WO1998019281A1

WO1998019281A1 - Detection of road boundaries

Info

Publication number: WO1998019281A1
Application number: PCT/AU1997/000720
Authority: WO
Inventors: Samuel N. Cubero, Jr.
Original assignee: Cubero Samuel N Jr
Priority date: 1996-10-28
Filing date: 1997-10-28
Publication date: 1998-05-07
Also published as: AUPO324996A0

Abstract

The proximity of a boundary of a roadway is detected optically by apparatus mounted to a vehicle (12). The apparatus includes an array of light sensitive elements (11) to receive light reflected from a transverse strip of the roadway. The output values of the light sensitive elements (11) are analysed by a microprocessor to detect substantial differences between neighbouring elements, indicative of the contrasting bright and dark portions on either side of the boundary, which may be a road edge or white line (13). The exposure time for the light sensitive elements is adjusted automatically, based on the minimum actual brightness level detected. The microprocessor is able to identify a white line by scanning the output values from the array of light sensitive elements to detect differences corresponding to a dark-to-bright transition and a bright-to-dark transition. If the distance between these transitions is within a predetermined range, the microprocessor identifies the feature as a line. A light shield (14) is mounted around the array of light sensitive elements, to minimise the effect of stray light.

Description

DETECTION OF ROAD BOUNDARIES THIS INVENTION relates to the automated detection of boundaries of a roadway. In particular, the invention is directed to a vehicle straying alarm which is responsive to proximity detection of a lane boundary.

BACKGROUND ART Each year, many people die or are seriously injured in road vehicle accidents caused by driver fatigue. Weary drivers are likely to lose concentration and veer off the road. Another major cause of traffic accidents is driver inattentiveness . Motorists often change road lanes without indicating or without checking for traffic. Careless crossing of lane boundary lines often leads to tragic consequences. Yet another cause of accidents is impairment of the driver's ability, e.g. by alcohol or drugs. An intoxicated driver may unwittingly veer off the road, or cross lanes into oncoming traffic.

Various methods and devices have been proposed for reducing accidents caused by driver fatigue, inattentiveness or intoxication. Known preventative measures include textured or contoured boundary lines which create audible sounds when traversed by a vehicle tyre; head tilt detectors worn by the motorist and adapted to generate a warning sound which is triggered by the lowering of the motorist's chin; a blinking detector which triggers a sound alarm when the driver's rate of eye blinking is below a threshold value; and vital sign monitoring systems which trigger an alarm if the driver's pulse rate, brain activity levels or breathing rate indicate sleepiness.

Although "audible" boundary lines are effective, they are considerably more expensive to lay than ordinary painted lane marker lines. Further, by the time the vehicle crosses the boundary line, it may be too late to take evasive action.

The other measures described above are complex and invasive, and are unpopular with motorists. In addition, although the systems may provide some preventative measure against weariness, they are largely ineffective against driver inattentiveness. It is also known to use automatically guided vehicles. Such vehicles have a guidance system which monitors lane boundaries and steers the vehicle within the lane. However, such automatically guided vehicles are expensive, and are more likely to lead to driver boredom and inattentiveness.

There are various known systems for detecting white lines on roadways, such as lane boundaries. Examples of such systems can be found in Japanese patent documents 8167023, 7141599, 7105473; and U.S. patents 4970653, 5301115, 5351044, 5517412, 5555312 and 5225827. Many of these known systems use a video camera mounted to a vehicle to scan the road ahead, and a computer to process the image data and detect lane boundary lines.

Generally, the prior art video techniques do not appear to be cost effective. One reason for the expense is that many of the known systems process the video input data in a very complex manner, e.g. using a Hough transform to analyse the image data. The cost of such systems renders them unsuitable for low cost application in popular cars.

As many of the known systems use complex and sophisticated processing of video images, they are unable to provide real time detection of boundary lines on a roadway . It is an object of this invention to provide apparatus for detecting road boundaries, which overcomes or ameliorates the abovedescribed disadvantages or which at least provides a useful choice.

It is another object of this invention to provide a vehicle straying alarm which uses the apparatus for detecting road boundaries. SUMMARY OF THE INVENTION In one broad form, the invention provides apparatus for automated detection of a roadway boundary, such as a road edge or a line, comprising a plurality of light sensitive elements adapted to be mounted to a vehicle to receive light reflected from respective successive portions of the roadway which extend transversely to the direction of travel of the vehicle, each light sensitive element providing an output, the value of which is dependent on the amount of light received by that element during an exposure period, and electronic control means for analysing the outputs of the light sensitive elements for the exposure period, said electronic control means being programmed to detect substantially different output values of neighbouring elements indicative of contrasting (bright and dark) portions on either side of a road boundary.

When the apparatus is used as a vehicle straying alarm, it includes alarm means, such as an audio alarm, which is actuated by the electronic control means when a road boundary is detected.

To optimise the dynamic range of the outputs of the light sensitive elements, the exposure period is preferably controlled by the electronic control means to ensure that the minimum output value from the light sensitive elements approximates a predetermined minimum value .

The electronic control means can be programmed to identify a particular boundary feature, such as a white line. In this embodiment, the electronic control means is programmed to calculate the maximum increase and decrease between output values of neighbouring elements for the period. Such increase and decrease correspond to the dark-to-bright and bright-to-dark transitions on either side of a white line on a dark roadway. The electronic control means also determines whether the distance between the increase and decrease is within a predetermined range indicative of the allowable width of the line.

The light sensitive elements may be arranged in two groups mounted under the vehicle, on opposite sides thereof. A light shield is preferably mounted under the vehicle, around each group of light sensitive elements. Each group of light sensitive elements may suitably comprise a CCD line array. A light source may also be incorporated within the light shield to illuminate the relevant portions of the roadway at night. The electronic control means is suitably a microprocessor or other programmable device.

In addition to detecting a boundary line, the apparatus may operate in "tracking" mode whereby the alarm is only sounded if the apparatus does not detect the line. In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings .

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a front elevational view of a vehicle to which a vehicle straying alarm is fitted,

Fig. 2 is a side elevational view of the vehicle of Fig. 1, Fig. 3 is a schematic view showing the use of a shield in conjunction with the light sensing means of Fig.

1,

Fig. 4 illustrates output levels of a light sensor array, Fig. 5 is a digital representation of the output levels of a light sensor array, and

Fig. 6 is a front perspective view of a vehicle having vehicle straying alarms according to other embodiments of the invention.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS

The road boundary detection apparatus of this invention comprises optical sensing means for receiving light reflected from one or more portions of the road adjacent a vehicle. The output of the optical sensing means, is fed to electronic control means which typically is a microprocessor or other computer device. The microprocessor is programmed to interpret the output of the optical sensing means to detect the presence of a lane boundary line and/or road edge within the field of view of the optical sensing means. By incorporating an alarm facility, the road boundary detection apparatus can function as a vehicle straying alarm. Thus, upon detecting a road boundary proximate the vehicle, the microprocessor triggers the alarm to notify the driver that the vehicle is straying too close to the boundary. The alarm signal should be sufficiently noticeable to wake up, or attract the attention of, the sleepy or inattentive driver, yet it should not be so disturbing that it creates a sudden reflex reaction which may cause the driver to steer the vehicle in a sudden dangerous manner. Preferably, the alarm means comprises a speaker or buzzer mounted inside the vehicle, and which generates an audio signal. Other forms of alarm signals may also be employed.

The electronic control means is preferably a microprocessor which is mounted at a suitable location in the vehicle, such as behind the dashboard.

The construction of the microprocessor and the alarm are known to persons skilled in the art, and need not be described in detail in this application. However, the operation of the microprocessor will be described hereafter, particularly with reference to its use in the operation of the vehicle straying alarm. Such operation can be achieved by suitable software .

In the embodiment illustrated in Figs. 1 and 2, the optical sensing means comprising dual light sensors 11 which are mounted under a vehicle 12 on opposite sides thereof, typically in front of the front wheels of the vehicle. Each light sensor 11 is aimed at the roadway and receives light reflected from an area of the road surface ("the field of view") extending transversely to the direction of travel of the vehicle. Typically, the field of view of each light sensor 11 is a strip extending transversely to the direction of travel. (However, the strip may extend obliquely to the direction of travel since an oblique strip still covers an area extending transversely to the direction of travel) . The area covered by each light sensor should preferably extend from outside the lateral boundary of the vehicle to just inside the inner tyre wall, as shown in Fig. 1.

Lane boundaries are normally marked by white or yellow lines painted on the darker road surface, which is typically bitumen or asphalt. Consequently, more light is reflected from the boundary line than the surrounding road surface. Similarly, the shoulder of a road (comprising gravel or grass) is normally more lightly coloured than the bitumen or asphalt road surface. Hence, more light is reflected from the road shoulder than the road, and the road edge marks a transition between "light" and "dark" areas .

Each light sensor 11 comprises a plurality of light sensitive elements. These elements may be discrete devices, such as photoresistors, solar cells or photodiodes, phototransistors, or any other suitable photosensitive elements. In the preferred embodiment, each light sensor comprises an integrated line array of charge coupled devices (CCD) , each CCD corresponding to one pixel in the output signal of the light sensor. Each CCD line scan is basically a large array of photosensitive elements which are sensitive to visible and near infrared light.

A suitable lens, such as a tubular lens, is placed at the appropriate focal distance in front of the CCD array to optimise collection of light from the road surface. If the CCD array is to be mounted close to the road, a 0.65 magnification or lower "wide angle" optical lens is recommended to enable each optical sensor to have a large field of view. In any event, each line scan camera should be such that the linear field of view covers the area of interest on the road surface . The width of the field of view for each light sensor should be no less than 30cm wide in order to reliably detect road lines (typically 8cm wide) and road edges.

Ideally, the only light detected by each light sensor should be the light reflected from the road surface in the field of view of that sensor. However, in practice, ambient light will be received by the light sensor. To reduce the effect of ambient light, a light shield 14 may be placed around each light sensor 11, as shown in Fig. 4.

The light shield 14 is typically of frusto- conical shape as shown in Fig. 4, but may alternatively be of a cylindrical shape. The light shield may be elongated in a direction transverse to the direction of travel of the vehicle, to correspond with the transverse strip monitored by the CCD array. The light shield 14 may be made from flexible rubber, plastic or any other suitable opaque material. As the light shield 14 is mounted beneath the vehicle 12, the light shield material should preferably be flexible to deflect around obstacles. The light shield should extend as close as possible to the roadway, but sufficient clearance should be left for everyday driving conditions.

A light 15 may be fitted in each shield to illuminate the field of view when driving at night. The light may be a halogen light source. At night time, the field of view should not be flooded with too much light, otherwise saturation may occur. A fixed size hole or aperture equivalent to fl6 or smaller is considered to be suitable, in conjunction with a dark tinted filter for restricting the intensity of light reaching the light sensitive elements. Alternatively, the lens may be made of tinted material. Dark tinted filters or lenses also help to smooth out the "spikes" and sharp irregularities which may appear in brightness data during particular sample periods. Some of the spikes may be large enough to trigger false alarms at low threshold settings if the general contrast detection method (described below) is used. A CCD line scan device collects packets of charge from each of the light sensitive elements in its pixel array, the amount of charge being dependent upon the light detected by that element. The amount of light detected by each element is dependent upon the amount of light reflected from the road surface at a respective portion of the field of view, as well as the period of time for which the element is exposed to the light from that surface. Hence, the output of the CCD elements is higher for light reflected from a (light) boundary line than it is for light reflected from the surrounding (dark) road surface. Similarly, the output of the CCD elements is higher for the (lighter coloured) road shoulder than the (darker) road surface itself.

The charge from each CCD element is transferred individually to the output of each CCD line scan array using a timing signal (clock pulses) from the microprocessor, and converted into a voltage which is proportional to that charge. The output voltages are converted to binary numbers using an analogue-to-digital converter. These binary numbers represent the brightness data for each element in the CCD line scan array, and are stored in RAM memory to be analysed by the microprocessor. In the preferred embodiment however, the data is analysed in real time by the microprocessor. The brightness data is based on the "grey scale" level. In the preferred embodiment, the brightness data is represented as an 8 bit number, indicating a brightness level of between 0 to 255 on a grey scale.

Fig. 4 illustrates the output of a CCD line array which has scanned an area of roadway including a bright region corresponding to a lane boundary line. The

CCD array comprises 64 photosensitive elements, the outputs of which are shown as 64 pixels numbered 0 to 63 in Fig. 4. The brightness of each pixel is represented as a number between 0 and 255.

In real situations, the brightest signal is less than the maximum 255 level, and the darkest signal is not as low as the 0 level. In the example illustrated in Fig. 4, the typical achievable range, or "static range" for the pixel outputs is between 35 and 230. (The 230 level corresponds to a bright "saturated" signal, and the 35 level corresponds a black surface which may not reflect any light, but some stray ambient light is still detected by the CCD element) . The static range should be kept as large as possible for fine resolution. A good static range should cover at least two thirds of the maximum number range. The static range is usually constant for a given circuit with fixed power supplies.

The "dynamic range" illustrated in Fig. 4 extends between the minimum and maximum brightness levels achieved in practice, (Bmin and Bmax, respectively) . Bmax typically corresponds to the light reflected from a white line. This is less than the maximum level of the static range since the white line is not 100% reflective. Similarly, Bmin typically corresponds to light reflected from a dark roadway. This is greater than the minimum level of the static range since the roadway is not pure black and does reflect some light.

A "sample period" is the time between successive data outputs for one photosensitive element in the CCD array, and is a measure of the charging or exposure time. The longer the sample period, the more charge is output by each CCD element and the higher the pixel value. In low light conditions, longer exposure times are required to avoid a reduction in data resolution, by keeping the "dynamic range" reasonably large. If the sample period is not long enough, even lightly coloured lines may appear as low brightness data values and may not be distinguishable from the data values representing the dark asphalt surface of the road. For bright light conditions, the sample period should be reduced to avoid saturation of the output signal from each photosensitive element.

By suitable timing signals, the microprocessor controls the sample period for reading brightness data from the CCD line arrays of the dual light sensors. The microprocessor thereby functions as an "electronic iris" or

"software aperture".

Although mechanical shutters or apertures could be used with the CCD arrays, such shutters are more expensive, slower and less reliable than software- implemented "aperture" control .

The dynamic range depends on ambient light levels, and may change during the day. The dynamic range should be kept as large as possible to optimise performance of the vehicle straying alarm. For example, on a bright day, the light reflected from a white line may cause the output signal to be saturated. The light reflected from the road surface may also be high due to the bright sunlight. Hence, the difference between Bmax and Bmin will be less, resulting in lower resolution. Similarly, in late afternoon or in low light conditions, the light reflective from a white line (Bmax) may be close to Bmin, resulting in a small dynamic range and poor resolution between light and dark coloured areas on the roadway .

The road boundary detection apparatus controls the minimum brightness level so that the dynamic range is not unduly small. Namely, a reference value for Bmin is set, e.g. level 80 in Fig. 4. The actual minimum brightness level during each sample period is then compared with the reference minimum value and the exposure time is adjusted accordingly. That is, if the actual minimum brightness level (Bmin) is greater than the reference minimum value, the exposure time for subsequent readings is reduced. Conversely, if the actual minimum brightness level is less than the reference value, the exposure time is increased. The exposure adjustment is repeated at each reading. This ensures that the dynamic range is kept at a desired range of levels within the static range. Consequently, the technique allows the CCD light sensors to provide large effective dynamic ranges, and high data resolution over a wide range of ambient lighting conditions so that road line and road edge detection can be performed more reliably.

The high speed of operation of the microprocessor software allows very short sample periods to be achieved, as low as 6 microseconds of charging time. Using this method, the frequency of "false alarms" caused by saturation (too much light) or total darkness (not enough light) is reduced. The detection of a road edge, or a road line, within the field of view of a light sensor 11 is performed by the microprocessor analysing the output data from the CCD array of that light sensor. A general contrast detection algorithm is the simplest method for detecting the presence of a road edge within the field of view of a light sensor. There are several different methods of detecting a contrast. In a basic method, a detection "flag" is set if a large enough difference exists between a high data value (bright zone) and a low data value (dark zone) within the brightness data obtained during one sample period. The "bright" data value and the "dark" data value can be determined as the maximum or minimum data values, respectively, found in the output data for the CCD array in one sample period. For example, the flag may be set if (Bmax-Bmin) ÷ Bmin>0.3, i.e. the difference between high and low values is greater than a threshold value equal to 30% of Bmin. Alternatively, the threshold value may be a fixed quantity. However, using a threshold difference in the form of a percentage of the "dark" data value makes the road boundary detection apparatus more sensitive to ambient light conditions as the brightness data values are compared on a relative basis, rather than in absolute numerical terms .

Although the general method described above is able to detect road edges by the light contrasts from the areas on either side of a road edge, it may give rise to an unduly high number of "false alarms" due to shadows on sunlit road surfaces. One way to avoid setting a detection flag by brief momentary appearances of shadows on roads, is to require the contrast to be present for a preset period of time (timed by the microprocessor) . Although this method reduces the incidence of false alarms, there is a slight penalty (of a few tenths of a second) in response time for the triggering of the alarm.

A particularly advantageous feature of the boundary detection apparatus is an ability to detect and identify a boundary line of a roadway. Referring to Fig. l,if the vehicle 12 veers to one side of a roadway, the white (or yellow) line 13 on that side will fall within the field of view of the nearest light sensor 11. The output of the CCD array will appear in the form shown generally in Fig. 5. That is, a set of values within the total array of values will be higher than the surrounding data values, representing the greater amount of light reflected from the boundary line than the surrounding dark road surface. The width of a painted road line may vary from country to country, but is generally constant within each country. In Australia, the typical width for a single boundary line is 8cm. If a wide angle camera lens is used in conjunction with the CCD array, this road line width corresponds to 25 pixels. To allow for variation in line widths, a range of acceptable widths is specified, say 20 to 30 pixels. The microprocessor analyses the output of the CCD array to detect the presence of any boundary line within the field of view of a light sensor, as it can be distinguished from other features which may give rise to high data values such as gutters, road shoulders, patches of bright sunlight, etc. In a simple "feature identification" algorithm, the brightness data values output from the CCD array are scanned to search for the largest "dark to bright" transition and the largest "bright to dark" transition between data values for two or more close (neighbouring) pixels. A road line is identified if these two transitions qualify as valid transitions, and the pixel distance between these two transitions is within the allowable range representative of a boundary line. A valid transition may be defined as one in which the difference in successive pixel values is greater than some predetermined value, or a predetermined percentage of the data value prior to, or after, the transition. A further criterion for valid identification, can be that the bright-to-dark transition is on the outside of the "dark-to-bright" transition.

To avoid a false identification of a road line as a result of a series of sharp "spikes" in the sequence of data values output from the CCD array, a further (optional) criterion for a valid identification may be based on average brightness value. Namely, the average brightness value of the pixels between the "dark-to-bright" and "bright-to-dark" transitions is compared with a brightness value representative of a white line. If this average value is higher than a specified threshold value, say, 0.75 (Bmax + Bmin), the identification of the white line can be considered to be valid.

The abovedescribed "feature identification" algorithm is only one example of the feature identification ability of the road boundary detection apparatus of this invention. Other algorithms can be devised to identify a boundary line, or other features such as double lines (non-overtaking lines) .

The abovedescribed road boundary detection apparatus is combined with an alarm facility to form a vehicle straying alarm. In use, if the vehicle 12 veers to one side of the road, and a road edge or boundary line comes within the field of view of the light sensor 11 on that side of the vehicle, the electronic control means will recognise the presence of the boundary line or the road edge from its analysis of the output data of the light sensor, and trigger the alarm to warn the motorist. The motorist is thereby able to take appropriate evasive or remedial action. Unintentional lane straying or road departure by a sleepy or inattentive driver usually occurs at a very slow rate. Hence, once the alarm is triggered, the driver should have ample time to correct the direction of the vehicle.

Once the alarm has been triggered, it can remain active until the road edge or boundary line is no longer within the field of view, or it may continue until a reset button is manually pressed by the driver. Alternatively, the alarm may be actuated for a preset time of, say, two seconds, each time a road edge or road line comes anew into the field of view of a light sensor. However, the preferred method is to sound the alarm whenever a road edge or road line is within the field of view of a light sensor, as this will avoid the annoying requirement of having to switch the alarm off manually, thereby enabling the driver to focus his/her attention solely on the task of correcting the deviation.

The alarm may be automatically disabled by the electronic control means if the vehicle's turn indicator has been actuated, thereby indicating that the driver is deliberately turning off the road or performing an overtaking manoeuvre. For motorists who have developed the dangerous habit of changing lanes or turning without first actuating the turn indicator, the vehicle straying alarm has the incidental benefit of serving as a reminder to the motorist to use the turn indicator. The microprocessor may analyse the "rate of change of drift" of the high pixel values within the array of values. If this rate of change is reasonably fast, the microprocessor algorithm assumes that the driver is in full control and is performing a deliberate lane change or overtaking maneuver. Otherwise, if the drift is slow, the alarm will be automatically triggered. The microprocessor may also disable the alarm if the light sensors on both sides of the vehicle detect a road line simultaneously, as this indicates that the vehicle is crossing a transverse line such as a pedestrian crossing, stop line or the like. The alarm may be used in both built-up areas and on highways. As the alarm has particular application on highways, country roads and other "high speed" areas, the microprocessor may be programmed not to actuate the alarm if the speed of the vehicle is less than say, 65kph. In this manner, the alarm would not operate when motoring in urban areas having a 60kph speed limit. To enable this automatic speed control operation, the microprocessor has an input connected to the vehicle ' s tachometer output .

The fields of view of the two light sensors are preferably kept close to the vehicle body so that false alarms are not triggered when the vehicle is travelling in a narrow lane and lane boundary markings are close to the vehicle. Positioning of the light sensors under the vehicle also reduces the likelihood of false alarms due to shadows from trees, signposts, or the like, and possibly even the vehicle's own shadow against a bright, sunlit road. Although the light sensors 11 have been shown mounted under the front of the vehicle, they may be mounted at any other suitable location on the vehicle, e.g. on opposite side view mirrors, or adjacent the headlights or turn lights at the front of the vehicle.

Furthermore, instead of using dual light sensors, a single light sensor with a wider field of view may be used. The single light sensor may be mounted at the top centre of the windscreen near the rear vision mirror or at the centre of the front bumper or grille of the vehicle, as shown in Fig. 6, to scan a transverse strip (marked "A") . The field of view is restricted to approximately the width of the vehicle so that boundary lines on the left and right sides of the road lane are not normally detected when the vehicle is "centred" with respect to the road lane. When the vehicle begins to stray out of its lane, a line or road edge will be detected in the field of view, and an alarm will be triggered. During nighttime driving, the road is illuminated by the vehicle's own headlights, and separate lighting for the vehicle straying alarm is not required.

Instead of using a one dimensional CCD array, a plurality of CCD arrays may be arranged to provide a two- dimensional scanning area.

In the abovedescribed operation of a vehicle straying alarm, the alarm is triggered if a feature, such as a road edge or boundary line comes within the field of view of a light sensor. In an alternative method of operation, the vehicle straying alarm can be used in a "tracking" mode. That is, the alarm is disabled whenever a white line appears in the field of view of one or both sensors. If however, the vehicle departs from a mid-lane position such that the white boundary line is no longer within the field of view of a light sensor, the alarm will be triggered. In its tracking mode, the vehicle may use a single light sensor having a wider two-dimensional field of view, as marked "B" in Fig. 6. By using this rectangular type of photosensor arrangement, the vehicle straying alarm can be made less susceptible to false alarms from stray light sources, such as white plastic bags, small road signs or numerals painted on the road, randomly shaped shadows from trees and passing vehicles, and light reflections from wet roads, since more information is available to the microprocessor. This arrangement of the light sensor also has the advantage of triggering the alarm if the vehicle is too close to another vehicle in front of it which is blocking the camera's vision of the road lanes.

The vehicle straying alarm may be installed in the vehicle at the time of its manufacture, or retrofitted by a user or technician.

The foregoing describes only some embodiments of the invention and modifications which are obvious to those skilled in the art may be made thereto without departing from the scope of the invention.

Claims

CLAIMS :

1. Apparatus for automated detection of a roadway boundary, such as a road edge or a line, comprising a plurality of light sensitive elements adapted to be mounted to a vehicle to receive light reflected from respective successive portions of the roadway which extend transversely to the direction of travel of the vehicle, each light sensitive element providing an output, the value of which is dependent on the amount of light received by that element during an exposure period, and electronic control means for analysing the outputs of the light sensitive elements for the exposure period, said electronic control means being programmed to detect substantially different output values of neighbouring elements indicative of contrasting (bright and dark) portions on either side of a road boundary.

2. Apparatus as claimed in claim 1, wherein the exposure period is controlled by the electronic control means, the electronic control means being programmed to adjust the exposure period such that the minimum output value from the light sensitive elements approximates a predetermined minimum value.

3. Apparatus as claimed in claim 1, wherein the roadway boundary is a light coloured line, the electronic control means being programmed to detect the presence of the line within the portions of the roadway by (i) determining the maximum increase and decrease in output values of neighbouring elements for the period, and (ii) ascertaining whether the distance between the maximum increase and decrease is within a predetermined range indicative of the allowable width of the line.

4. Apparatus as claimed in claim 1, further comprising alarm means actuated by the electronic control means upon detection of a road boundary by the electronic control means.

5. Apparatus as claimed in claim 4, wherein the alarm means comprises an audio alarm.

6. Apparatus as claimed in claim 4, wherein the light sensitive elements comprise a CCD line array.

7. Apparatus as claimed in claim 1, wherein the electronic control means determines whether the output values of neighbouring elements are substantially different by comparing the difference with a preset amount or a preset portion of the minimum, average or maximum value of the output values for the relevant exposure period.

8. Apparatus as claimed in claim 1, further comprising a lens mounted in front of the light sensitive elements to direct light reflected from the roadway onto the light sensitive elements.

9. Apparatus as claimed in claim 1, wherein the light sensitive elements are mounted under the vehicle.

10. Apparatus as claimed in claim 9, further comprising a light shield mounted to the vehicle around the light sensitive elements.

11. Apparatus as claimed in claim 1, further comprising a light source adapted to illuminate the portions of the roadway.

12. Apparatus as claimed in claim 4, wherein the light sensitive elements are arranged in two groups mounted on opposite sides of the vehicle.

13. Apparatus as claimed in claim 12, wherein the electronic control means is programmed not to actuate the alarm if a road boundary is detected by each group of light sensitive elements.

14. Apparatus as claimed in claim 4, wherein the electronic control means is connected to the output of a tachometer on the vehicle, and the electronic control means is programmed not to actuate the alarm unless the speed of the vehicle is above a predetermined speed.

15. Apparatus as claimed in claim 4, wherein the electronic control means is programmed not to actuate the alarm means unless a road boundary is detected for a predetermined number of successive exposure periods, or a predetermined period of time.

16. Apparatus as claimed in claim 6, wherein the electronic control means is programmed to detect shifting within the CCD array of the neighbouring elements having substantially different output values, over a plurality of exposure periods, and to disable the alarm means if the shift rate is greater than a predetermined rate.

17. Apparatus as claimed in claim 1, wherein the electronic control means is a microprocessor.

18. Apparatus as claimed in claim 1, wherein the light sensitive elements are arranged in a two-dimensional array.

19. A vehicle straying alarm comprising a plurality of light sensitive elements adapted to be mounted to a vehicle to receive light reflected from respective successive portions of the roadway which extend transversely to the direction of travel of the vehicle, each light sensitive element providing an output, the value of which is dependent on the amount of light received by that element during an exposure period, electronic control means for analysing the outputs of the light sensitive elements for the exposure period, such electronic control means being programmed to detect substantially different output values of neighbouring elements indicative of contrasting (bright and dark) portions on either side of a road boundary, an audio alarm actuated by the electronic control means upon detection of a road boundary by the electronic control means, and means for adjusting the exposure period such that the minimum output value from the lights sensitive elements approximates a predetermined minimum value.

20. Apparatus for automated detection of a lane boundary line of a roadway, comprising a plurality of light sensitive elements adapted to be mounted to a vehicle to receive light reflected from respective successive portions of the roadway which extend transversely to the direction of travel of the vehicle, each light sensitive element providing an output, the value of which is dependent on the amount of light received by that element during an exposure period, electronic control means programmed to detect the presence of the line within the portions of the roadway by (i) identifying a substantial increase and a substantial decrease between output values of neighbouring elements for the period, and (ii) ascertaining whether the distance between the increase and decrease is within a predetermined range indicative of the allowable width of the line, and an audio alarm actuated by the electronic control means upon detection of the line.