JPH11183622A - Rear-end collision warning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rear-end collision warning device matched with the driving feeling of the crew. SOLUTION: A rear-end collision warning device is provided with a laser radar 1 which detects distance between a vehicle and a preceding vehicle on the vehicle, with a vehicle speed sensor 2 which detects the speed of the vehicle and with an accelerometer 3. It is featured in such a way that a minimum deceleration which must be applied to the vehicle so as to prevent the vehicle from colliding with the preceding vehicle is computed on the basis of the distance, between the vehicle and the preceding vehicle, detected by the laser radar 1, on the basis of the speed, of the vehicle, detected by the vehicle speed sensor 2 and on the basis of the acceleration, of the vehicle, detected by the accelerometer 3 and that a warning is issued to the driver when the minimum deceleration which must be applied to the vehicle reaches a prescribed value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rear-end collision warning device for preventing a rear-end collision of a vehicle with a front vehicle.

[0002]

2. Description of the Related Art For example, Japanese Unexamined Patent Publication No. Hei 6-96396 discloses a conventional rear-end collision warning device that sets a grace period until a rear-end collision with a preceding vehicle based on the distance between the vehicle and the relative speed with the preceding vehicle. The deceleration assisting means is operated when this time is less than a predetermined value.

Further, Japanese Patent Application Laid-Open No. Hei 5-166097 detects a relative acceleration in addition to an inter-vehicle distance and a relative speed, and obtains a more accurate collision time based on the detected relative acceleration.

Further, Japanese Patent Application Laid-Open No. 6-180363 determines whether or not the sum of the idle distance and the braking distance of a vehicle is greater than the following distance.

However, these prior arts generate an alarm based on a grace period until a collision with a preceding vehicle or a relationship between a braking distance and an inter-vehicle distance. Even if the grace time or the braking distance is a fixed value, the degree of danger felt by the occupant is completely different.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a rear-end collision warning device that matches the driving feeling received by an occupant.

[0007]

According to a first aspect of the present invention, there is provided a radiating means for radiating a radiator in front of a vehicle, the radiating means being provided on the vehicle. A radiator detecting means for detecting the radiator reflected by a vehicle in front of the vehicle, and a radiator detecting means corresponding to a time difference between radiation of the radiator of the radiator and a radiator detection signal of the radiator detector. Distance detecting means for detecting the distance between the vehicle and the preceding vehicle; own vehicle speed detecting means installed on the vehicle to detect the speed of the vehicle; and own vehicle installed on the vehicle and detecting the acceleration of the vehicle Acceleration detection means;
The distance between the vehicle and the preceding vehicle detected by the distance detection unit, the speed of the vehicle detected by the vehicle speed detection unit, and the acceleration of the vehicle detected by the vehicle acceleration detection unit A collision avoidance deceleration calculating means for calculating a minimum deceleration that must be applied to the vehicle so that the vehicle does not collide with the vehicle ahead, and the vehicle calculated by the collision avoidance deceleration calculating means When the minimum deceleration that must be added to the vehicle reaches a first predetermined value, an alarm generating means for issuing an alarm to a driver is provided.

According to a second aspect of the present invention, there is further provided a brake force generating means for applying a brake force to a wheel brake installed on a wheel of the vehicle irrespective of a driver's braking operation, and When the minimum deceleration calculated by the rear-end collision avoidance deceleration calculating means to be applied to the vehicle reaches a second predetermined value larger than the first predetermined value, the brake force generating means is operated. The rear-end collision warning device according to claim 1, wherein a braking force is applied to a wheel brake of the vehicle.

According to a third aspect of the present invention, the rear-end collision avoidance deceleration calculating means is configured to determine whether the vehicle and the front vehicle are located on the basis of a distance between the vehicle and the preceding vehicle detected by the distance detecting means. A relative speed calculating unit that calculates a relative speed between the vehicle and the preceding vehicle, and a distance between the vehicle and the preceding vehicle based on a distance between the vehicle and the preceding vehicle detected by the distance detecting unit. 2. The rear-end collision warning device according to claim 1, further comprising a relative acceleration calculating means for calculating a relative acceleration.

According to a fourth aspect of the present invention, a radiating means for radiating a radiator on a vehicle in front of the vehicle, and reflecting by a vehicle on the vehicle and in front of the vehicle. Radiator detection means for detecting the radiator thus detected, and detecting a distance between the vehicle and the preceding vehicle in accordance with a time difference between radiator emission of the radiator and a radiator detection signal of the radiator detection means. Distance detecting means, a vehicle speed detecting means installed on the vehicle and detecting the speed of the vehicle, a vehicle acceleration detecting means installed on the vehicle and detecting the acceleration of the vehicle, and the distance detecting means Based on the detected distance between the vehicle and the preceding vehicle, the speed of the vehicle detected by the own vehicle speed detecting means, and the acceleration of the vehicle detected by the own vehicle acceleration detecting means. When it is determined that the vehicle does not collide with the preceding vehicle, when a predetermined collision avoidance deceleration is assumed for the vehicle, a collision margin deceleration calculating means for calculating a maximum deceleration allowed for the preceding vehicle, A rear-end collision warning device characterized in that the rear-end collision warning device is provided with an alarm generating unit that issues an alarm to a driver when the rear-end collision deceleration calculated by the rear-end collision deceleration calculating unit is smaller than a third predetermined value.

According to the rear-end collision warning device of the first aspect, when the minimum deceleration calculated by the rear-end collision avoidance deceleration calculating means to be applied to the vehicle reaches the first predetermined value, the driving is started. By issuing a warning to the driver, the driver can be warned before the inter-vehicle distance that requires deceleration that the driver feels dangerous is reached.

According to the second aspect of the present invention, when the minimum deceleration that must be applied to the vehicle reaches a second predetermined value that is larger than the first predetermined value, the vehicle is activated. By applying a braking force to the wheel brakes, the vehicle can be decelerated before the inter-vehicle distance that requires a deceleration that the driver feels dangerous.
According to the rear-end collision warning device described in claim 3,
By providing a relative speed calculating means for calculating a relative speed between the vehicle and the preceding vehicle, and a relative acceleration calculating means for calculating a relative acceleration between the vehicle and the preceding vehicle, The minimum deceleration that must be applied to the vehicle can be calculated.

Further, according to the rear-end collision warning device according to the fourth aspect, when it is determined that the vehicle does not collide with the front vehicle, and when a predetermined rear-end collision avoidance deceleration is assumed for the vehicle, the front collision warning device can be used. The maximum deceleration allowed for the vehicle is the third
By issuing a warning to the driver when the distance is smaller than the predetermined value, the driver can be warned before the inter-vehicle distance that requires deceleration that the driver feels dangerous is reached.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention proposes a rear-end collision warning device that matches the driver's driving feeling by defining the following rear-end collision avoidance deceleration and issuing an alarm based on this value.

[0015] When the collision avoidance deceleration is defined and the running state is maintained as it is, a rear-end collision is expected.
In order to avoid a rear-end collision, the minimum deceleration that must be applied on average from that point is referred to as a rear-end collision avoidance deceleration.

For example, while traveling at a speed of 50 km / h, 1
The rear-end collision avoidance deceleration when an obstacle stopped 00 m ahead is found is as follows.

The minimum condition for avoiding a rear-end collision is 100 m
The deceleration α [m / sec ² ] because the speed only needs to be 0 earlier.
And the braking distance should be 100 m.

Braking distance 100 m = (vehicle speed 13.89)
^{m / sec) 2/2 ·} α α = 192.9 / 200 = 0.96m / sec 2 Therefore, collision avoidance deceleration 0.96m / sec ² (0.09
8G).

Similarly, while traveling at 50 km / h,
Calculating the collision avoidance deceleration when an obstacle stopped 0 m ahead is found, the collision avoidance deceleration is 4.83 m / sec ²
(0.49G), which requires considerable sudden braking.

The warning on the basis of the collision avoidance deceleration determination scale is effective when a collision is expected, that is, when the danger is obvious. However, an extremely dangerous situation can be assumed even when it is impossible to predict a rear-end collision, and in such a case, an effective warning cannot be issued.

For example, at a speed of 100 km / h / h and a relative speed of 0
Consider two vehicles traveling at km / h. In this way, when the relative speed is 0 km / h, there is no danger of a rear-end collision regardless of the size of the inter-vehicle distance. However, in this situation, if the inter-vehicle distance is 1 m, it can be said that the vehicle is in an extremely dangerous state in consideration of the case where the preceding vehicle decelerates. Thus, the potential danger cannot be dealt with only by the collision avoidance deceleration.

Therefore, the concept of rear-end collision avoidance deceleration is extended to define a rear-end collision deceleration, and this value can be used to accurately alert a potential danger.

When there is no obvious danger, consider giving a virtual deceleration to the preceding vehicle. By giving the virtual deceleration α, the danger of a rear-end collision becomes apparent, so that the rear-end collision avoidance deceleration can be calculated. Therefore, a virtual deceleration α at which the rear-end collision avoidance deceleration becomes β is defined as a rear-end collision deceleration at the rear-end collision avoidance deceleration β.

Definition of Rear-end Collision Deceleration In a certain traveling state, if the state is maintained as it is, it is assumed that when the rear-end collision does not occur, the preceding vehicle decelerates at the deceleration α. At this time, the virtual deceleration α of the preceding vehicle such that the collision avoidance deceleration of the own vehicle becomes β is set as the collision margin deceleration in the collision avoidance deceleration β.

By using this collision margin deceleration as a measure to determine the potential danger and issue an alarm, it is possible to accurately issue an alarm even when the danger is potential. Hereinafter, a method of calculating the rear-end collision avoidance deceleration will be described. In order to calculate the rear-end collision avoidance deceleration, it is necessary to calculate an inter-vehicle distance between two vehicles and determine whether or not a rear-end collision occurs.

The position of the preceding vehicle is represented by Y (t), and the position of the own vehicle is represented by X.
When represented by one-dimensional coordinates as (t), the following distance R (t) = Y (t) −X (t).

In general, X (t) can be approximated as a polynomial up to the nth derivative.

[0028] X (t) = X (0 ) + X '(0) · t + X''(0) · t 2/2
+ X '''(0) · t 3/6 + ··· + X (n) (0) · t n / n! Since the rear-end collision avoidance deceleration is defined with respect to the deceleration (acceleration) of the own vehicle, it is necessary to calculate by an approximate expression including at least the term of the second derivative (acceleration). Here, the approximation is made up to the second derivative.

[0029] X (t) = X (0 ) + X '(0) · t + X''(0) · t 2/2 X (0) = X 0, X' (0) = V, X '' ( 0) = When G1, the _{X (t) = X 0 +} V · t + G1 · t 2/2. v and G1 are the current vehicle speed and the current vehicle acceleration.

Similarly, Y (t) can also be approximated as a polynomial up to its nth derivative, and here, it is similarly approximated up to the second derivative.

[0031] Y (t) = Y (0 ) + Y '(0) · t + Y''(0) · t 2/2 where, (a case of approaching positive) relative velocity RV between the preceding vehicle speed Since V−RV, if Y (0) = Y ₀ , Y ′ (0) = V−RV, Y ″ (0) = G2, then Y (t) = Y ₀ + (V−RV) a ^{t + G2 · t 2/2} . G2 is the acceleration of the preceding vehicle at the moment. Accordingly, the inter-vehicle distance R (t) becomes R (t) = Y (t ) -X (t) = Y 0 -X 0 -RV · t + (G2 -G1) · t 2/2. When Y ₀ -X ₀ initial value R ₀ of the inter-vehicle distance, the formula becomes _{R (t) = R 0 -RV} · t + (G2 -G1) · t 2/2 representing the inter-vehicle distance R (t) . Here, R ₀ > 0 since no rear-end collision occurs at first. RV is positive when approaching at relative speed.
G1 and G2 are accelerations of the own vehicle and the preceding vehicle, and a negative value indicates deceleration. Here, A = (G2 -G1) / 2, B = -RV, when put a C = R _0, the R (t) = A · t 2 + B · t + C. In this equation, when t (t> 0) where R (t) = 0 exists, that is, when there is a positive real solution where R (t) = 0, a collision will occur. In the case of a quadratic equation, since two solutions appear, the rear-end collision time is the minimum positive real solution.
(When G1 = G2, A = 0, which is a linear expression, but even in such a case, if a very small value is set to A, it can be approximately discussed only with a quadratic expression. In this case, both the own vehicle and the preceding vehicle were considered to have constant acceleration motion. Therefore, when it was determined whether or not the vehicle would collide based on this condition alone, the vehicle collided with both or the preceding vehicle moving backward. In some cases.

When considering actual vehicle motion, when decelerating by applying a brake, deceleration can be generated while the vehicle speed is positive, but once the vehicle speed becomes zero, acceleration (deceleration) also becomes zero. Is realistic.

Therefore, even when a minimum positive actual solution that satisfies R (t) = 0 is obtained, if the traveling speed at that time becomes negative (when the vehicle crashes backward while colliding), the speed becomes zero. Up to (constant acceleration motion) and after the speed becomes 0 (constant motion)
It needs to be reconsidered.

When the own vehicle stops first, there is no apparent collision, so it is sufficient to consider only the case where the preceding vehicle stops first. Preceding vehicle running speed VF (T) is because it is VF (T) = V -RV + T · G2, the time T _VF0 speed becomes 0 becomes _{T VF0 = (V-RV)} / G2. If CT is the smallest positive real solution where R (t) = 0, when CT> T _VF0 , the vehicle will collide while retreating, so it reaches T _VF0 (constant acceleration motion) and thereafter (constant velocity) Exercise).

Up to T _VF0 (constant acceleration motion) The state at t = T _VF0 is as follows.

_Inter- vehicle distance R (T _VF0 ) Own vehicle speed V + T _VF0 · G1 preceding vehicle speed 0 own vehicle acceleration G1 preceding vehicle acceleration 0 T _VF0 or later Since the preceding vehicle has stopped, the stopping distance of the own vehicle is equal to the following distance. _Should be shorter than the initial value R (T _VF0 ). T _VF0 subsequent braking distance T _VF0 subsequent braking distance _{= (V + T VF0 · G1} ) 2/2 · G1 this value is not to the rear-end is smaller than R (T _VF0) so as follows. _{That, R (T VF0) ≦ (} V + T VF0 · G1) when the ^2/2 · G1 will be rear-end.

As described above, when the initial conditions of the own vehicle and the preceding vehicle are given, it is possible to determine whether or not a rear-end collision occurs. From this, the rear-end collision avoidance deceleration and the rear-end collision deceleration are obtained as follows.

Calculation of rear-end collision avoidance deceleration (when it is determined that rear-end collision occurs) The deceleration of the own vehicle is slightly increased, and the collision determination is performed again.

2. Until the rear collision is determined by the rear collision judgment, 1. repeat.

3. The deceleration of the own vehicle applied until the time when the rear-end collision stops is the rear-end collision avoidance deceleration.

Calculation of rear-end collision deceleration (when it is determined that rear-end collision does not occur) A rear-end collision avoidance deceleration β of the own vehicle, which is a condition for calculating a rear-end collision margin, is set.

2. The rear-end collision determination is performed while slightly increasing the deceleration of the preceding vehicle by an inter-vehicle distance equation in which the deceleration of the own vehicle is increased by β.

3. 1. In the rear-end collision determination, until the rear-end collision is determined. repeat.

4. The maximum value added to the deceleration of the preceding vehicle in the determination that the vehicle does not collide is the collision margin deceleration in the collision avoidance deceleration β.

By performing warning / automatic deceleration when the collision avoidance deceleration and the collision allowance deceleration exceed certain conditions, it is possible to perform control that matches the driver's driving feeling.

An example of a rear-end collision warning device on which the present invention is based will be described with reference to FIG. Reference numeral 1 denotes a laser radar for measuring an inter-vehicle distance, which is configured as shown in FIG. There are various measurement methods for laser radar, but here we use a pulse laser method that measures the distance to the target by emitting pulse light and measuring the time until it returns to the target and returns. I have.

As shown in FIG. 2, the transmission of the pulse light is performed by the driving circuit 13, the laser diode 12, and the light transmitting lens 11. When the light emission trigger signal is transmitted from the distance measurement circuit 17 to the drive circuit 13, the drive circuit 13 supplies a pulse current to the laser diode 12. The laser diode 12 emits pulse light by the supplied pulse current, and is condensed and emitted by the light transmitting lens 11.

The pulse light reflected by the object is condensed by the light receiving lens 14 and converted into an electric pulse signal by the photodiode. The pulse signal is amplified by the amplification circuit 16 and input to the distance measurement circuit 17.

The distance measurement circuit 17 measures the time Δt from when the light emission trigger signal is sent to when the light reception pulse signal is received, and the distance R between the vehicles to the target is calculated from the value ΔR = light speed C · Δt / 2 light speed C = 2.998 × 10 ⁸ m / sec.

The inter-vehicle distance measured by the laser radar 1 is input to the danger determining device. The relative speed with respect to the preceding vehicle is determined from the differential (difference) of the measured inter-vehicle distance, and the danger determination device 7
(See FIG. 1).

The danger determination device 7 includes a laser radar 1
Signal from the vehicle speed sensor 2 for detecting the own vehicle speed, a signal from the acceleration sensor 3 for detecting the acceleration of the own vehicle, and ON / OF control by the driver.
A signal from the main switch 4 indicating F is also input.

Based on these input signals, the above-described rear-end collision avoidance deceleration and rear-end collision deceleration are obtained, and control is performed by comparing them with preset values. In the present embodiment, control is performed using the following determination values.

[0053]

[Table 1]

The danger determination device 7 makes the above determination and outputs an alarm output signal to the alarm device 5. The alarm device 5 sounds a buzzer based on the alarm output signal.

In the case of the system having the brake actuator 6, if the deceleration is not performed even if an alarm is issued, the brake actuator 6 can be operated to forcibly decelerate.

FIGS. 3 to 5 show flowcharts of control of the rear-end collision warning device according to the present invention. Here, a description will be given as a system up to an alarm without using a brake actuator.

After starting from S1, initialization is performed in S2. Here, initialization of variables and the like are performed. Next, the state of the main switch is read in S3, and the loop of S2 to S3 is performed until the main switch is turned ON. When the main switch is turned on, the process proceeds to S4, where the own vehicle information is acquired. The vehicle information includes the vehicle speed and the vehicle acceleration. In S5, the inter-vehicle distance to the preceding vehicle is measured and read by the laser radar, and in S6, the inter-vehicle distance is differentiated to calculate the relative speed RV. In S7, the preceding vehicle acceleration G2 is calculated by adding the value of the own vehicle acceleration to the value obtained by differentiating the relative speed RV.

In S8, a collision determination is performed based on the information obtained in S4 to S7. If the collision is continued as it is, the process proceeds to S10 to calculate the collision avoidance deceleration. If the collision is not performed, the process proceeds to S13. Calculate the deceleration.

As shown in FIG. 4, in the calculation of the collision avoidance deceleration in S10, a calculation parameter is set in S101, and in S102, a variable K that finally becomes the collision avoidance deceleration is set.
Is cleared to zero.

In S103, a value obtained by adding K to the own vehicle acceleration G1 is set as a new own vehicle acceleration G1, and a collision determination is performed in S104. At this time, the value of G1 is calculated by adding K only at the time of rear-end collision determination, and the value of G1 is not updated.

When it is determined in the rear-end collision determination that the vehicle is in a rear-end collision, S
Proceeding to 106, the value of K is increased by 0.01G and S103
To form a loop back to

If it is determined in the rear-end collision determination that no collision occurs, S
The routine proceeds to 107, where the calculation of the collision avoidance deceleration is completed. The value of K at this time is the required rear-end collision avoidance deceleration.

If the value of the rear-end collision avoidance deceleration is equal to or greater than 0.15 G in S11 of FIG. 3, an alarm is output in S12 and the process returns to S3.

In FIG. 5, in the calculation of the rear-end collision deceleration in S13, calculation parameters are set in S131. At this time, the virtual deceleration β, which is a prerequisite for the calculation, is set to the own vehicle deceleration G1.
Is added. In S132, the variable K that becomes the rear-end collision deceleration is cleared to zero.

In S133, a value obtained by adding K to the preceding vehicle acceleration G2 is set as a new G2, and a collision determination is performed in S134. At this time, the value of G2 is calculated by adding K only at the time of rear-end collision determination, and the value of G2 is not updated.

If it is determined in step S135 that the vehicle does not collide, the process proceeds to step S136, where the value of K is increased by 0.01G.
A loop back to 33 is formed.

If it is determined in the rear-end collision determination that the vehicle is in a rear-end collision, S1
Proceeding to 37, the value of K is returned to the previous value, and proceeding to S138, the calculation of the collision margin deceleration is completed. The value of K at this time is the rear-end collision margin deceleration obtained when the rear-end collision avoidance deceleration β is obtained.

If the value of the rear-end collision deceleration is equal to or less than 0.12 G in S14 of FIG. 3, an alarm is output in S15 and the process returns to S3.

[0069]

According to the present invention, a warning is issued to the driver based on the rear-end collision avoidance deceleration, so that a rear-end collision warning device that matches the feeling of the occupant can be provided.

[Brief description of the drawings]

FIG. 1 is an overall view of a rear-end collision warning device according to the present invention.

FIG. 2 shows a laser radar of the rear-end collision warning device according to the present invention.

FIG. 3 is an overall flowchart showing the operation of the rear-end collision warning device according to the present invention.

FIG. 4 is a flowchart showing a method for calculating a rear-end collision avoidance deceleration of the rear-end collision warning device according to the present invention.

FIG. 5 is a flowchart showing a method of calculating a rear-end collision deceleration of the rear-end collision warning device according to the present invention.

[Explanation of symbols]

Reference Signs List 1 laser radar 2 vehicle speed sensor 3 acceleration sensor 5 alarm device 6 brake actuator 7
Danger judgment device

Claims (4)

[Claims] 1. A radiating means on a vehicle for radiating a radiator in front of the vehicle, and a radiation for detecting the radiator reflected by a vehicle on the vehicle and in front of the vehicle. Body detecting means, distance detecting means for detecting a distance between the vehicle and the preceding vehicle in accordance with a time difference between radiation of the radiator of the radiating means and a radiant detection signal of the radiant detecting means, Own-vehicle speed detection means installed to detect the speed of the vehicle, own-vehicle acceleration detection means installed in the vehicle to detect the acceleration of the vehicle, and the vehicle and the preceding vehicle detected by the distance detection means The distance of the
On the basis of the speed of the vehicle detected by the vehicle speed detecting means, and the vehicle acceleration detected by the vehicle acceleration detecting means, the vehicle does not collide with the preceding vehicle. A collision avoiding deceleration calculating means for calculating a minimum deceleration to be applied, and a minimum deceleration calculated for the vehicle calculated by the collision avoiding deceleration calculating means reaches a first predetermined value. A rear-end collision warning device, comprising: a warning generation means for generating a warning to a driver. 2. The vehicle according to claim 1, further comprising: a braking force generating unit that applies a braking force to a wheel brake installed on a wheel of the vehicle regardless of a driver's braking operation. The minimum deceleration that must be applied to the vehicle, calculated by
2. The rear-end collision warning device according to claim 1, wherein when a second predetermined value larger than a predetermined value is reached, the braking force generating means is operated to apply a braking force to a wheel brake of the vehicle. 3. The rear-end collision avoiding deceleration calculating means calculates a relative speed between the vehicle and the preceding vehicle based on a distance between the vehicle and the preceding vehicle detected by the distance detecting means. Relative speed calculating means for calculating, and relative acceleration calculating means for calculating a relative acceleration between the vehicle and the preceding vehicle based on a distance between the vehicle and the preceding vehicle detected by the distance detecting means. The rear-end collision warning device according to claim 1, further comprising: 4. A radiating means for radiating a radiator on a vehicle in front of the vehicle, and radiating means for detecting the radiator reflected by a vehicle on the vehicle and in front of the vehicle. Body detecting means, distance detecting means for detecting a distance between the vehicle and the preceding vehicle in accordance with a time difference between radiation of the radiator of the radiating means and a radiant detection signal of the radiant detecting means, Own-vehicle speed detection means installed to detect the speed of the vehicle, own-vehicle acceleration detection means installed in the vehicle to detect the acceleration of the vehicle, and the vehicle and the preceding vehicle detected by the distance detection means The distance of the
When it is determined that the vehicle does not collide with the preceding vehicle, based on the speed of the vehicle detected by the vehicle speed detection unit, and the acceleration of the vehicle detected by the vehicle acceleration detection unit, Assuming a predetermined collision avoidance deceleration for the vehicle, a collision margin deceleration calculating means for calculating a maximum deceleration allowed for the preceding vehicle, and a collision margin reduction calculated by the collision margin deceleration calculating means A rear-end collision alarm device comprising: an alarm generating means for generating an alarm to a driver when the speed is lower than a third predetermined value.