JPH0717346A - Collision preventing device for vehicle and road surface friction coefficient calculating device - Google Patents

Abstract

PURPOSE:To effectively prevent the collision trouble of a vehicle by correctly estimating an inter-vehicle distance on the start of brake application by indirectly estimating the foad surface friction coefficient mu before the full brake application. CONSTITUTION:A calculation controller 11 calculates the degree of danger of collision etc., between the own vehicle and a vehicle ahead on the basis of the input signals supplield from a car speed sensor 12, range finder 13, etc., and outputs the instructions necessary for and alarm device 18 and an automatic brake device 19 according to the result of the calculation. In concrete, when the distance from the vehicle ahead becomes less than the first inter-vehicle distance, the gentle brakes are applied, and the road surface friction coefficient mu is calculated from the vehicle state in the gentle brake application, and set change is performed by obtaining the second inter-vehicle distance on the basis of the value mu, and when the distance from the vehicle ahead becomes less than the second inter-vehicle distance, the full brakes are applied. The road surface friction coefficient mu is found by carrying out the data sampling for the brake pressure, wheel acceleration speed, and the wheel slip rate in plural times during the gentle brake application and carrying out the regression analysis on the basis of the obtained data.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vehicle collision prevention device, for example, a vehicle collision prevention device for preventing a vehicle rear-end collision accident to a front obstacle and a road surface friction coefficient calculation device for indirectly estimating a road surface friction coefficient. Regarding

[0002]

2. Description of the Related Art With the increase in vehicle accidents in recent years, various devices have been proposed to prevent a vehicle collision accident such as a rear-end collision accident during traveling. As such a device, the distance from the own vehicle to an obstacle (for example, a preceding vehicle), the relative speed between the two,
There is also a computer that calculates and predicts the risk of a collision based on the vehicle speed and the like, and warns the driver and automatically brakes. In this case, for example, if automatic braking is performed so that a collision can be avoided immediately before, the higher the relative speed between the two or the own vehicle speed, the earlier the start time point at which braking should be started, and The hydraulic control for braking also differs depending on the distance.

However, it is generally known that the braking stop distance of the vehicle, that is, the distance from the start of braking to the stop of the vehicle greatly varies depending not only on the relative speed and the vehicle speed, but also on the condition of the traveling road surface. For example, when the road surface is wet or frozen, the friction coefficient μ is significantly different from that when it is dry, so the distance to the vehicle stop is accurately calculated and the collision is reliably prevented. Has
It is also necessary to consider the road surface condition.

To solve this problem, for example, Japanese Patent Laid-Open No.
As described in Japanese Patent Laid-Open No. 8-53544, there is a device in which a vehicle is provided with a special sensor for infrared rays or the like to directly measure a road surface state and a collision result is surely prevented by using the measurement result.

However, in this device, the road surface condition,
That is, a special sensor for measuring the friction coefficient μ is required, and there are problems in terms of cost and mountability.

[0006]

Therefore, instead of directly measuring the friction coefficient .mu., The wheel speed during braking, as is done in the antilock control device disclosed in Japanese Patent Application Laid-Open No. 3-67766, A method of indirectly estimating the friction coefficient μ from the brake oil pressure, the slip ratio, etc. can be considered.

However, this method estimates the friction coefficient μ only after the wheels are locked, and does not estimate the friction coefficient μ before the wheels are locked. That is, before the wheels are locked by automatic braking, the setting of the braking start inter-vehicle distance (that is, the limit inter-vehicle distance for starting braking) should be changed according to the friction coefficient μ of the road surface immediately before the wheels are locked. I can't. Therefore, for example, if the set value of the braking start inter-vehicle distance is too large, the full brake may be activated at an earlier point than necessary, and in this case, the wheels may lock, which may cause an unnecessary slip accident. There is also.
On the other hand, if the set value of the braking start inter-vehicle distance is too small, the collision may not be avoided due to the delay in the start of full brake operation.

Further, when the above method is used as it is, there is a problem that the wheels are suddenly locked when a certain degree of collision risk is exceeded and the driver is greatly shocked.

The present invention has been made to solve the above problems. A first object of the present invention is to accurately predict the braking start inter-vehicle distance before the wheels are locked by the full brake operation, and at the proper timing. An object of the present invention is to obtain a vehicle collision prevention device that can smoothly perform automatic braking.

A second object of the present invention is to provide a road surface friction coefficient which can obtain an accurate road surface friction coefficient according to a road surface condition during the operation of a gentle brake without locking a wheel by a full brake operation. To obtain a computing device.

[0011]

According to a first aspect of the present invention, there is provided a vehicle collision prevention device, comprising: (i) distance measuring means for measuring a relative distance to a front object; and (ii) a vehicle wheel. An automatic slow braking means for slow braking with a predetermined first deceleration force, (i
ii) automatic sudden braking means for suddenly braking the wheel with a second deceleration force larger than the first deceleration force; and (iv)
Slow braking control means for activating the automatic slow braking means when the relative distance obtained by the distance measuring means becomes equal to or less than the first inter-vehicle distance, and (v) calculating the road surface friction coefficient from the vehicle state at the time of slow braking. Road friction coefficient calculation means to
i) an inter-vehicle distance calculating means for calculating a second inter-vehicle distance based on the road surface friction coefficient obtained by the road surface friction coefficient calculating means; and (vii) a relative distance obtained by the distance measuring means is obtained by the inter-vehicle distance calculating means. And a sudden braking control means for activating the automatic sudden braking means when the calculated distance becomes equal to or less than the second inter-vehicle distance.

A road surface friction coefficient calculating device according to a second aspect of the present invention is a calculation device for calculating a road surface friction coefficient μ from a vehicle state during traveling, and (i) a brake pressure detecting means for detecting a brake pressure P. And (ii) a wheel acceleration detecting means for detecting a wheel acceleration α, (iii) a slip ratio detecting means for obtaining a wheel slip ratio S, and (iv) a brake pressure applied by each of the detecting means during operation of a slow brake. , P _i = C ₁ based on a set of data (P _i , α _i , S _i ; i = 1 to n) obtained by measuring the wheel acceleration and the wheel slip ratio multiple times.
The coefficient C ₁ is obtained by the method of least squares from the formula of × S _i + C ₂ × α _i, and μ = a × is calculated using the constant a having a predetermined value.
And a calculating means for calculating the road surface friction coefficient μ from C ₁ .

[0013]

In the vehicle collision prevention device according to the present invention, when the distance to the front object becomes equal to or less than the first inter-vehicle distance, the slow brake is activated, and the vehicle state at the time of the slow brake is changed to the road surface. The friction coefficient μ is calculated, the second inter-vehicle distance is calculated based on the value of μ, and the calculated result value is changed. Then, when the distance to the front object becomes equal to or less than the second inter-vehicle distance, the sudden braking is activated.

In the road surface friction coefficient calculating device according to the second aspect of the present invention, data sampling of brake pressure, wheel acceleration, and wheel slip ratio is performed a plurality of times before sudden braking, and regression analysis is performed from the data obtained as a result. Thus, the road surface friction coefficient μ is calculated and estimated.

[0015]

The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a vehicle collision prevention device according to an embodiment of the present invention. This apparatus is provided with an arithmetic and control unit 11 for controlling the functions of the entire apparatus and performing various arithmetic operations, and is connected to the following devices via an input / output interface (not shown).

(I) Vehicle speed sensor 12: a sensor for detecting the speed of the vehicle.

(Ii) Distance measuring sensor 13: a sensor for detecting the distance between the own vehicle and the preceding vehicle. For example, a laser or a millimeter wave is emitted toward the preceding vehicle and the reflected wave state is taken first. Detects the distance to the vehicle.

(Iii) Relative speed sensor 14: a sensor for detecting the relative speed between the own vehicle and the oncoming vehicle, and for example, a Doppler sensor is used. This Doppler sensor emits radio waves such as microwaves toward the front of the vehicle to receive the reflected wave from the preceding vehicle, and detects the relative speed of the preceding vehicle from the Doppler shift between the transmitted wave and the received wave. is there.
Note that the relative speed is, instead of using the Doppler sensor,
The relative distance obtained by the distance measuring sensor 13 may be differentiated with respect to time.

(Iv) Acceleration sensor 15: for detecting acceleration / deceleration from the motion state of the vehicle. The acceleration / deceleration of the vehicle may be obtained by time-differentiating the own vehicle speed obtained by the vehicle speed sensor 12, instead of independently providing the acceleration sensor.

(V) Wheel speed sensor 16: a sensor for detecting the rotational speed of the wheel.

(Vi) Brake oil pressure sensor 17 is for detecting the brake oil pressure during braking.

(Vii) Alarm device 18: In response to a command from the arithmetic and control unit 11, a driver is warned by a buzzer sound or lighting of a lamp.

(Viii) Automatic braking device 19: arithmetic and control unit 1
In response to a command from 1, automatic braking such as forcibly closing the throttle and forcibly turning on the brake is performed.

The arithmetic and control unit 11 calculates the risk of collision between the host vehicle and the preceding vehicle based on the signals obtained from the above devices, and the alarm device 18 and the automatic braking device 19 according to the result of the calculation. It outputs the necessary commands to.

The operation of the vehicle collision prevention device having the above-mentioned structure will be described with reference to FIG.

As shown in FIG. 2, at a certain time t ₀ , the own vehicle 20 travels at a speed V ₀ , and the preceding vehicle 21 travels at a position Δx ahead of it at a speed V ₁ and a deceleration a _1. It is assumed that

In FIG. 3, the arithmetic and control unit 11 first sets default values (initial setting values) of various parameters necessary for the following arithmetic processing. In addition, in the following description of the parameters, "'" is added to all default values. Specifically, the default value of the own vehicle deceleration used for the calculation of the first safe inter-vehicle distance x ₁ described later is a ₀ ′ [G], from the time of the positional relationship shown in FIG. 2 (hereinafter referred to as the initial time). The default value of the lag time until the automatic braking device 19 of the own vehicle starts braking is τ ′ [seconds], the maximum value of the maximum deceleration of the own vehicle that can be exhibited on the current road surface condition is a _0max ′, and the drying described later. The coefficient of the slip ratio P of the regression line obtained for the asphalt road surface is set to C ₁ ′ (step S101). The default value a ₀ ′ of the vehicle deceleration is about 0.1 [G], and the default value a _0max ′ of the maximum deceleration is about 0.6 [G]. The lag time τ ′ is, for example, about 0.2 to 0.4 [seconds], which is a value determined as the actual value of the response time of the automatic braking device 19.

Next, the arithmetic and control unit 11 controls the above (i) to (v).
Based on the output of each sensor in i), various parameter values required for the following arithmetic processing are obtained (step S102). Specifically, the vehicle speed V ₀ is measured by the vehicle speed sensor 12, the inter-vehicle distance Δx is measured by the distance measuring sensor 13, the relative speed ΔV is measured by the relative speed sensor 14, and the vehicle acceleration a is calculated by the acceleration sensor 15.
₀ , the wheel angular velocity ω is obtained from the wheel speed sensor 16, and the brake oil pressure P is obtained from the brake oil pressure sensor 17.

Next, the arithmetic and control unit 11 carries out step S1.
The first safe inter-vehicle distance x ₁ and the second safe inter-vehicle distance x ₂ are calculated based on the default value of each parameter set in 01 and each parameter value obtained in step S102 (step S103). Here, the first safe inter-vehicle distance x
₁ means that when the own vehicle 20 starts decelerating at the virtual deceleration a ₀ ′ after the lag time τ ′ from the present time, the preceding vehicle 2
The distance that can be stopped without colliding with 1. The second safe inter-vehicle distance x ₂ is the maximum deceleration a calculated from the maximum road surface μ obtained by the slow braking operation after a lag time τ ′ from a certain time point after the slow braking operation described later.
_When the own vehicle 20 starts decelerating at _0max , the preceding vehicle 21
The distance that can be stopped without colliding with. The calculation of x ₁ and x ₂ is performed as follows.

Calculation of _first safe inter-vehicle distance x _{1 As} shown in FIG. 2, at time t ₀ , the own vehicle 2
It is assumed that ₀ travels at the speed V ₀ and the preceding vehicle 21 travels at the speed V ₁ and the deceleration a ₁ . When calculating the _first safe inter-vehicle distance x ₁ in such a setting,
It is necessary to consider two cases: the case where the own vehicle 20 collides with the preceding vehicle 21 after the stop as shown in FIG. 5 and the case where the own vehicle 20 collides with the preceding vehicle 21 while the preceding vehicle 21 is traveling as shown in FIG.

In the case of FIG. 4, the vehicle 20 is at time t.
Begins to decelerate in the 'deceleration a ₀ after the seconds' from _{0 τ.} On the other hand, the preceding vehicle 21 continues to decelerate at the deceleration a ₁ at time t ₁
The vehicle speed reaches zero. Here, the condition when the own vehicle 20 collides after the preceding vehicle 21 is stopped is the following expression (1).

A ₁ ≧ a ₀ ′ or a ₁ <a ₀ ′ and V ₀ <a ₀ ′ × (ΔV + a ₁ × τ ′) / (a ₀ ′ −a ₁ ) ... (1) In this case, time t _The distance from ₀ to the time when both vehicles are stopped is equal to the area S ₁ of the shaded area in FIG. 4, which is the safe inter-vehicle distance. This is expressed by the following equation (2).

S ₁ = V ₀ × τ ′ + (V ₀ ² / 2a ₀ ′) − (V ₀ −ΔV) ² / 2a ₁ (2) However, in this equation, ΔV = V ₀ −V ₁ (3) a ₁ = a ₀ −d (ΔV) / dt (4) Here, a ₀ is the actual acceleration of the vehicle 20 obtained by the acceleration sensor 15, and d (ΔV) / dt is the relative acceleration obtained by differentiating the relative velocity ΔV with respect to time.

On the other hand, in the case of FIG. 5, the preceding vehicle 2
A rear-end collision occurs before the vehicle speed of 1 becomes 0. The condition in this case is the following expression (5).

[0036] In the case a ₁ <a _{0'Katsu V 0> a 0 '× (} ΔV + a 1 × τ') / (a 0 '-a 1) ...... (5) such, safe inter-vehicle distance Fig. It is equal to the area S ₂ of the shaded area 5 and is expressed by the following equation (6).

[0037] _{S 2 = V 0 × t 0} -a 0 '× (t 0 -τ') 2/2 - _{[(V 0 -ΔV) × t 0} -a 1 × t 0 2/2) ] ...... (6) However, in this equation, ΔV and a ₁ are respectively (3)
The equations are the same as the equation and the equation (4), and t ₀ = (ΔV + a ₀ ′ × τ ′) / (a ₀ ′ −a ₁ ) ... (7)

Calculation of _Second Safety Inter-Vehicle Distance x _{2 In} the flow of FIG. 3, the maximum road surface μ that is running is unknown at the beginning, that is, before the soft braking operation is performed. The speed a _{0max is} also unknown. Therefore, instead of a ₀ ′ in the equations (1), (2), (5), (6) and (7), the above step S1
The second safe inter-vehicle distance x ₂ is calculated using the default value a _0max ′ of the maximum deceleration set in 01.

Next, the arithmetic and control unit 11 calculates the first safe inter-vehicle distance x ₁ and the second safe inter-vehicle distance x calculated in step S103.
The safe inter-vehicle distance x ₂ is compared with the inter-vehicle distance Δx obtained in step S102. As a result, when Δx is greater than or equal to x ₂ and less than or equal to x ₁ (step S104; N, step S105; N), the primary alarm is issued to the alarm device 18 and the automatic braking device 19 is issued. It commands the operation of the gentle brake to the extent that the wheels are not locked (step S106).

The soft brake is, for example, as shown in FIG.
The brake oil pressure is actuated so that the maximum pressure is about 10 atm, and thereafter, at every minute time interval Δt, n (n> 3) data sets (brake oil pressure P _i , wheel slip ratio S _i ,
The wheel acceleration α _i ) is sampled. Where i =
1, 2, to n. Specifically, first, the brake oil pressure P _i is detected from the brake oil pressure sensor 17, then, in step S107, the wheel angular velocity ω _i detected by the wheel speed sensor 16 is differentiated with time to obtain the wheel acceleration α _i. In S108, the wheel slip ratio S _{i is} calculated from the following equation (8).
And store these data (P _i , S _i , α _i ) (step S109).

S _i = (V _B −V _W ) / V _B (8) Here, V _B is the speed of the host vehicle at the time of the sampling, and V _W is the wheel speed at the time of the sampling ( Peripheral speed). Here, if the wheel radius is r,
There is a relationship of V _W = r × ω.

When the sampling of n data sets (P _i , S _i , α _i ) is completed by repeating the processing of steps S102 to S109 (step S110; Y), A regression line is obtained by regression analysis using P _i as the dependent variable and S _i and α _i as the independent variables, and the maximum road surface μ _max related to the road surface is calculated (step S112). And the maximum road surface μ _max
The maximum deceleration a _0max is calculated from (step S11
3).

The method of deriving the regression line and the method of calculating the maximum road surface μ _max and the maximum deceleration a _0max will be described in detail below.

Now, consider a case where the wheel 25 having the radius r of the vehicle 20 is traveling on the road surface 26 at an angular velocity ω. Here, the moment of inertia I, the brake torque f _i when multiplied by the gentle brake, the vertical load F _T of the wheel, the ground frictional force and F _Bi, from _{F Bi = μ i × F T} , its motion The equation is the following equation (9).

I × α _i = −f _i −r × F _Bi = −f _i −r × μ _i × F _T (9) By the way, in general, between the road surface μ and the wheel slip ratio S, There is a relationship as shown in FIG. 8, and the road surface μ is measured immediately before the wheels are locked (slip ratio S = S _L = about 10%).
Is the maximum. The value of this road surface μ usually draws a maximum curve in the case of a dry asphalt road surface, and its peak value, that is, the maximum road surface μ is 1.0. The value of the road surface μ is greatly influenced by the traveling road surface. For example, a road surface wet with rain has a small curve, and a snowy road or a frozen road surface has a smaller curve.

From the relationship between the road surface μ and the wheel slip ratio S shown in FIG. 8, the value μ _max of the maximum road surface μ on the road surface is also considered to be the value when the slip ratio is S _L. Here, as shown, between the slip ratio S is 0～S _L S
And μ are considered to be in a proportional relationship, μ _i =
It may be regarded as k × S _i (k is a constant). Therefore, (9)
The formula looks like this:

I × α _i = −f _i −r × k × S _i × F _{T From} this, the following equation (10) is obtained.

P _i = C ₁ × S _i + C ₂ × α _i (10) Here, P _i = −f _i , C ₁ = r × k × F _T , C ₂ = I
Is.

Then, the n sets of data (P _i , S _i , α _i ) stored in step S109 are substituted into the equation (10),
C ₁ and C ₂ are obtained by the method of least squares (step S1
11). Derivation of the regression line by the least squares method is performed, for example, by “Modern Control Engineering” by Hideaki Kano, Nikkan Kogyo Shimbun.
Can be carried out according to the method described in P126 to P129.

In this way, the regression line shown in the following equation (11) is derived.

P = C ₁ × S + C ₂ × α (11) Here, C ₁ = r × k × F _T.

On the other hand, similarly to the above, the coefficient C ₁ ′ of the wheel slip ratio S is obtained in advance on the dry asphalt road surface, and the value of this coefficient C ₁ ′ is made to correspond to the case of the maximum road surface μ = 1.0. . Here, since the value of C ₁ is considered to be proportional to the road surface μ, the road surface μ _max is obtained by the following equation (12) (step S112).

Μ _max = C ₁ / C ₁ ′ (12) Therefore, the maximum deceleration a _0max is obtained from the following equation (13) (step S113).

A _0max = μ _max × g [G] = (C ₁ / C ₁ ′) × g [G] (13) Here, g is gravitational acceleration 9.8 [G].

Next, the maximum deceleration a obtained in this way
The second safe inter-vehicle distance x ₂ is calculated again based on _0max and the input of various sensor signals obtained in step S102 (step S103). At the same time, the first safe inter-vehicle distance x ₁ is calculated based on the new sensor input value (step S103). Second safe inter-vehicle distance x
_As described above, ₂ is the distance that can be stopped without colliding with the preceding vehicle 21 when the vehicle 20 starts decelerating at the maximum deceleration a _0max that can be exerted on the road surface after the lag time τ ′ from that point. Is.

Then, the inter-vehicle distance Δx at that time is compared with x ₂ and x _1, and the alarm device 18 is operated according to the result.
And, it issues a command to the automatic braking device 19. That is, when the inter-vehicle distance Δx is equal to or greater than the second safety inter-vehicle distance x ₂ and equal to or less than the first safety inter-vehicle distance x ₁ (step S10
4; N, step S105; N), 1 for the alarm device 18
The next alarm is instructed and the automatic braking device 19 is instructed to operate the gentle brake to the extent that the wheels are not locked (step S106).
According to 13, the maximum deceleration a _0max is calculated again.

On the other hand, when the inter-vehicle distance Δx is greater than or equal to the _first safe inter-vehicle distance x ₁ (step S104; Y), the risk of rear-end collision is low and the alarm and automatic braking are released or maintained in the released state (step). S114).

If the inter-vehicle distance Δx is less than or equal to the _second safety inter-vehicle distance x ₂ (step S105; Y), it is determined that the risk of a rear-end collision is high, and a secondary alarm is issued to the alarm device 18, and The automatic braking device 19 is instructed to operate the sudden braking (step S115). Hard braking is performed by forcibly closing the throttle and maximizing the brake hydraulic pressure. In addition, during sudden braking control,
It goes without saying that the well-known anti-lock brake device may be operated at the same time.

In the first loop, when the inter-vehicle distance Δx is already equal to or less than the second safe inter-vehicle distance x ₂ obtained from the maximum deceleration a _0max ′ assumed above, the secondary Of course, it issues warnings and commands for sudden braking.

In the present embodiment, the case of a rear-end collision with the preceding vehicle has been described, but the present invention is not limited to this, and it is needless to say that the present invention can also be applied to a case of a fixed object existing ahead in the traveling direction. .

[0061]

As described above, according to the first aspect of the invention, when the distance to the front object becomes equal to or less than the first inter-vehicle distance, the slow brake is operated and at the time of the slow braking. The road surface friction coefficient μ is calculated from the vehicle state, the second inter-vehicle distance is calculated based on the value of μ, and the setting is changed. When the distance to the front object is equal to or less than the second inter-vehicle distance, the full brake is applied. Since it was decided to operate it, the road friction coefficient μ
Can be estimated to accurately predict the braking start inter-vehicle distance. Therefore, there is an effect that automatic braking can be performed at an appropriate timing according to the road surface condition, and a vehicle collision can be effectively prevented. Further, it is not necessary to provide a dedicated sensor for directly measuring the road surface friction coefficient μ, and there is an effect that problems of cost and mountability can be solved.

In the road surface friction coefficient computing device according to the second aspect of the present invention, data sampling of brake pressure, wheel acceleration, and wheel slip ratio is performed a plurality of times during gentle braking operation, and the road surface is subjected to regression analysis from the data obtained by this sampling. Since the friction coefficient μ is calculated and estimated, it is possible to obtain an accurate road surface friction coefficient according to the road surface condition only by the gentle braking.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of a vehicle collision prevention device according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a positional relationship and the like between an own vehicle and a preceding vehicle in an initial state.

FIG. 3 is a flow chart for explaining the operation of the vehicle collision prevention device.

FIG. 4 is an explanatory diagram for explaining how to determine a safe inter-vehicle distance when a rear-end collision occurs after the preceding vehicle stops.

FIG. 5 is an explanatory diagram for explaining a method of obtaining a safe inter-vehicle distance when a rear-end collision occurs during deceleration of a preceding vehicle.

FIG. 6 is an explanatory diagram showing an example of how to apply a soft brake.

FIG. 7 is an explanatory diagram showing a relationship between a road surface and wheels.

FIG. 8 is an explanatory diagram showing a relationship between a slip ratio S and a road surface μ.

[Explanation of symbols]

 11 arithmetic and control unit 12 vehicle speed sensor 13 distance measuring sensor 14 relative speed sensor 15 acceleration sensor 16 wheel speed sensor 17 brake hydraulic pressure sensor 18 alarm device 19 automatic braking device

Claims (2)

[Claims] 1. A distance measuring means for measuring a relative distance to a front object, an automatic gentle braking means for gently braking a wheel of an own vehicle with a predetermined first deceleration force, and the first wheel for the wheel. An automatic sudden braking means that suddenly brakes with a second deceleration force larger than the deceleration force, and an automatic gentle braking means is activated when the relative distance obtained by the distance measuring means becomes equal to or less than the first inter-vehicle distance. A slow braking control means, a road surface friction coefficient calculating means for calculating a road surface friction coefficient from the vehicle state at the time of slow braking, and a second inter-vehicle distance based on the road surface friction coefficient obtained by the road surface friction coefficient calculating means. When the relative distance calculated by the inter-vehicle distance calculating means and the distance measuring means is less than or equal to the second inter-vehicle distance calculated by the inter-vehicle distance calculating means, the abrupt braking means is activated. A vehicle collision prevention device, comprising: a brake control means. 2. A computing device for computing a road surface friction coefficient μ from a running vehicle state, a brake pressure detecting means for detecting a brake pressure P, a wheel acceleration detecting means for detecting a wheel acceleration α, and a wheel slip ratio S. And a set of data (P _i , α _i , S) obtained by measuring the brake pressure, the wheel acceleration and the wheel slip ratio a plurality of times by the respective detecting means during the operation of the slow brake. _i ; i = 1
~n) based on, from _{P i = C 1 × S i} + C 2 × obtains the coefficients C ₁ by the least square method from the equation consisting of _{α i, μ = a × C} 1 with a constant a with a predetermined value A road surface friction coefficient calculation device comprising: a calculation unit that calculates a road surface friction coefficient μ.