Alarm system for vehicle

To provide an alarm system for a vehicle that can reduce alarms that can irritate a driver. An alarm system 1 for a vehicle includes an alarm line setting part 51 that sets a right alarm line 61 and a left alarm line 62 in front of a vehicle 2, and an alarm unit control part 55 that activates a speaker 41 and a display 42 when the velocity of the vehicle 2 is equal to or less than a predetermined velocity and a collision determination part 53 determines that an object will cross the right alarm line 61 or the left alarm line 62 within a predetermined time. When the vehicle 2 is decelerating, the alarm unit control part 55 performs an alarm reduction control that suppresses operation of the speaker 41 compared with when the vehicle 2 is not decelerating.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an alarm system for a vehicle. In particular, it relates to an alarm system for a vehicle that raises an alarm in response to an object approaching the vehicle.

Description of the Related Art

Systems that raise an alarm in response to an object approaching have been proposed. For example, Japanese Patent Laid-Open No. 2012-160103 discloses a system that determines the possibility of an object located at the side of a vehicle colliding with the vehicle based on the distance between the object and the vehicle or the relative velocity of the object with respect to the vehicle. When the system determines that the object can collide with the vehicle, the system raises an alarm to the driver of the vehicle.

The system described in Japanese Patent Laid-Open No. 2012-160103 determines the stopping distance of each object existing around the vehicle (that is, the distance the object moves before the object is stopped). The system activates an alarm unit when the stopping distance of an object is greater than the distance between the object and the vehicle. Such a system has an advantage that the system can alert the driver to an object that is less conspicuous to the driver to avoid collision with the object.

The system described in Japanese Patent Laid-Open No. 2012-160103 determines that an object around the vehicle can collide with the vehicle and raises an alarm when the object satisfies a physical condition relating to collision. However, such a system can raise a false alarm when the physical condition does not accurately reflect the actual possibility of collision. That is, even when the object is actually unlikely to collide with the vehicle, such a system can raise an alarm, and such an alarm can irritate the driver.

The present invention has been made to solve the problem described above, and an object of the present invention is to provide an alarm system for a vehicle that can reduce alarms that can irritate a driver.

SUMMARY OF THE INVENTION

To solve the problem described above, the present invention provides an alarm system for a vehicle, comprising: an alarm line setting part that sets an alarm line extending forward from the vehicle; an object detection part that detects an object on a side of the vehicle; a collision determination part that determines whether or not the object crosses the alarm line within a predetermined time; and an alarm unit control part that activates an alarm unit when the collision determination part determines that the object crosses the alarm line within the predetermined time, wherein the alarm unit control part performs an alarm reduction control to reduce operation of the alarm unit when the vehicle is decelerating compared with when the vehicle is not decelerating.

For example, when the vehicle is decelerating to temporarily stop, the possibility of an object on a side of the vehicle colliding with the vehicle is lower than when the vehicle is not decelerating. For this reason, with the arrangement described above, the alarm unit control part performs the alarm reduction control to reduce operation of an alarm unit when the vehicle is decelerating compared with when the vehicle is not decelerating. As a result, more alarms can be raised to the driver when the possibility of the object colliding with the vehicle is relatively high, whereas alarms that can irritate the driver can be reduced when the possibility of the object colliding with the vehicle is relatively low.

According to the present invention, preferably, the alarm unit includes a first alarm device and a second alarm device, and the alarm unit control part activates the first alarm device and the second alarm device when the vehicle is not decelerating; and the alarm unit control part does not activate the first alarm device but activates the second alarm device as the alarm reduction control when the vehicle is decelerating.

With this arrangement, when the vehicle is decelerating (that is, when the possibility of the object colliding with the vehicle is relatively low), the alarm unit control part does not activate the first alarm device and thereby can reduce alarms that can irritate the driver.

According to the present invention, preferably, the first alarm device audibly raises an alarm, and the second alarm device visually raises an alarm.

An audible alarm is more likely to irritate the driver than a visual alarm. With the arrangement described above, when the vehicle is decelerating (that is, when the possibility of the object colliding with the vehicle is relatively low), the audible alarm is not raised, so that the driver is less likely to be irritated by alarms.

According to the present invention, preferably, the alarm unit control part activates the alarm unit when a velocity of the vehicle is equal to or greater than a predetermined velocity; and the alarm unit control part does not perform the alarm reduction control when the vehicle starts after having decelerated from a velocity greater than the predetermined velocity and stopped.

For example, when the vehicle temporarily stops at the intersection and then starts to enter the intersection, the possibility of the vehicle colliding with another vehicle entering the intersection from another direction increases. With this arrangement, the alarm reduction control is not performed in such a situation, so that the driver can be alerted with reliability.

According to the present invention, preferably, the alarm unit control part performs the alarm reduction control when a deceleration of the vehicle becomes equal to or greater than a predetermined deceleration.

With this arrangement, the alarm unit control part can be prevented from performing the alarm reduction control in response to a slight deceleration of the vehicle and thus failing to alert the driver.

According to the present invention, preferably, the alarm system for a vehicle further comprises a traffic signal detection part that detects a traffic signal emitted by a traffic light in the direction of traveling of the vehicle, and the alarm unit control part performs the alarm reduction control when the traffic light is emitting a stop signal.

With this arrangement, when the vehicle is decelerating in response to the stop signal, the alarm reduction control can be performed with reliability.

Advantages of the Invention

The present invention can provide an alarm system for a vehicle that can reduce alarms that can irritate a driver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, embodiments will be described with reference to the accompanying drawings. To facilitate understanding of the description, the same components are denoted by like reference numerals throughout the drawings, and redundant descriptions thereof will be omitted.

First, with reference toFIG. 1, a configuration of an alarm system1for a vehicle (referred to simply as an alarm system1, hereinafter) according to an embodiment will be described.FIG. 1is a block diagram showing the alarm system1.

The alarm system1is mounted on a vehicle and raises an alarm to the driver of the vehicle to alert the driver. In this specification, the vehicle on which the alarm system1is mounted will be referred to as a “vehicle2”. The alarm system for a vehicle according to the present invention can be applied not only to a four-wheeled vehicle but also to a two-wheeled vehicle.

In this specification, the direction in which the vehicle2runs forward is defined as “forward”, and the direction in which the vehicle2runs backward is defined as “backward”. The left side of the vehicle running forward is defined as “left”, and the right side of the vehicle running forward is defined as “right”.

The alarm system1includes a side radar31, an external camera32, a navigation device33, a speed sensor34, and an acceleration/deceleration sensor35. The alarm system1further includes a speaker41, a display42and an electronic control unit (ECU)5.

The side radar31is used to detect the presence of an object outside of the vehicle2, the velocity of the object, and the distance between the object and an alarm line, which will be described later. Detectable objects include a vehicle, a structure fixed on a road, and a pedestrian, for example. The vehicle is not limited to a four-wheeled vehicle but may be any running body, such as a two-wheeled vehicle or a bicycle. As described later, the detections described above performed by the side radar31are targeted to an object in a detection region set at a side of the vehicle2. The side radar31is a millimeter wave radar (which has an operating frequency of 76 GHz to 77 GHz), for example, and has antennas directed to the left and right of the vehicle2. Each antenna may be a single antenna used for both transmission and reception or a set of a transmitting antenna element and a receiving antenna element. The side radar31transmits a measuring wave from the antennas to the sides of the vehicle2and receives a reflection wave from an object. The side radar31transmits a signal corresponding to the received reflection wave to the ECU5.

The external camera32takes an image of a range including the detection region of the side radar31and obtains image information. The external camera32is an image sensor, for example, and is installed on a rear view mirror (not shown) or the like of the vehicle2. The external camera32transmits a signal corresponding to the obtained image information.

The navigation device33provides predetermined information to a passenger of the vehicle2. The navigation device33stores map information or obtains map information by communicating with a server outside the vehicle2. The map information includes information about roads, intersections, traffic lights, buildings or the like. The navigation device33has a sensor that detects the location of the vehicle2, such as a global positioning system (GPS) sensor or a self-contained navigation sensor. The navigation device33determines the location of the vehicle2on a map, audibly or visually provides information about the location, and transmits a signal corresponding to the information to the ECU5. The “intersections” include not only crossroads but also T junctions, roundabout intersections and the like.

The speed sensor34detects the velocity of the vehicle2. The speed sensor34determines the velocity of the vehicle2based on the rotational speed of a wheel (not shown) of the vehicle2, the engine speed or the like, and transmits a signal corresponding to the velocity to the ECU5.

The acceleration/deceleration sensor35detects the acceleration or deceleration of the vehicle2. The acceleration/deceleration sensor35transmits a signal corresponding to the detected acceleration or deceleration to the ECU5.

According to this embodiment, the velocity and the acceleration or deceleration of the vehicle2are separately detected by the speed sensor34and the acceleration/deceleration sensor35. However, the present invention is not limited to this implementation. For example, the alarm system1may not be provided with the acceleration/deceleration sensor but may determine the acceleration by performing the time derivative of the velocity of the vehicle2detected by the speed sensor.

The speaker41and the display42are examples of alarm devices of an alarm unit according to the present invention. More specifically, the speaker41is an example of a first alarm device according to the present invention, and the display42is an example of a second alarm device according to the present invention. The speaker41operates based on a received control signal and raises an alarm by outputting an alarm or other sound. The display42is a liquid crystal panel, for example. The display42operates based on a received control signal and raises an alarm by displaying a picture, text or the like.

The ECU5is a controller that controls equipment by transmitting and receiving signals. The ECU5may be partially or wholly formed by an analog circuit or formed as a digital processor. The ECU5includes an alarm line setting part51, an object detection part52, a collision determination part53, a traffic signal detection part54, and an alarm unit control part55.

FIG. 1shows functions of the ECU5in the form of blocks. However, the analog circuit or the software module incorporated in the digital processor of the ECU5is not necessarily divided as shown inFIG. 1. That is, the functional blocks shown inFIG. 1may be further divided, or some of the functional blocks may be integrated into a single functional block. Those skilled in the art can modify the internal configuration of the ECU5as appropriate, as far as the processes described later can be performed.

The alarm line setting part51sets an alarm line. The alarm line is a virtual line set in the vicinity of the vehicle2, which is used for determination of collision by the collision determination part53. The alarm line will be described in more detail later.

The object detection part52detects the direction of movement of an object outside the vehicle2, the distance of the object from an alarm line, and the relative velocity of the object with respect to the alarm line. Specifically, the object detection part52performs a predetermined calculation based on a signal received from the side radar31and performs the detections based on the calculation result.

The collision determination part53performs determination of collision. In determination of collision, it is determined whether or not an object outside the vehicle2will cross the alarm line within a predetermined time. The determination of collision will be described in more detail later.

The traffic signal detection part54detects a traffic signal emitted by a traffic light in the direction of traveling of the vehicle2. Specifically, the traffic signal detection part54determines the location of the traffic light located in the direction of traveling of the vehicle2based on a signal received from the external camera32or the navigation device33. The traffic signal detection part54further detects the signal emitted by the traffic light based on the image information obtained by the external camera32. The detection of the signal emitted by the traffic light can be achieved by various techniques, such as pattern matching. The determination of the location of the traffic light and the detection of the signal can be achieved by other various techniques, such as road-to-vehicle communication or vehicle-to-vehicle communication.

The alarm unit control part55transmits a control signal to the speaker41or the display42based on the result of the determination of collision by the collision determination part53or the result of the detection by the traffic signal detection part54. Specifically, the alarm unit control part55transmits a control signal to activate the speaker41or the display42when the collision determination part53determines that the object can collide with the vehicle2.

Next, with reference toFIG. 2, the determination of collision performed by the alarm system1will be described.FIG. 2is a diagram for illustrating the determination of collision performed by the alarm system1.

When the vehicle2is stopped or running at a relatively low velocity (10 km/h or less, for example), the alarm line setting part51(seeFIG. 1) of the ECU5sets a right alarm line61and a left alarm line62, as shown inFIG. 2. The right alarm line61and the left alarm line62are invisible virtual lines. The right alarm line61is set at a predetermined distance (1 m or less, for example) in the right direction from the right edge of the vehicle2, and the left alarm line62is set at a predetermined distance (1 m or less, for example) in the left direction from the left edge of the vehicle2. That is, the distance between the right alarm line61and the left alarm line62is greater than the width of the vehicle2.

The right alarm line61and the left alarm line62extend in the forward direction of the vehicle2from a base line BL as a base end. The base line BL is a virtual line set rearward from the front edge of the vehicle2by a predetermined distance (2 m, for example). The right alarm line61and the left alarm line62extend straight substantially in parallel with each other in the longitudinal direction of the vehicle2. The alarm line setting part51sets the length of the right alarm line61and the left alarm line62at L1(7 m, for example).

The object detection part52(seeFIG. 1) of the ECU5sets a detection region A1at the right side of the right alarm line61, and a detection region A2at the left side of the left alarm line62. The detection region A1is defined by the right alarm line61, a line at a predetermined distance in the right direction from the right alarm line61, and lines extending from the right alarm line61at approximately 135° with respect to the right alarm line61. The detection region A2is defined by the left alarm line62, a line at a predetermined distance in the left direction from the left alarm line62, and lines extending from the left alarm line62at approximately 135° with respect to the left alarm line62. The object detection part52detects an object in the detection regions A1and A2based on the signal received from the side radar31.

In the following, the determination of collision of another vehicle91in the detection region A1approaching the vehicle2will be described. When a virtual line L91extending in the direction of movement of the other vehicle91intersects with the right alarm line61, the collision determination part53(seeFIG. 1) of the ECU5calculates a time to collision (TTC) for the other vehicle91with respect to the right alarm line61based on information detected by the object detection part52. In general, the TTC is expressed by the following formula f1, provided that the distance between relevant objects is defined as L0, and the relative velocity between the objects is defined as V0. The relative velocity V0of the object is positive when the object is approaching the alarm line. The formula f1is derived from the equation of motion on the condition that the object is moving at a constant velocity.

The collision determination part53determines whether or not the other vehicle91will cross the right alarm line61within a predetermined time based on the calculated TTC. Specifically, when the TTC is equal to or less than a preset threshold (2 seconds, for example), the collision determination part53determines that the other vehicle91will cross the right alarm line61within a predetermined time (2 seconds, for example). Then, the other vehicle91can collide with the vehicle2.

As described above, the alarm system1determines whether the object at the right side of the vehicle2can collide with the vehicle2or not with respect to the right alarm line61. Similarly, the alarm system1determines whether the object at the left side of the vehicle2can collide with the vehicle2or not with respect to the left alarm line62.

The alarm system1configured described above is particularly advantageous in a situation where there is a blind spot for the driver of the vehicle2. An example of the situation where there is a blind spot for the driver is a situation where there is a wall near the lane in which the vehicle is running or a situation where there is another vehicle parked around the vehicle2. That is, when an object in the blind spot is approaching the vehicle2and can collide with the vehicle2, at least one of the speaker41and the display42raises an alarm to alert the driver to the object so that the driver can operate the vehicle to avoid collision.

Next, with reference toFIG. 3, determination of collision at an intersection will be described.FIG. 3is a diagram for illustrating determination of collision at an intersection83. As an example,FIG. 3shows a situation where the traffic laws prescribe that vehicles run in the left lane, such as in Japan.

At the intersection83, a lane80intersects with lanes81and82. In the following description, the lane81, which is closer to the vehicle2when the vehicle2enters the intersection83, will be referred to as the “first lane81”, and the opposite lane to the first lane81will be referred to as the “second lane82”.

Consider a case where the vehicle2running in a straight line in the lane80is approaching the intersection83and there is a traffic light85in the direction of traveling of the vehicle2as shown inFIG. 3. When a lamp85aof the traffic light85is emitting a stop signal (the so-called “red signal”), the vehicle2decelerates to stop at a stop line83a. When the vehicle2decelerates (to 10 km/h or less, for example), the alarm system1sets the right alarm line61and the left alarm line62as described above.

At this point, traffic lights86and87along the first lane81and the second lane82are probably emitting a signal (the so-called “green signal”) that allows the vehicles in the lanes to move. Therefore, when there is another vehicle95running in the first lane81and entering the intersection83, the virtual line in the direction of traveling of the other vehicle95may intersect with the right alarm line61.

When the virtual line intersects with the right alarm line61, the alarm system1calculates the TTC for the other vehicle95. When the TTC is equal to or less than a threshold, the alarm system1determines that the other vehicle95can cross the right alarm line61within a predetermined time, that is, the other vehicle95can collide with the vehicle2and raises an alarm to the driver of the vehicle2.

However, such an approach of the other vehicle95to the vehicle2is inevitable because of the structure of the intersection83and does not necessarily lead to collision of the other vehicle95with the vehicle2with high possibility. In particular, when the vehicle2decelerates to stop at the stop line83aas in this example, the possibility of the other vehicle95colliding with the vehicle2is very low. When the alarm system1raises an alarm to alert the driver to the other vehicle95in this case, the alarm may irritate the driver. For this reason, when the vehicle2is decelerating, the alarm system1performs an alarm reduction control to suppress the operation of the speaker41compared with when the vehicle2is not decelerating.

Next, with reference toFIG. 4, the alarm reduction control will be described.FIG. 4is a timing chart of alarms from the speaker41and alarms on the display42.FIG. 4shows a process in which the vehicle2having been stopped starts, decelerates to stop, and then starts again.FIG. 4also shows whether to activate or deactivate the speaker41and the display42for alarm when there is an object that can cross the right alarm line61or the left alarm line62(seeFIG. 2) within a predetermined time.

The vehicle2is stopped until a time t1. In other words, the velocity of the vehicle2is 0 km/h until the time t1. In this phase, the alarm line setting part51of the ECU5has set the right alarm line61and the left alarm line62. The collision determination part53performs determination of collision with respect to the right alarm line61or the left alarm line62. Even when there is an object that can cross the right alarm line61or the left alarm line62within the predetermined time, the alarm unit control part55does not activate the speaker41. That is, in this phase, the alarm unit control part55does not activate the speaker41but activates the display42.

At the time0, the vehicle2starts and accelerates. While the velocity of the vehicle2is equal to or less than 10 km/h, the right alarm line61and the left alarm line62remain set, and the collision determination part53performs determination of collision with respect to these lines. The value “10 km/h” mentioned here is an example of a predetermined velocity according to the present invention. In this phase, the alarm unit control part55can activate both the speaker41and the display42.

At a time t2, the velocity of the vehicle2becomes greater than 10 km/h, and the alarm line setting part51cancels the right alarm line61and the left alarm line62. As a result, the collision determination part53no longer performs determination of collision, and the alarm unit control part55no longer activates any of the speaker41and the display42.

The vehicle2then decelerates, and at a time t3, the velocity of the vehicle2becomes equal to or lower than 10 km/h. Then, the alarm line setting part51sets the right alarm line61and the left alarm line62. The collision determination part53performs determination of collision with respect to the right alarm line61or the left alarm line62. The alarm unit control part55can activate both the speaker41and the display42.

At a time t4, the deceleration of the vehicle2becomes equal to or greater than 5 m/s2. Then, the alarm unit control part55no longer activates the speaker41. That is, when there is an object that can cross the right alarm line61or the left alarm line62within the predetermined time, the alarm unit control part55does not activate the speaker41but activates the display42. The value “5 m/s2” is an example of a predetermined deceleration according to the present invention.

The control of the alarm unit control part55suppressing operation of the speaker41compared with when the vehicle2is not decelerating (the phase from the time t1to the time t2) is referred to as the “alarm reduction control”. After that, the vehicle2is stopped at a time t5, and while the velocity of the vehicle2is 0 km/h, the alarm unit control part55continues performing the alarm reduction control.

At a time t6, the vehicle2starts again and accelerates. After that, until a time t7at which the velocity of the vehicle2becomes greater than 10 km/h, the alarm unit control part55can activate both the speaker41and the display42, as in the phase from the time t1to the time t2described above.

Next, with reference toFIGS. 5 and 6, processes performed by the ECU5(seeFIG. 1) will be described.FIGS. 5 and 6are flowcharts showing processes performed by the ECU5. The processes are repeatedly performed at a predetermined period. In the following description, for ease of explanation, any processing that is performed by a functional block of the ECU5in a strict sense will be described as being performed by the ECU5.

First, in Step S10shown inFIG. 5, the ECU5determines whether or not the velocity v of the vehicle2falls within a range from 0 km/h to 10 km/h inclusive. The ECU5determines the velocity v of the vehicle2based on the signal received from the speed sensor34(seeFIG. 1). When it is determined that the velocity v falls within the range (YES in Step S10), the ECU5proceeds to Step S11.

In Step S11, the ECU5sets the right alarm line61and the left alarm line62(seeFIG. 2). After completing setting of the right alarm line61and the left alarm line62, the ECU5proceeds to Step S12.

In Step S12, the ECU5determines whether or not, in the detection region A1or the detection region A2(seeFIG. 2), there is an object (referred to simply as a “crossing object”, hereinafter) whose virtual line extending in the direction of traveling thereof intersects with the right alarm line61or the left alarm line62. The ECU5performs the determination based on the signal received from the side radar31(seeFIG. 1). When it is determined that there is a crossing object (YES in Step S12), the ECU5proceeds to Step S13.

In Step S13, the ECU5calculates the TTC for the crossing object. The ECU5calculates the distance between the crossing object and the right alarm line61or the left alarm line62and the velocity of the crossing object with respect to the alarm line (that is, the relative velocity) based on the signal received from the side radar. The ECU5further calculates the TTC according to the formula f1described above. When a plurality of crossing objects are detected, the ECU5calculates the TTC for each crossing object. After completing calculation of the TTC, the ECU5proceeds to Step S14.

In Step S14, the ECU5determines whether or not the TTC calculated in Step S13is equal to or less than 2 seconds. When it is determined that the TTC is equal to or less than 2 seconds (YES in Step S14), the ECU5proceeds to Step S15.

In Step S15, the ECU5determines whether or not the TTC calculated in Step S13is more than 1 second. When the TTC is more than 1 second, the necessity for alarm is relatively low. When it is determined that the TTC is more than 1 second (YES in Step S15), the ECU5proceeds to Step S16.

In Step S16, the ECU5activates the display42(seeFIG. 1). Specifically, the ECU5transmits a control signal to the display42, and in response to the control signal, the display42provides an indication that an object outside the vehicle2can collide with the vehicle2. After making the display42display the alarm, the ECU5ends the process. That is, when the TTC is equal to or less than 2 seconds (YES in Step S14) and more than 1 second (YES in Step S15), the necessity for alarm is relatively low, so that the ECU5does not activate the speaker41(seeFIG. 1) but activates the display42to raise an alarm to the driver of the vehicle2.

On the other hand, when the TTC is equal to or less than 1 second, the necessity for alarm is relatively high. When it is determined in Step S15that the TTC calculated in Step S13is not more than 1 second (NO in Step S15), the ECU5proceeds to Step S17.

In Step S17, the ECU5determines whether or not the velocity v of the vehicle2is 0 km/h. In other words, the ECU5determines whether or not the vehicle2is stopped. When the vehicle2is stopped, the necessity for alarm is relatively low compared with when the vehicle2is running. Therefore, when it is determined that the velocity v of the vehicle2is 0 km/h (YES in Step S17), the ECU5proceeds to Step S16. As described above, in Step S16, the ECU5makes the display42display an alarm and ends the process. That is, when the TTC is equal to or less than 1 second (NO in Step S15), and the vehicle2is stopped (YES in Step S17), the ECU5does not activate the speaker41but activates the display42to raise an alarm to the driver of the vehicle2.

On the other hand, when it is determined in Step S17that the velocity v of the vehicle2is not 0 km/h (NO in Step S17), or in other words, when the vehicle2is not stopped, the ECU5proceeds to Step S18.

When the vehicle2is not stopped, the necessity for alarm is higher than when the vehicle2is stopped. In Step S18, the ECU5determines whether or not the alarm from the speaker41is necessary. With reference toFIG. 6, a process of the ECU5determining the necessity will be described.

In Step S30shown inFIG. 6, the ECU5determines whether or not the vehicle2is located in the vicinity of an intersection. The ECU5performs the determination based on the signal received from the external camera32or the navigation device33(seeFIG. 1). When it is determined that the vehicle2is located in the vicinity of an intersection (YES in Step S30), the ECU5proceeds to Step S31.

In Step S31, the ECU5determines whether or not a traffic light in the direction of traveling of the vehicle2is emitting the stop signal. The ECU5performs the determination based on the signal received from the external camera32or the navigation device33(seeFIG. 1). When it is determined that the traffic light is emitting the stop signal (YES in Step S31), the ECU5proceeds to Step S32.

In Step S32, the ECU5determines whether or not the vehicle2is decelerating. The ECU5performs the determination based on the signal received from the speed sensor34or the acceleration/deceleration sensor35(seeFIG. 1). When it is determined that the vehicle2is decelerating (YES in Step S32), the ECU5proceeds to Step S33.

In Step S33, the ECU5determines whether or not the deceleration of the vehicle2is equal to or higher than 5 m/s2. The ECU5performs the determination based on the signal received from the acceleration/deceleration sensor35. When it is determined that the deceleration of the vehicle2is equal to or higher than 5 m/s2(YES in Step S33), the vehicle2is highly likely to stop. Then, the ECU5proceeds to Step S34. In Step S34, the ECU5determines that the alarm from the speaker41is unnecessary.

On the other hand, when it is determined in Step S32that the vehicle2is not decelerating (NO in Step S32), the ECU5proceeds to Step S35.

In Step S35, the ECU5determines whether or not the vehicle2is in a state where the vehicle2has started again after having been running at a velocity greater than 10 km/h and then decelerated and stopped. That is, the ECU5determines whether or not the vehicle2is in the state in the phase from the time t6to the time t7shown inFIG. 4. When it is determined that the vehicle2is in that state (YES in Step S35), the ECU5proceeds to Step S36. In Step S36, the ECU5determines that an alarm from the speaker41is necessary.

On the other hand, when it is determined in Step S35that the vehicle2is not in the state where the vehicle2has started again after having been running at a velocity greater than 10 km/h and then decelerated and stopped (NO in Step S35), the ECU5proceeds to Step S34, where the ECU5determines that an alarm from the speaker41is unnecessary.

On the other hand, when it is determined in Step S30that the vehicle2is not located in the vicinity of an intersection (NO in Step S30), when it is determined in Step S31that the traffic light in the direction of traveling of the vehicle2is not emitting the stop signal (NO in Step S31), or when it is determined in Step S33that the deceleration of the vehicle2is not equal to or higher than 5 m/s2(NO in Step S33), the ECU5proceeds to Step S36. In Step S36, the ECU5determines that the alarm from the speaker41is necessary.

Referring toFIG. 5again, the process performed by the ECU5will be described. When it is determined in Step S18that the alarm from the speaker41is unnecessary (YES in Step S18), the ECU5proceeds to Step S16.

In Step S16, the ECU5activates the display42. Specifically, the ECU5transmits a control signal to the display42, and in response to the control signal, the display42provides an indication that an object outside the vehicle2can collide with the vehicle2. After making the display42display the alarm, the ECU5ends the process. That is, even when the TTC is equal to or less than 1 second (NO in Step S15), when conditions (seeFIG. 6) such as that the vehicle2is decelerating in the vicinity of the intersection are satisfied, the ECU5does not activate the speaker41but activates the display42.

On the other hand, when it is determined in Step S18that the alarm from the speaker41is necessary (NO in Step S18), the ECU5proceeds to Step S19.

When the TTC is equal to or less than 1 second (NO in Step S15), and conditions (seeFIG. 6) such as that the vehicle2is decelerating in the vicinity of the intersection are not satisfied, the necessity for alarm is relatively high. Then, the ECU5activates the display42to display an alarm in Step S19, and activates the speaker41to produce an alarm or other sound in Step S20.

When it is determined in Step S10that the velocity v of the vehicle2does not fall within the predetermined range (NO in Step S10), when it is determined in Step S12that there is no crossing object (NO in Step S12), or when it is determined in Step S14that the TTC is not equal to or less than 2 seconds (NO in Step S14), the ECU5activates neither the display42nor the speaker41and ends the process.

According to this embodiment, when the vehicle2is decelerating, the alarm unit control part55performs the alarm reduction control to reduce operation of the alarm unit compared with when the vehicle2is not decelerating. As a result, more alarms can be raised to the driver of the vehicle2when the possibility of the object colliding with the vehicle2is relatively high, whereas alarms that can irritate the driver can be reduced when the possibility of the object colliding with the vehicle2is relatively low.

The alarm unit includes the speaker41, which is the first alarm device, and the display42, which is the second alarm device. The alarm unit control part55activates the speaker41and the display42when the vehicle2is not decelerating. When the vehicle2is decelerating, the alarm unit control part55does not activate the speaker41but activates the display42as the alarm reduction control.

With this arrangement, when the vehicle2is decelerating (that is, when the possibility of the object colliding with the vehicle2is relatively low), the alarm unit control part55does not activate the speaker41, which is the first alarm device, and thereby can reduce alarms that can irritate the driver.

The speaker41, which is the first alarm device, raises an audible alarm. The display42, which is the second alarm device, raises a visual alarm.

The audible alarm is more likely to irritate the driver than the visual alarm. With the arrangement described above, when the vehicle2is decelerating (that is, the possibility of the object colliding with the vehicle2is relatively low), the audible alarm is not raised, so that the driver is less likely to be irritated by alarms.

When the velocity of the vehicle2is equal to or less than 10 km/h, the alarm unit control part55activates the alarm unit. When the vehicle2starts after decelerating from a velocity greater than 10 km/h to stop, the alarm unit control part55does not perform the alarm reduction control.

When the vehicle2temporarily stops at the intersection and then starts to enter the intersection, the possibility of the vehicle colliding with another vehicle entering the intersection from another direction increases. In such a situation, the alarm reduction control is not performed, so that the driver can be alerted with reliability.

The alarm unit control part55performs the alarm reduction control when the deceleration of the vehicle2is equal to or higher than 5 m/s2.

With this arrangement, the alarm unit control part55can be prevented from performing the alarm reduction control in response to a slight decrease in velocity of the vehicle2and thereby failing to alert the driver.

The alarm system1includes the traffic signal detection part54that detects a traffic signal emitted by a traffic light in the direction of traveling of the vehicle2. The alarm unit control part55performs the alarm reduction control when the traffic light is emitting the stop signal.

With this arrangement, when the vehicle2is decelerating in response to the stop signal, the alarm reduction control can be performed with reliability.

According to this embodiment, as an example of the alarm reduction control, of the speaker41and the display42, operation of the speaker41is suppressed. However, the alarm reduction control according to the present invention is not limited to this implementation. For example, the alarm reduction control may include changing the volume or pattern of the audible alarm or the color or contents of the visual alarm to reduce irritation to the driver. Furthermore, the alarm unit may be a vibrator that makes the steering wheel or driver's seat in the vehicle vibrate to alert the driver by the vibration generated by the alarm unit. In that case, as the alarm reduction control, the strength or pattern of the vibration may be changed to reduce irritation to the driver, for example.

REFERENCE SIGNS LIST