JP3186475B2 - Inter-vehicle distance alarm - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inter-vehicle distance alarm device which detects an inter-vehicle distance from a host vehicle to a preceding vehicle and issues a warning to a driver when the vehicle approaches excessively.

[0002]

2. Description of the Related Art Conventional inter-vehicle distance warning devices include:
"" Automotive Technology "Vol. 43, No. 2, 1989, p.
p.65-73 ",""Automotive Engineering Extra Edition" 1992-
1993 Technology of the Year, pp32.
57 "and other publications.
There is one described in Japanese Utility Model Laid-Open No. 1-152282. In the above device, an electromagnetic wave (for example, a laser beam) is emitted in front of the vehicle, a reflected wave of the electromagnetic wave is received, and a distance D from the propagation delay time from output to reception is detected from the reflector. Then, the warning distance Dw is calculated based on the relative speed detected from the change in the distance D, the warning distance Dw is compared with the measured distance D, and the actual distance D is calculated.
Is shorter than the warning distance Dw, the driver is warned of excessive approach. The above-mentioned warning distance corresponds to the minimum distance that can be safely stopped or avoided with a margin, and if it is less than that, it will be in an excessively close state. The warning distance Dw is, for example, the following (Equation 1)
It is determined by the formula.

[0003]

(Equation 1)

[0004] Here, Vf: vehicle speed of the own vehicle Td: idle running time α: deceleration Vr: relative speed of the own vehicle and the preceding vehicle Note that the idle running time Td is determined after the driver determines that the vehicle is approaching excessively. This is the time until deceleration or braking actually operates.

[0005] In addition, when the driver is driving with a safe distance between the vehicles under the circumstances, or when the driver continues approaching immediately before passing, the above-mentioned one-step alarm is issued. In this case, the warning sound will continue to sound, preventing comfortable driving and reducing the driver's attention to the warning due to the warning being issued too much, to alert the driver to the excessive approach which is the purpose of the warning. Can not be done. Therefore, in the above-described conventional preceding vehicle approach warning device, a second warning distance Dwm shorter than the warning distance Dw is set, and a primary warning is issued when the actual inter-vehicle distance D becomes equal to or less than the warning distance Dw. Second
The alarm is divided into two stages so as to generate a secondary alarm when the alarm distance becomes equal to or less than the alarm distance Dwm. In the first stage, after a primary alarm signal for alerting is generated for a certain period of time, the alarm is stopped and the alarm continues to sound. In the second stage where no further approach is required, an emergency secondary alarm is continuously generated.

[0006]

Such a conventional inter-vehicle distance warning device calculates a relative speed from distance data between a host vehicle and a target such as a preceding vehicle and calculates the relative speed based on the calculated value and the host vehicle speed. Since the warning distance is calculated, there is a problem that a false alarm is issued in the following cases. For example, as shown in FIG. 7, when the own vehicle 10 is traveling while following the preceding vehicle 11, the interrupting vehicle 12 suddenly interrupts the middle and enters the radar detection area 13 of the own vehicle 10. In the case where the target vehicle of the radar suddenly changes because the own vehicle 10 has changed lanes, the inter-vehicle distance decreases instantaneously, and the calculated value of the relative speed becomes a very large value. As can be seen from the above equation (1), the value of the warning distance Dw increases rapidly, so that a warning is issued. That is, in the above case, it is erroneously determined that the host vehicle 10 is approaching the preceding vehicle 11 suddenly, and an alarm is generated. In order to cope with the above problems, for example, an alarm is issued for the first time when a determination that the current inter-vehicle distance is shorter than the alarm distance is continuously obtained for a plurality of alarm calculation cycles (for example, five cycles). It is conceivable to configure as follows. However, in the above-described configuration, the issuance of an alarm is always delayed for a plurality of predetermined periods even in a normal state without interruption or the like, so that an alarm may be delayed. For example, if one cycle of the alarm calculation is 100
If the alarm issuance is delayed for 5 cycles in msec, the alarm is always delayed by about 400 msec. If the vehicle speed is, for example, 100 km / h, the alarm issuance is delayed. The vehicle would travel 11.1 meters.

The present invention has been made to solve the problems of the prior art as described above, and an object of the present invention is to provide an inter-vehicle distance alarm device that does not cause a false alarm or an alarm delay.

[0008]

Means for Solving the Problems In order to achieve the above object, the present invention is configured as described in the claims. That is, the invention described in claim 1 is:
As shown in the functional block diagram of FIG.
An inter-vehicle distance measuring means 100 for measuring an inter-vehicle distance between the own vehicle and the preceding vehicle, a relative speed calculating means 101 for calculating a relative speed between the own vehicle and the preceding vehicle from a change in the inter-vehicle distance, and an own vehicle for detecting the own vehicle speed Speed detecting means 102; warning distance calculating means 103 for calculating a warning distance based on at least the own vehicle speed and the relative speed; and an alarm is issued basically when the inter-vehicle distance is equal to or less than the warning distance. If the difference between the current inter-vehicle distance and the inter-vehicle distance before a predetermined time is equal to or more than a predetermined value, the alarm generation judging means 104 for suppressing the alarm issuance during a predetermined grace period; Alarm generating means 105 for generating an alarm according to the determination of the means. The above-described units correspond to, for example, the following units in the embodiment of FIG. That is, the inter-vehicle distance measuring means 100 is connected to the inter-vehicle distance sensor 1.
The own vehicle speed detecting means 102 corresponds to the vehicle speed sensor 3 and the alarm generating means 105 corresponds to the alarm buzzer 4. The relative speed calculating means 101 corresponds to a relative speed calculating section 2c which is a function of the CPU 2, the alarm distance calculating means 103 corresponds to an alarm distance calculating section 2e, and the alarm occurrence determining means 104 corresponds to an alarm issuing determining section 2f. The arithmetic processing in the above configuration is as shown in, for example, a flowchart of FIG.

Next, as set forth in claim 2, the inter-vehicle distance measuring means repeatedly measures the inter-vehicle distance between the host vehicle and the preceding vehicle at a predetermined cycle. The relative speed is calculated by a least squares processing method using K average inter-vehicle distance data obtained by averaging consecutive J inter-vehicle distance data sequentially obtained by the distance measurement means. If the difference between the latest average inter-vehicle distance data and the average inter-vehicle distance data measured one cycle before is equal to or greater than a predetermined value, the grace time is set to (J + K−
2) The warning issuance is suppressed during the period. Note that, in this case, the relative speed calculating means 101
Of the moving average processing calculation unit 2b and the relative speed calculation unit 2c.

In addition, as described in claim 3, the alarm generation judging means determines that the inter-vehicle distance is equal to or greater than a predetermined value even if the difference between the current inter-vehicle distance and the inter-vehicle distance before a predetermined time is equal to or greater than a predetermined value. If the distance is equal to or shorter than a predetermined minimum inter-vehicle distance determined in accordance with the own vehicle speed, it is immediately determined that an alarm is issued without providing the grace period. The arithmetic processing in the above configuration is, for example, as shown in the flowchart of FIG.

[0011]

As described above, according to the present invention, in the calculation of the alarm generation judging means 104, it is basically determined that an alarm is issued when the inter-vehicle distance becomes less than the alarm distance. When the difference from the inter-vehicle distance before the time is equal to or more than a predetermined value, the warning issuance is suppressed during a predetermined grace period. With the above-described configuration, the difference between the current inter-vehicle distance and the inter-vehicle distance before a predetermined time is less than a predetermined value in a normal state where the inter-vehicle distance changes continuously (when there is no interruption or the like). The alarm can be issued immediately without any time delay. Also, if the inter-vehicle distance changes suddenly due to interruption, etc.,
Since the difference between the current inter-vehicle distance and the inter-vehicle distance before a predetermined time is equal to or greater than a predetermined value, the issuance of an alarm is suppressed during a predetermined grace period, and an alarm is issued only when the alarm condition is satisfied even after that period. Is issued, and occurrence of a false alarm can be prevented.

[0012] Claim 2 shows a method of determining the delay time. As will be described later in detail, when the relative speed is calculated by the least squares processing method using K pieces of average inter-vehicle distance data obtained by averaging J consecutive inter-vehicle distance data, the discontinuity is once added to the distance data. When a large step occurs, (J-1) is thereafter applied to the average distance data.
The period and the relative speed data are affected during the period (J + K-2). Therefore, in order to prevent the erroneous determination except for the above-described influence, the grace time may be set to (J + K−2) cycles to suppress the alarm issuance during that time.

According to a third aspect of the present invention, when the inter-vehicle distance is equal to or less than a predetermined minimum inter-vehicle distance determined in accordance with the own vehicle speed, even when the condition for providing the grace period is satisfied. This is configured to immediately determine that an alarm has been issued without providing a grace period. The minimum inter-vehicle distance Dm is a value of about Dm = Vf / 5 when the own vehicle speed Vf is displayed in km / h, for example, when the own vehicle speed is 50 km.
/ H is about 10 m, and 100 km / h is about 20 m, and it is necessary to secure at least this inter-vehicle distance.
Therefore, when the inter-vehicle distance becomes equal to or less than the minimum inter-vehicle distance Dm, an alarm is immediately issued without providing a grace period. The minimum inter-vehicle distance Dm is defined as the second warning distance Dwm described in the conventional example, and when the inter-vehicle distance becomes equal to or less than this value, an emergency secondary alarm is continuously generated. You may.

[0014]

FIG. 2 is a block diagram showing an embodiment of the present invention. In the present embodiment, the target that the own vehicle may collide with on the road is equivalent to a falling object, a preceding vehicle, or a stopped vehicle.
All the targets are described as preceding vehicles. In FIG.
The inter-vehicle distance sensor 1 is, for example, a distance measuring device such as a laser radar or a Doppler radar, transmits a transmission signal to the front of the vehicle, receives reflection from a preceding vehicle or the like as a reception signal, and receives the transmission signal and the reception signal. And a function for calculating the inter-vehicle distance D based on the transmission / reception time difference between the two. The vehicle speed sensor 3
Measures the speed Vf of the host vehicle from, for example, the number of revolutions of the wheels. The CPU 2 performs various signal processing such as determination of excessive approach with a preceding vehicle, and is a central processing unit of a computer. In addition, the alarm buzzer 4 is
Is a device that receives an alarm signal that is output when it is determined that an alarm has occurred as a result of the information processing in and generates an alarm.
In addition to the buzzer, for example, an acoustic alarm using a chime or the like, an audio alarm using a recorded or synthesized voice, a light alarm using a light emitting device, or the like can be used.

The CPU 2 stores the distance data sent from the inter-vehicle distance sensor 1 in the first memory 2a. Then, in the moving average processing operation unit 2b, the first memory 2a
The moving average processing calculation is performed on the distance data group that fluctuates with time, which is stored in. Note that the moving average processing is to shift the calculation for calculating the average value of a predetermined number of continuous data among a plurality of distance data sequentially measured in time series from old data to latest data by one data at a time. It is done while doing. The inter-vehicle distance value (average inter-vehicle distance) obtained by the moving average processing calculation unit 2b is stored in the second memory 2d. Further, the relative speed calculation unit 2c calculates the relative speed as the amount of change per unit time of the average inter-vehicle distance obtained by the moving average processing calculation unit 2b.

Next, in the alarm distance calculation section 2e, an alarm is issued based on, for example, the above equation (1) or the following equation (5) according to the relative speed and the own vehicle speed inputted from the vehicle speed sensor 3. The distance Dw is calculated. The warning issuance determination unit 2f basically determines to issue a warning when the current average inter-vehicle distance is equal to or less than the alarm distance Dw. 2 If the difference from the average inter-vehicle distance one cycle before (the value of the immediately preceding calculation) stored in the memory 2d is greater than or equal to a predetermined value, it is determined to be an abnormal value due to an interrupt or the like, and Suppress alarm issuance during the grace period. The above-mentioned delay time is specifically measured as a predetermined period of the above-described arithmetic processing that is periodically repeated. For example, when calculating the relative speed by the least squares processing method using K average inter-vehicle distances obtained by averaging J consecutive inter-vehicle distance data, the grace time is set to (J + K−2) cycles ( Details will be described later).

The details of the arithmetic processing of this embodiment will be described below in detail with reference to the flowchart shown in FIG. In FIG. 3, when the power is turned on and the system operates, first,
In step S1, the alarm suppression counter N is reset to zero. Then, the measurement of the distance is started in step S2. For example, the inter-vehicle distance sensor 1 in FIG. 2 is a pulse light emission type sensor, and calculates the distance to the preceding vehicle from the delay time until the transmitted light is scattered by the preceding vehicle and returns to the sensor. When the distance measurement is started, in step S3, pulse light is emitted from the light transmitting unit of the following distance sensor 1 toward the front of the host vehicle. Then, in step S4, the above-mentioned pulse light is scattered and reflected by the preceding vehicle ahead and returns to the light receiving unit.

Next, in step S5, an inter-vehicle distance d (t) to the preceding vehicle is calculated from the elapsed time Δt from when the light is transmitted to when the light is received by the following equation (2). d (t) = C × Δt / 2 (Equation 2) where C: light speed The distance between vehicles is indicated by d (t) because the distance d
Changes depending on the measured time t. Next, in step S6, the inter-vehicle distance d (t) calculated as described above is stored in the first memory 2a. The steps so far are the distance measurement processing, and the distance measurement processing ends in step S7. Next, in step S8, a moving average process is performed on the data group of the calculated inter-vehicle distance d (t). This is a process for smoothing the data by removing an instantaneous distance variation. In the present embodiment, for example, the average value of the past four distance data is obtained. Therefore, the average inter-vehicle distance data D (t) after the averaging process is obtained by the following equation (3).

[0019]

(Equation 3)

Here, d (t): present (latest) distance data d (t−δt): distance data obtained by calculation one cycle before d (t−2δt): distance obtained by calculation two cycles before Data d (t−3δt): distance data obtained by calculation three cycles before The distance measurement calculation cycle δt is, for example, 100 msec. Next, in step S9, the average inter-vehicle distance data D (t) obtained as described above is stored in the second memory 2d. Next, in step S10, using the latest average inter-vehicle distance data obtained this time and the average inter-vehicle distance data for the past three times, the average inter-vehicle distance data for a total of four times, based on equation (4) below. Relative speed V between vehicle and preceding vehicle
Calculate r. Expression (4) is an operation expression based on the least squares method.

[0021]

(Equation 4)

However, D (t): present (latest)
Average inter-vehicle distance data D (t−δt): Average inter-vehicle distance data obtained by calculation one cycle before D (t−2δt): Average inter-vehicle distance data obtained by calculation two cycles before D (t−3δt): Average inter-vehicle distance data obtained by calculation three cycles earlier Next, in step S11, based on the own vehicle speed Vf (not shown in the flow of FIG. 3) read from the vehicle speed sensor 3 and the relative speed Vr. The warning distance Dw is calculated. In addition, as an example of the arithmetic expression of the alarm distance Dw, the above (Equation 1)
Some of the equations are shown, but the following equation (5) may be used.

[0023]

(Equation 5)

Here, Tf and Tr are time-related constants, for example, values of about 0.5 to 1.0 sec. Also,
α is a deceleration (constant), for example, a value of about 7.0 m / sec ² . FIG. 4 is a characteristic diagram showing an example of the relationship between the warning distance Dw and the host vehicle speed Vf. This characteristic is a characteristic when the relative speed Vr is set to 20 km / h. The illustrated straight line indicates the warning distance Dw, and the lower side of the straight line (the shorter inter-vehicle distance) is the warning distance range.

Next, in step S12, it is determined whether or not the value of the alarm suppression counter N is 0. If "YES", that is, if the alarm is not being suppressed, the process proceeds to step S13. In step S13, the current average inter-vehicle distance D (t)
And the warning distance Dw. If the current average inter-vehicle distance D (t) is longer than the warning distance Dw, a process of "no warning" is performed in step S14, and the process returns to step S2. The process of “no alarm” is a process of continuing the state when the alarm is stopped, and stopping the alarm when the alarm is issued. In step S13,
If the current average inter-vehicle distance D (t) is less than or equal to the warning distance Dw, the process proceeds to step S15. In step S15, the difference | D (t) -D (t-δt) | between the current average inter-vehicle distance D (t) and the immediately preceding average inter-vehicle distance D (t−δt) is determined, and the difference is determined as a predetermined value. Compared to the value (for example, 4 m), if the difference is 4 m or less, the process proceeds to step S16 to perform the processing of issuing an alarm, and returns to step S2. This activates the alarm buzzer 4 and issues an alarm. Step S
If the difference is larger than 4 m at 15, the process goes to step S17, sets the value of the alarm suppression counter N to 1, and returns to step S2.

If the value of the alarm suppression counter N is 1, "NO" is determined in step S12.
Then, the process goes to step S18. In step S18, the value of N is incremented by one. Next, in step S19, it is determined whether or not the number of N is a predetermined value (for example, 6). If "YES", the value of N is reset to 0 in step S20, and the process returns to step S2. Also, “N
In the case of "O", the process returns to step S2 as it is. Therefore, when the value of the alarm suppression counter N once becomes 1, until N reaches a predetermined value (6 in the above example), that is, a predetermined operation Until the number of times of processing is repeated, the processing of steps S13 to S17 is passed.

Thus, the current average inter-vehicle distance D (t)
Is less than the warning distance Dw, and the difference between the current average inter-vehicle distance D (t) and the immediately preceding average inter-vehicle distance D (t−δt) is equal to or greater than a predetermined value even if the alarm should be issued. In such a case, that is, when the inter-vehicle distance changes suddenly, no alarm is issued. At this time, the time during which the calculation corresponding to the predetermined value 6 is repeated is the grace time. For example, if one calculation cycle is 100 msec, the grace time is 600 msec. Note that the predetermined value of 4 m in the above step 15 is a value selected from the speed dependence of the idling distance and the braking distance of the vehicle, and if only the limit width is set, the sudden speed up to the relative speed of 140 km / h is obtained. It is possible to effectively issue a warning assuming approach. It should be noted that, in a normal state where the inter-vehicle distance does not change suddenly, an alarm is issued immediately after going from step S15 to step S16, so that there is no delay in issuing the alarm in the normal state.

The value 6 of the predetermined value in step S19, that is, the value of the number of repetitions of the arithmetic processing for determining the grace time is determined as follows. That is, as described above, in the present embodiment, the average inter-vehicle distance is obtained using the four inter-vehicle distance data in the moving average processing in step S8. Therefore, once a discontinuous level difference occurs in the inter-vehicle distance data, the influence is exerted on the average inter-vehicle distance data for three periods and for the relative speed data for six periods. FIG. 5 is a characteristic diagram showing the above-described state. If a step occurs in the fifth data in the following distance data, up to the seventh in the average following distance data,
It can be seen that the relative speed data has an effect up to the tenth. Also, when the distance step is large, the calculated value of the relative speed becomes large, so that the alarm distance Dw calculated by the equation (5) becomes long, and for a certain period (in the above example,
The possibility of false alarm being issued is increased.
For this reason, in the present embodiment, the predetermined value in step S19 is set to 6, and the grace period is set for six cycles. As can be understood from the above description, in the general case, K pieces of average inter-vehicle distance data D (t) obtained by averaging J consecutive inter-vehicle distance data d (t) sequentially obtained by the inter-vehicle distance sensor 1 are used. When calculating the relative speed Vr using the least squares processing method, the delay time is set to (J + K−2).
What is necessary is just to set it for a period. In the case of the above example, J = 4, K
= 4, 4 + 4-2 = 6.

FIG. 6 is a flowchart of another embodiment showing the arithmetic processing. In the flow of FIG. 6, step S21 is provided between steps S13 and S15 in the flow of FIG. Step S2
In step 1, it is determined whether the average inter-vehicle distance D (t) is equal to or less than the minimum inter-vehicle distance Dm. If "YES", the flow goes to step S16 to immediately issue an alarm. Also, “N
In the case of "O", the process goes to step S15 to perform the same processing as in Fig. 3. As described above, this embodiment is a case where the condition for providing the grace period is satisfied. Even if the inter-vehicle distance is a predetermined minimum inter-vehicle distance Dm
In the following cases, it is configured to immediately determine that an alarm has been issued without providing a grace period.

The minimum inter-vehicle distance Dm is, for example, a value determined in accordance with the own vehicle speed.
Dm = Vf / 5 value, for example, 10 m at 50 km / h and 20 m at 100 km / h
The value of the degree. Since this inter-vehicle distance needs to be secured at a minimum, when the inter-vehicle distance becomes equal to or less than the minimum inter-vehicle distance Dm, an alarm is issued immediately without a grace period. . The minimum inter-vehicle distance Dm is defined as the second warning distance Dwm described in the conventional example, and when the inter-vehicle distance becomes equal to or less than this value, an emergency secondary alarm is continuously generated. You may.

[0031]

As described above, in the present invention,
Basically, it is determined that an alarm is issued when the following distance is less than the warning distance.However, if the difference between the current following distance and the following distance before the predetermined time is equal to or more than a predetermined value, a predetermined grace period is set. In the event that the inter-vehicle distance suddenly changes due to an interruption, etc., the alarm issuance is suppressed during the predetermined grace period, and the alarm condition is not exceeded even after that period. The alarm is issued only when the following conditions are satisfied, so that a false alarm can be prevented. In addition, when the distance between vehicles changes continuously (when there is no interruption, etc.), a time delay occurs. The effect is that an alarm can be issued immediately without the need. Also, when the inter-vehicle distance is less than the minimum inter-vehicle distance, a warning is issued immediately regardless of other conditions, and the inter-vehicle distance becomes less than the minimum inter-vehicle distance required for safe driving. In such a case, an alarm is issued promptly to warn that the vehicle is in an excessively close state.

[Brief description of the drawings]

FIG. 1 is a functional block diagram of the present invention.

FIG. 2 is a block diagram of one embodiment of the present invention.

FIG. 3 is a first flowchart illustrating a calculation process according to the present invention;
FIG.

FIG. 4 is a characteristic diagram showing a relationship between a warning distance and a vehicle speed.

FIG. 5 is a characteristic diagram showing an influence of a step generated in distance data on average distance data and relative speed data.

FIG. 6 is a second flowchart illustrating the arithmetic processing according to the present invention;
FIG.

FIG. 7 is a diagram for explaining a state at the time of vehicle interruption.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Inter-vehicle distance sensor 2 ... CPU 3 ... Vehicle speed sensor 2a ... First memory 4 ... Warning buzzer 2b ... Moving average processing calculation unit 10 ... Own vehicle 2c ... Relative speed calculation unit 11 ... Previous vehicle 2d ... Second memory 12 ... Interruption Vehicle 2e: Warning distance calculation unit 13: Radar detection area 2f: Warning issuance determination unit 100: Inter-vehicle distance measurement unit 103: Warning distance calculation unit 101: Relative speed calculation unit 104: Warning generation determination unit 102: Own vehicle speed detection unit 105 Alarm generation means

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G08G 1/00-9/02 G01S 13/93 G01S 17/93 G08B 3/10

Claims (3)

(57) [Claims] An inter-vehicle distance measuring means mounted on a vehicle for measuring an inter-vehicle distance between the host vehicle and a preceding vehicle; a relative speed calculating means for calculating a relative speed between the own vehicle and the preceding vehicle from a change in the inter-vehicle distance; Own vehicle speed detecting means for detecting the own vehicle speed, based on at least the own vehicle speed and the relative speed,
An alarm distance calculating means for calculating an alarm distance; basically, it is determined that an alarm is issued when the inter-vehicle distance is equal to or less than the alarm distance, but a difference between the current inter-vehicle distance and the inter-vehicle distance before a predetermined time is determined. When the value is equal to or more than the value, an alarm generation judging means for suppressing the alarm issuance for a predetermined grace period, and an alarm generation means for generating an alarm according to the judgment of the alarm generation judging means, are provided. Inter-vehicle distance warning device. 2. The inter-vehicle distance measuring means repeatedly measures the inter-vehicle distance between the host vehicle and the preceding vehicle at a predetermined cycle, and the relative speed calculating means comprises a continuous J obtained by the inter-vehicle distance measuring means. And calculating the relative speed by a least squares processing method using K pieces of the average inter-vehicle distance data obtained by averaging the inter-vehicle distance data. If the difference from the average inter-vehicle distance data measured is equal to or more than a predetermined value, the grace period is set to (J + K-2) cycles, and the warning issuance is suppressed during the period. The following distance warning device. 3. The inter-vehicle distance is determined in accordance with the own vehicle speed even when the difference between the present inter-vehicle distance and the inter-vehicle distance before a predetermined time is equal to or greater than a predetermined value. The inter-vehicle distance alarm device according to claim 1 or 2, wherein when the distance is equal to or shorter than a predetermined minimum inter-vehicle distance, an alarm issuance is immediately determined without providing the grace time.