1
`
`On-Road Vehicle Detection Using
`Optical Sensors: A Review
`
`Zehang Sun1, George Bebis2 and Ronald Miller3
`1eTreppid Technologies, LLC, Reno, NV
`2Computer Vision Laboratory, University of Nevada, Reno, NV
`3Vehicle Design R & A Department, Ford Motor Company, Dearborn, MI
`(zehang,bebis)@cs.unr.edu, rmille47@ford.com
`
`Abstract—As one of the most promising applications of computer vision,
`vision-based vehicle detection for driver assistance has received consider-
`able attention over the last 15 years. There are at least three reasons for the
`blooming research in this field: first, the startling losses both in human lives
`and finance caused by vehicle accidents; second, the availability of feasible
`technologies accumulated within the last 30 years of computer vision re-
`search; and third, the exponential growth of processor speed has paved the
`way for running computation-intensive video-processing algorithms even
`on a low-end PC in realtime. This paper provides a critical survey of recent
`vision-based on-road vehicle detection systems appeared in the literature
`(i.e., the cameras are mounted on the vehicle rather than being static such
`as in traffic/driveway monitoring systems).
`
`I. INTRODUCTION
`
`Every minute, on average, at least one person dies in a vehicle
`crash. Auto accidents also injure at least 10 million people each
`year, and two or three million of them seriously. The hospital
`bill, damaged property, and other costs are expected to add up
`to 1%-3% of the world’s gross domestic product [1]. With the
`aim of reducing injury and accident severity, pre-crash sensing
`is becoming an area of active research among automotive man-
`ufacturers, suppliers and universities. Vehicle accident statistics
`disclose that the main threats a driver is facing are from other
`vehicles. Consequently, developing on-board automotive driver
`assistance systems aiming to alert a driver about driving environ-
`ments, and possible collision with other vehicles has attracted a
`lot of attention.
`In these systems, robust and reliable vehicle
`detection is the first step — a successful vehicle detection algo-
`rithm will pave the way for vehicle recognition, vehicle track-
`ing, and collision avoidance. This paper provides a survey of
`on-road vehicle detection systems using optical sensors. More
`general overviews on intelligent driver assistance systems can
`be found in [2].
`
`II. VISION-BASED INTELLIGENT VEHICLE RESEARCH
`WORLDWIDE
`
`With the ultimate goal of building autonomous vehicles,
`many government institutions have lunched various projects
`worldwide, involving a large number of research units work-
`ing cooperatively. These efforts have produced several proto-
`types and solutions, based on rather different approaches [2].
`In Europe, the PROMETHEUS program (Program for European
`Traffic with Highest Efficiency and Unprecedented Safety) pio-
`neered this exploration. More than 13 vehicle manufactures and
`several research institutes from 19 European countries were in-
`volved. Several prototype vehicles and systems (i.e., VaMoRs,
`VITA, VaMP, MOB-LAB, GOLD) were designed as a result of
`
`this project. Although the first research efforts on developing
`intelligent vehicles were seen in Japan in the 70’s, significant
`research activities were triggered in Europe in the late 80s and
`early 90s. MITI, Nissan and Fujitsu pioneered the research
`in this area by joining forces in the project “Personal Vehicle
`System” [3].
`In 1996, the Advanced Cruise-Assist Highway
`System Research Association (AHSRA) was established among
`automobile industries and a large number of research centers
`[2]. In the US, a great deal of initiatives have been launched
`to address this problem.
`In 1995, the US government estab-
`lished the National Automated Highway System Consortium
`(NAHSC) [4], and launched the Intelligent Vehicle Initiative
`(IVI) in 1997. Several promising prototype vehicles/systems
`have been investigated and demonstrated within the last 15 years
`[5].
`In March 2004, the whole world was stimulated by the
`“grand challenge” organized by DARPA [6]. In this competi-
`tion, 15 fully-autonomous vehicles attempted to independently
`navigate a 250-mile (400 km) desert course within a fixed time
`period, all with no human intervention whatsoever - no driver,
`no remote-control, just pure computer-processing and naviga-
`tion horsepower, competing for a $1 million cash prize. Al-
`though, even the best vehicle (i.e., “Red Team” from Carnegie
`Mellon) made only 7 miles, it is a very big step towards building
`autonomous vehicles in the future.
`
`III. ACTIVE VS. PASSIVE SENSORS
`
`The most common approach to vehicle detection is using
`active sensors such as lasers, lidar, or millimeter-wave radars.
`They are called active because they detect the distance of an
`object by measuring the travel time of a signal emitted by the
`sensors and reflected by the object. Their main advantage is
`that they can measure certain quantities (e.g., distance) directly
`requiring limited computing resources. Prototype vehicles em-
`ploying active sensors have shown promising results. However,
`active sensors have several drawbacks, such as low spatial reso-
`lution, and slow scanning speed. Moreover, when a large num-
`ber of vehicles are moving simultaneously in the same direction,
`interference among sensors of the same type poses a big prob-
`lem.
`Optical sensors, such as normal cameras, are usually referred
`to as passive sensors because they acquire data in a non-intrusive
`way. One advantage of passive sensors over active sensors is
`cost. With the introduction of inexpensive cameras, we can
`have both forward and rearward facing cameras on a vehicle, en-
`
`VALEO EX. 1008
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`abling a nearly 360o field of view. Optical sensors can be used
`to track more effectively cars entering a curve or moving from
`one side of the road to another. Also, visual information can be
`very important in a number of related applications, such as lane
`detection, traffic sign recognition, or object identification (e.g.,
`pedestrians, obstacles), without requiring any modifications to
`road infrastructures. On the other hand, vehicle detection based
`on optical sensors is very challenging due to huge within class
`variabilities. For example, vehicles may vary in shape, size, and
`color. Vehicle appearance depends on its pose and is affected by
`nearby objects. Illumination changes, complex outdoor environ-
`ments (e.g. illumination conditions), unpredictable interactions
`between traffic participants, and cluttered background are diffi-
`cult to control.
`To address some of the above issues, more powerful optical
`sensors are currently being investigated such as cameras oper-
`ating under low light (e.g., Ford proprietary low light camera
`[7]) or cameras operating in the non-visible spectrum (e.g., In-
`frared (IR) camera [8]). Building cameras with internal process-
`ing power (i.e., vision chip) has also attracted great attention.
`In conventional vision systems, data processing takes place at a
`host computer. Vision chips have many advantages over conven-
`tional vision systems, for instance high speed, small size, lower
`power consumption, etc. The main idea is integrating photo-
`detectors with processors on a very large scale integration [9].
`
`IV. THE TWO STEPS OF VEHICLE DETECTION
`In driver assistance applications, vehicle detection algorithms
`need to process the acquired images at real-time or close to real-
`time. Searching the whole image to locate potential vehicle
`locations is not realistic. The majority of methods reported in
`the literature follow two basic steps: (1) Hypothesis Generation
`(HG) where the locations of potential vehicles in an image are
`hypothesized, and (2) Hypothesis Verification (HV) where tests
`are performed to verify the presence of a vehicle in an image
`(see Fig. 1).
`
`Fig. 1. Illustration of the two-step vehicle detection strategy
`V. HYPOTHESIS GENERATION
`The objective of the HG step is to find candidate vehicle lo-
`cations in an image quickly for further exploration. HG ap-
`proaches can be classified into one of the following three cat-
`egories: (1) knowledge-based, (2) stereo vision based, and (3)
`motion-based.
`
`A. Knowledge-based methods
`Knowledge-based methods employ a-priori knowledge to hy-
`pothesize vehicle locations in an image. We review below some
`
`2
`
`approaches using information about symmetry, color, shadow,
`corners, horizontal/vertical edges, texture, and vehicle lights.
`
`A.1 Symmetry
`
`Vehicle images observed from rear or frontal view are in gen-
`eral symmetrical in horizontal and vertical directions. This ob-
`servation was used as a cue for vehicle detection in the early 90s
`[10]. An important issue that arises when computing symmetry
`from intensity, however, is the presence of homogeneous areas.
`In these areas, symmetry estimation is sensitive to noise. In [11],
`information about edges was included in the symmetry estima-
`tion to filter out homogeneous areas. When searching for local
`symmetry, two issues must be considered carefully. First, we
`need a rough indication of where a vehicle is probably present.
`Second, even when using both intensity and edge maps, symme-
`try as a cue is still prone to false detections, such as symmetrical
`background objects, or partly occluded vehicles.
`
`A.2 Color
`
`Although few existing systems use color information to its
`full extent for HG, it is a very useful cue for obstacle detection,
`lane/road following, etc. Several prototype systems investigated
`the use of color information as a cue to follow lanes/roads, or
`segment vehicles from background [12]. Similar methods could
`be used for HG, because non-road regions within a road area are
`potentially vehicles or obstacles. The lack of deploying color in-
`formation in HG is largely due to the difficulties of color-based
`object detection or recognition methods in outdoor settings. The
`color of an object depends on illumination, reflectance prop-
`erties of the object, viewing geometry, and sensor parameters.
`Consequently, the apparent color of an object can be quite dif-
`ferent during different times of the day, under different weather
`conditions, and under different poses.
`
`A.3 Shadow
`
`Using shadow information as a sign pattern for vehicle de-
`tection was initially discussed in [13]. By investigating im-
`age intensity, it was found that the area underneath a vehicle
`is distinctly darker than any other areas on an asphalt paved
`road. A first attempt to deploy this observation can be found
`in [14], though there was no systematic way to choose appro-
`priate threshold values. The intensity of the shadow depends on
`the illumination of the image, which in turn depends on weather
`conditions. Therefore the thresholds are not, by no means, fixed.
`In [15], a normal distribution was assumed for the intensity of
`the free driving space. The mean and variance of the distribution
`were estimated using Maximum Likelihood (ML). It should be
`noted that the assumption about the distribution of road pixels
`might not always hold when true. For example, rainy weather
`conditions or bad illumination conditions will make the color of
`road pixels dark, causing this method to fail.
`
`A.4 Corners
`
`Exploiting the fact that vehicles in general have a rectangular
`shape, Bertozzi et al. proposed a corner-based method to hy-
`pothesize vehicle locations [16]. Four templates, each of them
`corresponding to one of the four corners, were used to detect all
`the corners in an image, followed by a search method to find the
`
`

`

`matching corners. For example, a valid upper-left corner should
`have a matched lower-right corner.
`
`A.5 Vertical/horizontal edges
`Different views of a vehicle, especially rear views, contain
`many horizontal and vertical structures, such as rear-window,
`bumper etc. Using constellations of vertical and horizontal
`edges has shown to be a strong cue for hypothesizing vehicle
`presence. Matthews et al. [17] applied horizontal edge detec-
`tor on the image first, then the response in each column was
`summed to construct the profiles, and smoothed using a trian-
`gular filter. By finding the local maximum and minimum peaks,
`they claimed that they could find the horizontal position of a
`vehicle on the road. A shadow method, similar to that in [15],
`was used to find the bottom of the vehicle. Goerick et al. [18]
`proposed a method called Local Orientation Coding (LOC) to
`extract edge information. Handmann et al. [19] also used LOC,
`together with shadow information, for vehicle detection. Parodi
`et al. [20] proposed to extract the general structure of a traf-
`fic scene by first segmenting an image into four regions:
`the
`pavement, the sky, and two lateral regions using edge grouping.
`Groups of horizontal edges on the detected pavement were then
`considered for hypothesizing the presence of vehicles. Betke et
`al. [21] utilized edge information to detect distant cars. They
`proposed a coarse-to-fine search method looking for rectangu-
`lar objects through analyzing vertical and horizontal profiles. In
`[22], vertical and horizontal edges were extracted separately us-
`ing the Sobel operator. Then, a set of edge-based constraint fil-
`ters were applied on those edges to segment vehicles from back-
`ground. The edge-based constraint filters were derived from a
`prior knowledge about vehicles. Assuming that lanes have been
`successfully detected, Bucher et al. [23] hypothesized vehicle
`presence by scanning each lane starting from the bottom, trying
`to find the lowest strong horizontal edge.
`Utilizing horizontal and vertical edges as cues can be very ef-
`fective. However, an important issue to be addressed, especially
`in the case of on-line vehicle detection, is how the choice of
`various parameters affects system robustness. These parameters
`include the threshold values for the edge detectors, the thresh-
`old values for picking the most important vertical and horizontal
`edges, and the threshold values for choosing the best maxima
`(i.e., peaks) in the profile images. Although a set of parameter
`values might work perfectly well under some conditions, they
`might fail in other environments. The problem is even more se-
`vere for an on-road vehicle detection system since the dynamic
`range of the acquired images is much bigger than that of an in-
`door vision system. A multi-scale driven method was investi-
`gated in [7] to address this problem. Although it did not root out
`the parameter setting problem, it did alleviate it to some extend.
`
`A.6 Texture
`The presence of vehicles in an image cause local intensity
`changes. Due to general similarities among all vehicles, the in-
`tensity changes follow a certain pattern, referred to as texture in
`[24]. This texture information can be used as a cue to narrow
`down the search area for vehicle detection. Entropy was first
`used as a measure for texture detection. Another texture-based
`segmentation method suggested in [24] used co-occurrence ma-
`
`trices. The co-occurrence matrix contains estimates of the prob-
`abilities of co-occurrences of pixel pairs under predefined ge-
`ometrical and intensity constraints. Using texture for HG can
`introduce many false detections. For example, when we drive
`a car outdoor, especially in some downtown streets, the back-
`ground is very likely to contain textures.
`
`A.7 Vehicle lights
`
`Most of the cues discussed above are not helpful for night
`time vehicle detection — it would be difficult or impossible to
`detect shadows, horizontal/vertical edges, or corners in images
`obtained at night conditions. Vehicle lights represent a salient
`visual feature at night. Cucchiara et al. [25] used morphological
`analysis for detecting vehicle light pairs in a narrow inspection
`area.
`
`B. Stereo-vision based methods
`
`There are two types of methods using stereo information for
`vehicle detection. One uses disparity map, while the other
`uses an anti-perspective transformation (i.e., Inverse Perspective
`Mapping (IPM)).
`
`B.1 Disparity map
`
`The difference in the left and right images between corre-
`sponding pixels is called disparity. The disparities of all the im-
`age points form the so-called disparity-map. If the parameters
`of the stereo rig are known, the disparity map can be converted
`into a 3-D map of the viewed scene. Computing the disparity
`map, however, is very time consuming. Hancock [26] proposed
`a method employing the power of the disparity while avoiding
`some heavy computations. In [27], Franke et al. argued that, to
`solve the correspondence problem, area-based approaches were
`too computationally expensive, and disparity maps from feature-
`based methods were not dense enough. A local feature extractor
`“structure classification” was proposed to solve the correspon-
`dence problem easier.
`
`B.2 Inverse perspective mapping
`
`The term “Inverse Perspective Mapping” does not correspond
`to an actual inversion of perspective mapping [28], which is
`mathematically impossible. Rather, it denotes an inversion un-
`der the additional constraint that inversely mapped points should
`lie on the horizontal plane. Assuming a flat road, Zhao et al. [29]
`used stereo vision to predict the image seen by the right camera,
`given the left image, using IPM. Specifically, they used the IPM
`to transform every point in the left image to world coordinates,
`and re-projected them back onto the right image, which were
`then compared against the actual right image. In this way, they
`were able to find contours of objects above the ground plane.
`Instead of warping the right image onto the left image, Bertozzi
`et al.
`[30] computed the inverse perspective map of both the
`right and left images. Although only two cameras are required
`to find the range and elevated pixels in an image, there are sev-
`eral advantages to use more than two cameras [31]. Williamson
`et al. investigated a triocular system [32]. Due to the additional
`computational costs, binocular system is more preferred in the
`driver assistance system.
`
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`

`In general, stereo-vision based methods are accurate and ro-
`bust only if the stereo parameters have been estimated accu-
`rately, which is really hard to guarantee in the on-road scenario.
`Since the stereo rig is on a moving vehicle, vibrations from car
`motion can shift the cameras while the height of the cameras can
`keep changing due to the suspension. Suwa et al. [33] proposed
`a method to adjust the stereo parameters to compensate for the
`error caused by camera shifting. Broggi et al. [34] analyzed
`the parameter drifts and argued that vibrations affect mostly the
`extrinsic camera parameters and not the intrinsic ones. A fast
`self-calibration method was investigated in that study.
`
`C. Motion-based methods
`
`All the cues discussed so far use spatial features to distinguish
`between vehicles and background. Another important cue that
`can be used is the relative motion obtained via the calculation
`of optical flow. Optical flow information can provide strong
`information for HG. Approaching vehicles at an opposite di-
`rection produce a diverging flow, which can be quantitatively
`distinguished from the flow caused by the car ego-motion [35].
`On the other hand, departing or overtaking vehicles produce a
`converging flow. Giachetti et al. [35] developed first-order and
`second-order differential methods and applied them to a typi-
`cal image sequence taken from a moving vehicle along a flat
`and straight road. The results were discouraging. Three factors
`causing poor performance were summarized in [35]: (a) dis-
`placement between consecutive frames, (b) lack of textures, and
`(c) shocks and vibrations. Given the difficulties faced by mov-
`ing camera scenario, getting a reliable dense optical flow is not
`an easy task. Giachetti et al. [35] managed to re-map the corre-
`sponding points between two consecutive frames, by minimiz-
`ing a distance measure. Kruger et al. [36] estimated the optical
`flow from spatio-temporal derivatives of the grey value image
`using a local approach. They further clustered the estimated op-
`tical flow to eliminate outliers. In contrast to dense optical flow,
`“sparse optical flow” utilizes image features, such as corners
`[37], local minima and maxima [38], or “Color Blob” [39]. Al-
`though it can only produce a sparse flow, feature based method
`can provide sufficient information for HG. In contrast to pixel-
`based optical flow estimation methods where pixels are pro-
`cessed independently, feature based methods utilize high level
`information. Consequently, they are less sensitive to noise.
`In general, motion-based methods can detect objects based on
`relative motion information. Obviously, this is a major limita-
`tion, for example, this method can not be used to detect static
`obstacles, which can represent a big threat.
`
`VI. HYPOTHESIS VERIFICATION
`
`The input to the HV step is the set of hypothesized locations
`from the HG step. During HV, tests are performed to verify the
`correctness of a hypothesis. HV approaches can be classified
`into two main categories: (1) template-based methods and (2)
`appearance-based methods.
`
`A. Template-based methods
`
`Template-based methods use predefined patterns of the ve-
`hicle class and perform correlation between the image and the
`template. Some of the templates in the literature are very
`
`“loose”, while others very strict. Parodi et al. [20] proposed
`a hypothesis verification scheme based on license plate and rear
`windows detection using constraints based on vehicle geometry.
`Handmann et al. [19] proposed a template based on the obser-
`vation that the rear/frontal view of a vehicle has a “U” shape.
`During verification, they considered a vehicle to be present in
`the image if they could find the “U” shape (i.e., one horizontal
`edge, two vertical edges, and two corners connecting the hor-
`izontal and vertical edges).
`Ito et al.
`[40] used a very loose
`template to recognize vehicles. They hypothesized vehicle loca-
`tion using active sensors and verified those locations by check-
`ing whether pronounced vertical/horizontal edges and symmetry
`existed. Regensburger et al. [41] utilized a template similar to
`[40]. They argued that the visual appearance of an object de-
`pends on its distance from the camera. Consequently, they used
`two slightly different generic object (vehicle) models, one for
`nearby objects and the other for distant objects. A rather loose
`template was also used in [42], where the hypothesis was gen-
`erated on the basis of road position and perspective constraints.
`The template contained a priori knowledge about vehicles: “a
`vehicle is generally symmetric, characterized by a rectangular
`bounding box which satisfies specific aspect ratio constraints”.
`
`B. Appearance-based methods
`
`Appearance-based methods learn the characteristics of the ve-
`hicle class from a set of training images which capture the vari-
`ability in vehicle appearance. Usually, the variability of the non-
`vehicle class is also modelled to improve performance. First,
`each training image is represented by a set of local or global
`features. Then, the decision boundary between the vehicle and
`non-vehicle classes is learned either by training a classifier (e.g.,
`Neural Network (NN)) or by modelling the probability distri-
`bution of the features in each class (e.g., using the Bayes rule
`assuming Gaussian distributions).
`In [17], Principal Component Analysis (PCA) was used for
`feature extraction and Neural Networks (NNs) for classification.
`All the vehicle candidates were scaled to 20x20, then this 20x20
`scaled image was divided into 25 4x4 small windows. PCA was
`applied on every sub window and the output of the “local PCA”
`was provided to a NN to verify the hypothesis. Different from
`[17], Wu et al. [43] used standard PCA for feature extraction
`method for vehicle detection, together with a nearest-neighbor
`classifier. Goerick et al. [18] used a method called Local Orien-
`tation Coding (LOC) to extract edge information. The histogram
`of LOC within the area of interest was then provided to a NN for
`classification. Kalinke et al. [24] designed two models for vehi-
`cle detection: one for sedans, and the other for trucks. Hausdorrf
`distances between the hypothesized vehicles and the models in
`terms of LOC were the input to a NN. The outputs were sedans,
`trucks or background. Similar to [18], Handmann et al. [19]
`utilized the histogram of LOC, together with a NN, for vehicle
`detection. Moreover, the Hausdorrf distance was used for the
`classification of trucks and cars such as in [24]. A statistical
`model for vehicle detection was investigated by Schneiderman
`et al. [44]. A view-based approach using multiple detectors was
`employed to cope with viewpoint variations. The statistics of
`both object and “non-object” appearance were represented using
`the product of two histograms with each histogram represent-
`
`4
`
`

`

`ing the joint statistics of a subset Haar wavelet features in [44]
`and their position on the object. A different statistical model
`was investigated by Weber et al. [45]. They represented each
`vehicle image as a constellation of local features and used the
`Expectation-Maximization (EM) algorithm to learn the parame-
`ters of the probability distribution of the constellations. An over-
`completed dictionary of Haar wavelet features was utilized in
`[46] for vehicle detection. They argued that the over-completed
`representation provided a richer model and spatial resolution
`and was more suitable for capturing complex patterns. Sun et
`al. [47][7] went one step further by arguing that the actual val-
`ues of the wavelet coefficients are not very important for vehicle
`detection. In fact, coefficient magnitudes indicate local oriented
`intensity differences, information that could be very different
`even for the same vehicle under different lighting conditions.
`Following this observation, they proposed using quantized co-
`efficients to improve detection performance. Feature extraction
`using Gabor filters was investigated in [48]. Gabor filters pro-
`vide a mechanism for obtaining orientation and scale tunable
`edge and line detectors. Vehicles contain strong edges and lines
`at different orientation and scales, thus, this type of features are
`very effective for vehicle detection.
`
`VII. CHALLENGES AHEAD
`
`Although many efforts have been put into the vehicle detec-
`tion research area, many algorithms/systems have already been
`reported, many prototype vehicles have already been demon-
`strated, a highly robust and reliable system is yet to be built. In
`general, surrounding vehicles can be classified into three cate-
`gories according to their relative positions to the host vehicle:
`(a) overtaking vehicles, (b) mid-range/distant vehicles, and (c)
`close-by vehicles (see Fig. 2).
`
`Fig. 2. Detecting vehicles in different regions requires different methods. A1:
`Close by regions; A2: Overtaking regions; A3: Mid-range/distant regions.
`
`In the close-by regions, we may only see part of the vehi-
`cle. In this case, there is no free space in the captured images,
`which makes the shadow/edge based methods inappropriate. In
`the overtaking regions, only the side view of the vehicle is visi-
`ble while appearance changes fast. Methods detecting vehicles
`in these regions might be better to employ motion information
`or dramatic intensity changes [21]. Detecting vehicles in the
`mid-range/distant region is relatively easier since the full view
`of a vehicle is available and appearance is more stable.
`Real-time on-road vehicle detection is so challenging, that
`none of the HG methods discussed in Section V can solve it
`alone completely. Different cues/methods would be required to
`
`5
`
`handle different cases. We discuss below several research direc-
`tions for moving this area forward.
`
`A. Vehicle classification
`
`The majority of reported works aim only at detecting/tracking
`vehicles without differentiating among vehicle types. Given
`many different participants on the road (sedan, trail truck, mo-
`torbikes, etc.), knowing exactly what kind of participants are
`around the host vehicle will benefit driver assistance systems.
`
`B. Feature selection
`
`Building accurate and robust vehicle detection algorithms, es-
`pecially in the framework of supervised learning, requires em-
`ploying a good set of features. In most cases, a large number
`of features are extracted to compensate for the fact that rele-
`vant features are unknown a − priori.
`It would be ideal if
`we could use only those features which have great separability
`power while ignoring or paying less attention to the rest. For ex-
`ample, to allow a vehicle detector to generalize nicely, it would
`be nice to exclude features encoding fine details which might
`be present in particular vehicles only. Finding out what features
`to use for classification/recognition is referred to as feature se-
`lection. Sun et al.
`[49][50] have investigated various feature
`selection schemes in the context of vehicle detection, showing
`significant performance improvements. However, selecting an
`optimum feature subset (i.e., leading to high generalization per-
`formance) is still an open problem.
`
`C. Sensor fusion
`
`Information from a single sensor is not enough for a driver
`assistance system to manage high level driving tasks in dense
`traffic environments. Substantial research efforts are required to
`develop systems employing information from multiple sensors,
`both active and passive, effectively.
`
`D. Failure detection
`
`An on-board vision sensor will face adverse operating con-
`ditions, and it may reach a point where it might not be able to
`provide good quality data to meet minimum system performance
`requirements. In these cases, the driver assistance system may
`not be able to fulfil its desired responsibilities correctly (e.g.,
`issuing severe false alerts). A reliable driver assistance system
`should be able to evaluate its performance and disable its op-
`eration when it can not provide reliable traffic information any
`more.
`
`E. Hardware implementation
`
`Vehicle detection systems should be able to process informa-
`tion very fast to allow enough time for the drivers to react in
`case of an emergency. Among many options, real-time perfor-
`mance based on hardware implementations stand out for their
`simplicity and efficiency.
`
`VIII. CONCLUSIONS
`
`We presented a critical survey of vision-based on-road vehicle
`detection systems — one of the most important components of
`a driver assistance system. Judging from the research activities
`
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`

`underway worldwide, it is certain that this area will continue to
`be among the hottest research areas in the future. Major motor
`companies, government agencies, and universities, are all ex-
`pected to work together to make significant progress in this area
`over the next few years.
`
`Acknowledgements
`
`This work was supported by Ford Motor Company under
`grant No.2001332R, the University of Nevada, Reno under an
`Applied Research Initiative (ARI) grant, and in part by NSF un-
`der CRCD grant No.0088086.
`
`[3]
`
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