, it is determined that the nodes N_Nand N_N+1exist on the same curve. If I_N≦I_maxNis not established, it is determined that the nodes N_Nand N_N+1exist on different curves (see FIG. 8B).

[0060] If it is determined in the second determining means M4₁that the nodes N_Nand N_N+1exist on the same curve, then passing-state determination amounts θ₁/L₁. . . θ_N/L_Nof an N number of nodes N₁. . . N_Nexisting on the same curve are determined at step S6 in the following manner. For a first N−1 number of nodes N₁to N_N−1, passing-state determination amounts θ_K/L_K(K=1 to N−1) are determined according to

[0061] θ_K/L_K←θ_K/I_K

[0062] A passing-state determination amount θ_N/L_Nof a final N-th node N_Nis determined according to

[0063] θ_N/L_N←θ_N/(I_N−1or a shorter value of I_maxN)  (4)

[0064] wherein I_maxNis equal to 2d/tan(θ_N/4) defined in the equation (2).

[0065] Specifically, when two consecutive nodes N₁and N₂exist on the same curve, as shown in FIG. 9A, a passing-state determination amount θ₁/L₁for the first node N₁is determined as θ₁/I₁, and a passing-state determination amount θ₂/L₂for the second (last) node N₂is determined as θ₂/(I₂or a shorter value of I_max2). When three consecutive nodes N₁, N₂and N₃exist on the same curve, as shown in FIG. 9B, passing-state determination amounts θ₁/L₁and θ₂/L₂, respectively, for the first and second nodes N₁and N₂are determined as θ₁/I₁and as θ₂/I₂, and a passing-state determination amount θ₃/L₃for the third node N₃is determined as θ₃/(I₂or a shorter value of I_max3).

[0066] The last node N_Nof a plurality of consecutive nodes N₁, . . . N_Non the same curve can be determined at the time when the answer at step S5 is changed from YES to NO.

[0067] The direction of the crossed axes angle θ_Nof the node N_Nis compared with the direction of the crossed axes angle θ_Nof the node N_N+1at subsequent step S7. If these directions are the same, it is determined at step S8 that the two nodes N_Nand N_N+1exist on a simple curve (a curve having a constant curving direction). If the directions are opposite, it is determined at step S9 that the two nodes N_Nand N_N+1exist on an S-shaped curve (a curve having a direction of curvature change from the right to the left or from the left to the right).

[0068] On the other hand, if it is determined by the second determining means M4₁(at step S5) that the nodes N_Nand N_N+1do not exist on the same curve, the passing-state determination amount θ_N/L_Nfor the sole node N_Nexisting on a curve is determined by the third determining means M4₂at step S10 as

[0069] θ_N/L_N=θ_N/I_maxN  (5)

[0070] (see FIG. 9C).

[0071] At subsequent step S11, a fourth determining reference value I_maxNS is calculated using L_Nand L_N+1in equation (5) according to the following equation:

[0072] I_maxNS=L_N+L_N+1  (6)

[0073] The meaning of the fourth determining reference value I_maxNS will be described hereinbelow.

[0074] Next, at step S12, the direction of the crossed axes angle θ_Nof the node N is compared with the direction of the crossed axes angle θ_N+1of the node N_N+1. If these directions are the same, it is determined at step S14 that the two nodes N_Nand N_N+1exist on two different curves which curve in the same direction.

[0075] On the other hand, if the crossed axes angles θ_Nand θ_N+1are opposite at step S12, the link length I_Nis further compared with the fourth determining reference value I_maxNS at step S13. If I_N≦I_maxNS, it is determined at step S13 that the two nodes N_Nand N_N+1exist on two different curves which are curved in the opposite directions.

[0076] The significance of step S13 is as follows. Suppose that a node N_Nexists on a first curve portion of an S-shaped curve made by two continuous arcs in opposite directions and a node N_N+1exists on a second curve portion, as shown in FIG. 10A. In this case, a passing-state determination amount θ_N/L_Nin order for the vehicle to pass through the node N_Nand a passing-state determination amount θ_N+1/L_N+1in order for the vehicle to pass through the node N_N+1are θ_N/I_maxNand θ_N+1/I_maxN+1, respectively, using equation (5). Therefore, when the first and second curve portions are directly continuous to each other in an S-shape, the link length I_Nbetween the nodes N_Nand N_N+1must be equal to or smaller than I_maxN+I_maxN+1=I_maxNS, as shown in FIG. 10B. Conversely, if the link length I_Nbetween the nodes N_Nand N_N+1exceeds I_maxN+I_maxN+1=I_maxNS, the first and second curve portions must be separate curve portions continuous to each other through a straight road portion disposed therebetween.

[0077] If the state of the node N_Non the curve is classified into five types at steps S8, S9, S13, S14 and S15 in the above manner (FIG. 5), the passing-state determination amount θ_N/L_Ncalculated at steps S6 and S10 is corrected by the first and second correcting means M4₄and M4₅at subsequent step S16.

[0078] First, the correction carried out by the first correcting means M4₄when a plurality of nodes N_Nexist on a simple curve (as shown in step S8) will be described. The passing-state determination amount θ_N/L_Nin this case is calculated at step S6, but even if the calculated value of the passing-state determination amount θ_N/L_Nis the same, it is more difficult for the vehicle to pass, because the total of crossed axes angles θ_Non the curve is relatively large. The reason can be understood from the fact that even with curves having the same radius of curvature, it is more difficult for the vehicle to pass through the curve with the direction of movement of the vehicle being changed through 90°, than for the vehicle to pass through the curve with the direction of movement of the vehicle being changed through 30°. Thereupon, a sum Σθ_Nof crossed axes angles θ_Nof a plurality of nodes N_Nexisting on the curve is calculated as shown in FIG. 11A, and a correcting factor K_Cis searched from a map, using this sum Σθ_Nas a parameter. Then, using the correcting factor K_C, the passing-state determination amount θ_N/L_Nis corrected according to the following expression:

[0079] θ_N/L_N←(θ_N/L_N)×(1+K_C)  (7)

[0080] As the sum Σθ_Nof the crossed axes angles θ_Nis increased, the correcting factor K_Cis increased from 0.0 to 0.5. Therefore, the maximum corrected passing-state determination amount θ_N/L_Nis 1.5 times the original passing-state determination amount θ_N/L_N, whereby the value of the passing-state determination amount θ_N/L_Nis compensated to take into account the difficulty of passage when the vehicle actually passes through the curve.

[0081] The correction carried out by the second correcting means M4₅when two nodes N_Nand N_N+1exist on an S-shaped curve (as shown in steps S9 and S13) will be described below. The passing-state determination amount θ_N/L_Nin this case is calculated at steps S6 and S10, but even if the calculated values of the passing-state determination amounts θ_N/L_Nare the same, it is more difficult for the vehicle to pass through the S-shaped curve than to pass through a curve curved in only one direction. Thereupon, a correcting factor K_Sis searched from the map, using, as a parameter, a sum |θ_N|+|θ_N+1| of the absolute values of the crossed axes angles θ_Nand θ_N+1of the two nodes N_Nand N_N+1, as shown in FIG. 11B. Then, the passing-state determination amount θ_N/L_Nis corrected using the correcting factor K_Saccording to the following expression:

[0082] θ_N/L_N←(θ_N/L_N)×(1+K_S)  (8)

[0083] As the sum |θ_N|+|θ_N+1| of the absolute values of the crossed axes angles θ_Nand θ_N+1is increased, the correcting factor K_Sis increased from 0.0 to 1.0. Therefore, the maximum corrected passing-state determination amount θ_N/L_Nis 2.0 times the original passing-state determination amount θ_N/L_N, whereby the value of the passing-state determination amount θ_N/L_Ncan be compensated to take into account the difficulty of passage when the vehicle actually passes through the curve.

[0084] A yaw rate YR of the vehicle at a node N_Nis given by θ_N/t resulting from division of the crossed axes angle θ_Nwhich is an amount of variation in direction of movement of the vehicle by a time t required to generate such yaw rate. The time t is given by I_N/V resulting from division of the link length I_Nby a vehicle speed V at which the vehicle passes therethrough. By combining these equations, a final yaw rate YR is calculated as a product of the passing-state determination amount θ_N/L_Nand the vehicle speed V. Yaw rate YR is thus seen as

[0085] YR=θ_N/t=θ_N/(I_N/V)=(θ_N/I_N)×V  (9)

[0086] In addition, a lateral acceleration G of the vehicle is given by a product of the yaw rate YR and the vehicle speed V.

[0087] G=YR×V  (10)

[0088] Thus, the vehicle speed V is calculated at step S17 according to the following equation:

[0089] V={G/(θ_N/L_N)}^½  (11)

[0090] given from the equations (9) and (10). Equation (11) for vehicle speed indicates that if a preset limit lateral acceleration G permissible when the vehicle passes through a curve is defined, a passable speed V_maxNfor passage of the vehicle through the curve is provided based on the preset limit lateral acceleration G and the passing-state determination amount θ_N/L_N. The passable speed V_maxNis a maximum vehicle speed at which the vehicle can pass through the curve with a lateral acceleration of the vehicle not exceeding the preset limit lateral acceleration G.

[0091] On the other hand, a passage-predicting speed V_Nfor passage of the vehicle through the node N_N, when it is supposed that the vehicle has been decelerated at a reference deceleration β from the subject vehicle position P, is calculated at step S18 according to the following equation:

[0092] V_N=(V²−2βS_N)^½  (12)

[0093] wherein S_Nrepresents a distance from the position P of the subject vehicle to the node N_N.

[0094] At subsequent step S19, the passage-predicting speed V_Nis compared with the passable speed V_maxN. If V_N≦V_maxN, it is determined that the vehicle can pass through the node N_N. If V_N>V_maxN, it is determined that it is difficult for the vehicle to pass through the node N_N. When it is determined that it is difficult for the vehicle to pass through the node N_N, the warning means M9 is operated at step S20 to alert the driver to decelerate the vehicle, and at the same time the vehicle speed regulating means M10 is operated to automatically decelerate the vehicle. Thus, the spontaneous braking by the driver or the automatic deceleration is performed to reduce the vehicle speed, thereby enabling the vehicle to safely and reliably pass through the curve.

[0095] When it is determined at steps S3 and S4 that the node N_Nexists on the curve, as described above, the passing-state determination amount θ_N/L_Nfor passage through the node N_Nexisting on the curve is calculated. The present system avoids the unnecessary calculation of passing state determination amount θ_N/L_Nwhen the vehicle travels on a straight road section, thereby reducing calculation load and the size of the electronic control unit (ECU). In addition, since the determination of whether the vehicle can pass is carried out using the passing-state determination amount θ_N/L_Nwhich is a parameter accurately representing a degree of difficulty in the passage of the vehicle through the node, even if only one node or two nodes N_Nexist on a curve, the determination of whether the vehicle can pass through the node or nodes can be correctly performed.

[0096] Although the embodiment of the present invention has been described in detail, it will be understood that the present invention is not limited to the above-described embodiment, and various modifications may be made without departing from the spirit and scope of the invention defined in claims.

[0097] For example, the passable speed V_maxNhas been calculated based on the preset limit lateral acceleration G in the embodiment, but the passable speed V_maxNmay be calculated based on the preset limit yaw rate YR in place of the preset limit lateral acceleration G. In other words, the passable speed V_maxNmay be calculated from the equation (9) according to the following equation:

[0098] V_maxN=YR/(θ_N/L_N)  (13)

[0099] A first modification to the embodiment will be described below with reference to FIGS. 13 to 17.

[0100] A prior vehicle control system is disclosed in Japanese Patent Application Laid-open No.8-147598, wherein when it is determined that a vehicle cannot pass a curve ahead of the vehicle, a warning is first provided to a driver to alert the driver to decelerate the vehicle, and when it is determined that the vehicle still cannot pass through the curve, an automatic deceleration is carried out.

[0101] The above known system employs the following two techniques.

[0102] In the first technique, when the vehicle is moving at an excessive speed when approaching a curve, as shown in FIG. 16, a warning is first emitted at a point a and then automatic deceleration of the vehicle is started at a point b. The point a is a location where if the deceleration of the vehicle is carried out at a reference deceleration rate or value β from such point, the vehicle speed is reduced at the curved road section to a value suitable for issuing the warning. The point b is a location where if the deceleration of the vehicle is carried out at a reference deceleration vale β from such point, the vehicle speed is reduced at the curve to a value suitable for automatic vehicle deceleration which is at a speed that is greater than the velocity value suitable for only receiving a warning. In this case, a distance between the points a and b is determined in accordance with the vehicle speed suitable for the warning, the vehicle speed suitable for automatic deceleration and the reference deceleration value or rate β, irrespective of the vehicle speed at the position of the subject vehicle. In other words, if the vehicle speed at the position of the subject vehicle is varied, the time taken until the vehicle passes through the point b after passing through the point a, namely, the time taken until the automatic deceleration is started after emission of the warning, is varied. Thus, this potential time variation provides a level of inconsistency in the way the prior system addresses vehicles approaching a curved road section.

[0103] In the second technique, when the vehicle is moved at an excessive speed when approaching a curve, as shown in FIG. 17, a warning is first emitted at a point a and then automatic vehicle deceleration is started at a point b. The point a is a location where if the deceleration of the vehicle is carried out at a reference deceleration value β₁from such point, the vehicle speed is reduced at the curved road section to an appropriate value. The point b is a location where if the deceleration of the vehicle is carried out at a reference deceleration value β₂from point b, with β₂being greater than β₁, vehicle speed is reduced at the curve to the appropriate value. In this case, a distance between points a and b is varied depending upon the appropriate vehicle speed, the reference deceleration values β₁and β₂and vehicle speed at the position of the subject vehicle. However, the time taken until the vehicle passes through the point b after passing through the point a, namely, the time taken until the automatic deceleration is started after emission of the warning, is likewise not constant and hence, a level of inconsistency exists in the way in which the prior system responds to vehicles approaching a curved road section.

[0104] Therefore, the first modification of the present invention ensures that the time taken from the emission of a warning to the start of the automatic deceleration of the vehicle is constant in order to prevent any inherent inconsistencies in handling vehicles safely passing through curved road sections. The first modification will be described below.

[0105] The first modification includes temporary subject vehicle position establishing means M12 being connected between the subject vehicle position detecting means M2 and the passage-predicting speed calculating means M7, as shown in FIG. 13. The temporary subject vehicle position establishing means M12 is means for establishing a temporary position P′ (see FIG. 14) of the subject vehicle at a location which the vehicle existing at the subject vehicle position P reaches after a predetermined time t₀. Predetermined time period t₀is preferably but not necessarily approximately 1.5 seconds.

[0106] Thus, the passage-predicting speed calculating means M7 calculates a passage-predicting speed V_Nand a temporary passage-predicting speed V_N′ for passage of the vehicle through the node N_N, based on the vehicle speed V, the subject vehicle position P or the temporary subject vehicle position P′ and a previously set reference deceleration value β for the vehicle. As shown by a solid line in FIG. 14, the passage-predicting speed V_Nat each node N_Nsteadily decreases as a secondary function with an increase in distance from the subject vehicle position P. When it is supposed that the subject vehicle is at the temporary subject vehicle position P′, the temporary passage-predicting speed V_N′ at each node N_Nis decreased as a secondary function, as shown by a dashed line in FIG. 14, but a line drawn by the temporary passage-predicting speed V_N′ is displaced in parallel from a line drawn by the passage-predicting speed V_Nby a distance corresponding to the predetermined time t₀(1.5 seconds in the embodiment).

[0107] The passability determining means M8 compares the temporary passage-predicting speed V_N′ with the passable speed V_maxN. If V_N′≦V_maxN, the passability determining means M8 determines that the vehicle can pass through the node N_N. If V_N′>V_maxN, the passability determining means M8 determines that it is difficult for the vehicle to pass through the node N_N, and the warning means M9 comprised of a buzzer or a lamp is operated to alert the driver to decelerate the vehicle. Further, the passability determining means M8 compares the passage-predicting speed V_Nwith the passable speed V_maxN. If V_N≦V_maxN, it is determined that the vehicle can pass through the node N_N. If V_N>V_maxN, it is determined that it is difficult for the vehicle to pass through the node N_N, and the vehicle speed regulating means M10 comprised of means for automatically braking the vehicle or means for automatically reducing vehicle engine output is operated to automatically decelerate the vehicle.

[0108] The above-described operation will be further described with reference to a flow chart in FIG. 15. The contents of steps S1 to S17 of the flow chart are the same as the contents of steps S1 to S17 of the flow chart shown in FIGS. 4 to 6.

[0109] At step S21 of the flow chart in FIG. 15, a passage-predicting speed V_Nfor passage of the vehicle through the node N_Nis calculated assuming that the vehicle has been decelerated at the reference deceleration value β from the subject vehicle position P, using equation (12), i.e.,

[0110] V_N=(V²−2βS_N)^½

[0111] A temporary passage-predicting speed V_N′ for passage of the vehicle through the node N_Nis also calculated at step 21, assuming that the vehicle has been decelerated at the reference deceleration value β from the temporary subject vehicle position P′, according to the following equation:

[0112] V_N′=(V²−2βS_N′)^½  (14)

[0113] wherein S_N′ represents a distance from the temporary subject vehicle position P′ to the node N_N. At this time, because S_Nis greater than S_N′, V_N′ is greater than V_N. Specifically, because the temporary subject vehicle position P′ is nearer to the node N_Nthan the subject vehicle position P, the deceleration is correspondingly retarded, resulting in an increased temporary passage-predicting speed V_N′ for passage of the vehicle through the node N_N.

[0114] At subsequent step S22, the temporary passage-predicting speed V_N′ is compared with the passable speed V_maxN. If V_N′≦V_maxN, it is determined that the vehicle can pass through the node N_N. If V_N′>V_maxN, it is determined that it is difficult for the vehicle to pass through the node N_N. When it is determined to be difficult for the vehicle to pass through the node N_N, the warning means M9 is operated at step S23 to alert the driver to decelerate the vehicle.

[0115] When the warning means M9 is operated at step S23, the passage-predicting speed V_Nis compared with the passable speed V_maxNat step S24. If V_N≦V_maxN, it is determined that the vehicle can pass through the node N_N. If V_N>V_maxN, it is determined that it is difficult for the vehicle to pass through the node N_N. When it is determined to be difficult for the vehicle to pass through the node N_N, the vehicle speed regulating means M10 is operated to automatically decelerate the vehicle at step S25, so that the vehicle can pass through the node N_N.

[0116] Thus, in determining the timing for activating warning means M9, it is assumed that the vehicle exists at the temporary subject vehicle position P′ located ahead of the subject vehicle position P by a distance corresponding to a predetermined time t₀, as shown in FIG. 14. Therefore, if the timing for operating warning means M9 and the timing for operating vehicle speed regulating means M10 are determined by comparison of each with the passable speed V_maxN, the timing for operating warning means M9 precedes the timing for operating vehicle speed regulating means M10 by the predetermined time t₀. Thus, the lag of time from the operation of the warning means M9 to the operation of the vehicle speed regulating means M is not varied in accordance with a variation in vehicle speed V, thereby preventing any inconsistencies in system operation.

[0117] A second modification to the present invention will be described with reference to FIGS. 18 to 20.

[0118] The system described in Japanese Patent Application Laid-open No. 8-147598 calculates a passage-predicting speed for determining whether the vehicle can pass through a curve, based on the assumption that the driver has decelerated the vehicle at a previously set constant deceleration value from the subject vehicle position. The prior system then calculates the passable speed based on the determined shape of a road, and compares the calculated passable speed with the passage-predicting speed in order to determine whether the vehicle can pass through the curve. However, decelerating the vehicle travelling on the curve is more difficult, as compared with the decelerating the vehicle on a straight road. As a result, the deceleration rate actually generated tends to be larger on the straight road and smaller on the curve. Therefore, if it is supposed that the vehicle decelerates at a constant deceleration rate irrespective of the shape of a road, as in the prior art, it is difficult to calculate an appropriate and accurate passage-predicting speed, and there is a possibility that the timing of the activation of the warning and the automatic vehicle deceleration is displaced so as to provide inconsistent operation.

[0119] The second modification ensures that both of the warning and the automatic deceleration are carried out at an appropriate time to prevent operational inconsistencies and driver confusion. The second modification will be described below.

[0120] The second modification includes reference deceleration setting means M13 connected between the curve section determining means M3 and the passage-predicting speed calculating means M7, as shown in FIG. 18.

[0121] The reference deceleration setting means M13 sets reference deceleration rate or values β₁and β₂for calculating a passage-predicting speed V_Nby the passage-predicting speed calculating means M7. The reference deceleration values β₁and β₂are deceleration rates presumed to be generated when the driver initiates spontaneous braking at the subject vehicle position P to allow the vehicle to pass through a curve ahead of the vehicle. In a straight road section having a shape determined in the curve section determining means M3, the deceleration is set at a value β₁and in a curve section, the deceleration is set at a value β₂, which is less than β₁. A lesser deceleration is used for braking on a curved road section because of the increase in braking difficulty when the vehicle is travelling on a curved section, relative to the vehicle travelling on a straight road section.

[0122] The operation of the reference deceleration setting means M13 will be further described with reference to a flow chart in FIG. 19. The contents of steps S1 to S17 in this flow chart are the same as the contents of the steps S1 to S17 in the flow chart shown in FIGS. 4 to 6.

[0123] At step S17 in the flow chart shown in FIG. 19, a passage-predicting speed V_Nis calculated according to V_N=(V²−2βS_N)^½ and then the road shape determining means M11 determines a road section ahead of the subject vehicle position as being either a straight road section or a curve section at step S26. Following such determination, the reference deceleration setting means M13 sets the reference deceleration value β₁for the straight road section at step S27, and sets the reference deceleration value β₂for the curve section at step S28. At step S29, a passage-predicting speed V_Nfor passage of the vehicle through the node N_Nis calculated based on the set reference deceleration value β₁or β₂.

[0124] For example, on a road having a straight road portion and a curved road portion connected to the straight road portion, as shown in FIG. 20, the larger reference deceleration value β₁is assigned to the straight road portion, and the smaller reference deceleration value β₂is assigned to the curved road portion. If the vehicle having a speed V at the subject vehicle position P is decelerated at the reference deceleration value β₁on the straight road section, a speed V₁of the vehicle at a terminal end of the straight road section is calculated according to the following equation:

[0125] V₁=(V²−2β₁S₁)^½  (15)

[0126] wherein S₁represents a distance from the subject vehicle position P to the terminal end of the straight road section.

[0127] Further, if the vehicle is decelerated at the reference deceleration value β₂on the curve section, a speed V₂of the vehicle at a terminal end of the curve section is calculated according to the following equation:

[0128] V₂={V₁²−2β₂(S₂−S₁)}^½  (16)

[0129] wherein S₂represents a distance from the subject vehicle position P to the terminal end of the curve section.

[0130] Therefore, if the larger reference deceleration value β₁for the straight road section is employed on the curve road section in FIG. 20 as is employed in the prior systems, the passage-predicting speed V_Nexceeds the passable speed V_maxNat a point a, and the warning and/or the automatic deceleration of the vehicle may be activated despite being unnecessary. Conversely, if the smaller reference deceleration value β₂is employed on the curve section, the passage-predicting speed V_N(described as a dotted line in FIG. 20) cannot exceed the passable speed V_maxN, thereby preventing the warning and/or the automatic deceleration from being unnecessarily carried out.

[0131] In this manner, even on a road having a straight section and a curve section combined with each other, a passage-predicting speed V_Nat any node N_Ncan be appropriately calculated by using the corresponding reference deceleration values β₁and β₂.

[0132] At subsequent step S30, the passage-predicting speed V_Nis compared with the passable speed V_maxN. If V_N≦V_maxN, it is determined that the vehicle can pass through the node N_N. If V_N>V_maxN, it is determined that it is difficult for the vehicle to pass through the node N_N. When it is difficult for the vehicle to pass through the node N_N, the warning means M9 is operated to alert the driver to decelerate the vehicle, while at the same time, the vehicle speed regulating means M10 is operated to automatically decelerate the vehicle, at step S31. This causes the driver's spontaneous braking and the automatic deceleration to be performed to reduce the vehicle speed, so that the vehicle can reliably pass through the curve.

[0133] The scope of the invention is indicated by the appended claims, rather than by the foregoing discussion of presently preferred embodiments.