Title: PREDICTION OF POTENTIAL HUMAN INTENTION US-ING SUPERVISED COMPETITIVE LEARNING

Abstract: We propose a learning method to quantify human intention. Generally, a human being will imagine several potential actions for a given scene, but only one of these actions will subsequently be taken. This makes it difficult to quantify human intentions.
To solve this problem, we apply competitive learning to human behavior prediction as supervised learning. In our approach, competitive learning generates several outputs that are then associated with several potential situations imagined by a human. We applied the proposed method to human driving behavior and extracted three potential driving patterns. Results showed a squared error is reduced to 1/25 that of a conventional method . We also found that competitive learning can distinguish valid data from disturbance data in order to train a model.

1. INTRODUCTION:

Progress in advanced driving assistance systems (ADASs) has led to an autonomous driving function applicable for highway driving . The vehicle control logic of ADAS is typically developed by hand. There are a few related works on controlling vehicles comfortably , but the complex nature of vehicle environments prevents development by hand. Therefore, recent research has focused on the application of machine learning to autonomous driving systems. The convolutional neural network (CNN) shows particular promise as a core algorithm (Krizhevsky et al. (2012) ). C. Chen & Xiao (2015) estimates affordance for driving directly from a front camera image. They train a CNN to generate key perception indicators from images to easily control a vehicle. Similarly, Bojarski et al. (2016) predict the desired steering command directly from front camera images. They train a CNN with a human command as training data and drive in traffic on local roads with or without lane marking and on highways.

Our objective is to provide drivers with a comfortable trip featuring automatic vehicle control . The above methods provide a uniform control for various scenarios, but drivers have an assortment of preference behaviors, and individuals may drive in different ways even if the scenario or situation is the same. For example, on the highway, one person might drive hard to arrive at a destination quickly while another might drive relatively slowly to arrive at a destination safely. Therefore, we want to provide autonomous driving adapted to each individual to improve ease of driving and to generate control targets that imitate individual behavior. To imitate individual behavior, we predict future vehicle states resulting from an individualfs decision.

There are many studies on the imitation of human behavior within the context of driving. Ma & Andréasson (2006) proposed a vehicle interaction model that predicts future acceleration on the basis of current acceleration, velocity, relative velocity, and relative distance of the preceding vehicle. Moon & Choi (2011) proposed a method to predict human steering behavior. Their model considers path planning, feed-forward steering, and feedback steering. While these methods consider only acceleration or steering, Gindele et al. (2010) proposed a more complex model, the dynamic Bayesian network model, to estimate driver behavior and vehicle trajectory. This model considers vehicle model, trajectory, driver decision, and situation context. Wada et al. (2007) pro- posed a method to predict braking behavior. This method monitors the size of a preceding vehicle by means of front camera images to detect collision risk.

These methods provide one output for one scene. However, we assume that one output might represent just one part of a driverfs behavior. We assume a driver will imagine several potential situations for one scene and have several potential actions to take associated with each situation. This means the driver is making decisions all the time. Therefore, we want to extract several of the driver's candidate actions or intentions for the autonomous control of a vehicle while considering various potential situations.

In this paper, we propose a method to extract a driver's potential intention by utilizing competitive learning (Ahalt et al. (1990) , Shinozaki & Naruse (2013) , Osoba & Kosko (2013) ). Originally, competitive learning was used for unsupervised neural network algorithms and functioned as clustering. In such competitive learning, neural networks have several output layers and only one path that outputs the smallest loss to be trained . Consequently, neural networks predict the cluster with the most activated units . Ahalt et al. (1990) compare several competitive learning algorithms. In this paper, competitive learning algorithms is dealt as a kind of vector quantization algorithms. Shinozaki & Naruse (2013) utilize competitive learning for pre-training of deep neural networks and they also use competitive learning as unsupervised learning. Osoba & Kosko (2013) consider supervised competitive learning. However, they use also competitive learning as a kind of clustering methods. Therefore, in our best knowledge, competitive learning is dealt as a kind of unsupervised learning algorithms or clustering algorithms. We apply competitive learning to time series supervised learning, especially, regression task. And we train neural networks to output several patterns associated with a driver's potential intentions. In this paper, we describe how to model the driving behavior with competitive learning architecture . Additionally, we show the experimental results of tests performed with an actual vehicle.

2. DRIVER BEHAVIOR MODEL:

Here, we describe how to model driving behavior and how to implement the neural network architecture.

Our driving behavior model is a dynamic system that includes both a driver and a vehicle system. In this model, the driver observes the environment and then decides on a driving action, e.g., steering or pedal operation. The vehicle system then changes its state depending on the driving action and the state of the vehicle at a previous time. Finally, we observe the various vehicle states. For example, the environment observed by the driver is the same as that shown by the front camera and the vehicle states include travel speed, accelerations, gas pedal position, brake pedal position, engine speed, engine load, engine temperature, fuel level, fuel temperature, cooling water temperature, etc.

A graphical model of driving behavior is shown in Fig. 1 . Here, e stands for the environment observed by the driver, d stands for the driving action decided by the driver, s stands for vehicle states, To ensure comfortable driving, we estimate the driver's intention by predicting vehicle states in k step future . The relations between each variable are shown in the following equations.

EQUATION

Additionally, we compensate for latent variable s t by observable variable x t . The compensated s t are shown in Eq. 5:

s t = f s (s t-1 ; θ s ) + f d (, d t ; θ d ) + f -1 x (x t ; θ -1 x (5)

We design the neural network architecture in accordance with the driving behavior model shown in Fig. 1 . The designed architecture is shown in Fig. 2 . First, we select convolutional neural network (CNN) layers to approximate driving action f e , as the driver decides which action to take depending on the front view and CNN is the best layer to process images. The front camera images are resized to 256×256 with RGB channels. Second, we select a recurrent neural network (RNN) layer to approximate vehicle states f s , as our driver model is a dynamic system (Graves (2013) ).

Finally, we select a fully connected layer for other layers f d , f -1 x , f x+k because of its usability. This model predicts observed variables and we train the driver behavior model by loss function. We select squared error as loss function L(x t+k , xt+k ) and update parameters by backpropagation (Hecht-NielsenWerbos (1990)).

L(x t+k , xt+k ) = |x t+k -xt+k | 2 (6)

The CNN consists of three layers. In the first layer, kernel size is 11×11 with 64 channels and stride 4 . In the second layer, kernel size is 5×5 with 128 channels. In the third layer, kernel size is 3×3 with 128 channels. After all CNN layers, we apply batch normalization, ReLU, and max pooling (Ioffe & Szegedy (2015) ). The RNN layer has 1024 units. We use the ADAM algorithm for optimization (Kingma & Ba (2014) ).

The architecture shown in Fig. 2 is a baseline architecture and cannot extract potential driver intentions. In the next section, we introduce a competitive learning architecture that can extract potential intentions and discuss how to train it. 3 COMPETITIVE LEARNING FOR DRIVER BEHAVIOR MODEL

3.1. COMPETITIVE LEARNING ARCHITECTURE:

The competitive learning architecture is shown in Fig. 3 . It has N o output layers. Additionally, we set up N o RNN layers, as the driver's potential intentions are separated depending on the driving actions . Therefore, a separated RNN will be affected by separated intentions. The i-th RNN i and the output layer generate i-th prediction xi t+k at time t+k. We calculate i-th loss using Eq. 7 and decide the loss for backpropagation using Eq. 9. Finally, we update the parameters depending on this loss for backpropagation.

EQUATION

)

EQUATION

Backpropagation in the competitive learning architecture is shown in Fig. 4 . In the competitive learning architecture, we train only the computation path that output minimum loss. In Fig. 4 , the computation path to be trained is indicated by solid lines and the layer whose parameters are not updated is indicated by dashed lines. In this case, the i-th output layer has minimum loss and is trained. The other output layers (i.e., whose losses are bigger than i-th) are not trained and the losses are compensated as zero. Since each output layer is trained by different data, each prediction reflects different intentions. Therefore, competitive learning can extract several potential intentions. 

4. EXPERIMENTAL RESULTS:

In order to evaluate the proposed method, we record the front camera view using a smartphone and collect actual vehicle data using OBD2 (Birnbaum & Truglia (2000) , International ( 2003)). The data on vehicle states are whitened by removing means and scaled variances. Front camera images and vehicle data are resampled to 2 Hz. Six steps (equivalent to three seconds in the future) are predicted. We collect 80 minutes of data and use 40 minutes to train the neural networks. We set three competitive layers and train 300 iterations for pre-training and 2000 iterations for fine-tuning. We train neural networks with TITAN Z and use the Chainer framework (Tokui et al. (2015) ).

4.1. COMPARISON OF COMPETITIVE LEARNING ARCHITECTURE WITH BASELINE ARCHITECTURE:

Before we show the competitive learning results, we present the results of the baseline architecture shown in Fig. 6 . These data are predictions of vehicle speed over ten minutes. The left figure is the prediction on training data and the right one is on test data. Red line indicates the measured speed and blue line indicates the prediction by baseline architecture. In the training data, the baseline architecture could predict accurately in the low speed band under 30 km/h. However, the prediction error became bigger in the middle-high speed band over 30 km/h. In the higher speed band, the driverfs action had several variations even when the vehicle was operating in the same environment. This variation made prediction difficult. In the test data prediction, prediction error became bigger in not only the high speed band but also the very low speed band around 0 km/h . Additionally, the timing of the acceleration or deceleration shifted later, too. This is because the baseline architecture cannot extract potential driving intentions.

The competitive learning results are shown in Fig. 7 . This figure is written up the same way as Fig. 6 , except the blue line refers to integrated predictions by the competitive learning architecture. These predictions are a combination of three outputs derived by selecting minimum loss. In the training data prediction, the competitive learning architecture predicted a value quite close to the measured vehicle speed before three seconds . In the test data prediction, the competitive learning architecture could also predict the speed accurately. Additionally, the timing shift of acceleration or deceleration became smaller. The loss summation in both the training and test data is listed in Table 1 . Competitive learning architecture loss was about 1/25 smaller in the training data and about 1/3 smaller in the test data compared with the baseline architecture.

4.2. COMPETITIVE LEARNING PREDICTION ASSOCIATED WITH POTENTIAL INTENTIONS:

Here, we show the driving intentions extracted using the competitive learning architecture. The intentions extracted from the training data are shown in Fig. 8 . Four time series data are included: upper left is the integrated prediction, upper right is the first output layer's prediction (indicated by green line), lower left is the second output prediction (blue line), and lower right is the third output prediction (purple line). The red lines are the measured vehicle speed. The upper left figure depicts the winning output prediction, where the minimum error prediction is output for each time and the line color refers to the winning output layer . For example, the purple line in the upper left figure is the prediction by the third output layer . We can see that each output layer is affected by other driving intentions. The first output layer accurately predicted the stop timing associated with the stop intention, the second output layer accurately predicted the acceleration or high speed band associated with a rapid intention, and the third output layer accurately predicted the deceleration timing associated with a careful intention. We also show the results of the test data in Fig. 9 , which are written the same as the training data in Fig. 8 . Trained intentions are also valid in test data because the first output was accurate at stop time, the second output was accurate at acceleration or the high speed band, and the third output was accurate at the time of deceleration. Therefore, extracting these potential intentions enables accurate prediction. The competitive learning result without pre-training is shown in Fig. 10 . We can train "only one" output layer to predict driver intention even if we have three output layers. Potential intentions of the driver have similar trends. Therefore, we require the initialization of several of the same output layers . We can extract potential intentions by training each output layer using other data.

We show the summation loss in Table 2 . Surprisingly, competitive learning without pre-training significantly improved its losses by about 1/2 for training data and about 1/3 for test data. We consider the second and third output layers to play an important role, even though they cannot learn potential intentions. There is usually a lot of inadequate data to train, and these data disrupt the training of the model. However, in most cases we cannot distinguish useful data from disturbance data. In the competitive learning architecture, each output layer automatically selects data to train its computation path. This selection might distinguish useful data from disturbance data, and the first output layer can be trained using just the useful data, even though other output layers are trained using inadequate data. In this way, competitive learning can train the model such that it is robust against noisy data.

5. CONCLUSION:

We proposed supervised competitive learning to imitate a driver's potential intentions. Competitive learning was applied to supervised learning and the squared error was reduced to 1/25 that of a conventional method. We also demonstrated that competitive learning can distinguish valid data from disturbance data to train a model.

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Figures and Tables:

Figure 1: Graphical model of driving behavior.

Figure 2: Neural network architecture designed to imitate driving model.

Figure 3: Competitive learning architecture.

Figure 4: Backpropagation in competitive learning architecture.

Figure 5: Pre-training for competitive learning architecture.

Figure 6: Prediction of baseline architecture on training data (left) and test data (right).

Figure 7: Integrated prediction of competitive architecture on training data (left) and test data (right).

Figure 8: Driving intentions extracted by competitive learning architecture; training data.

Figure 9: Driving intentions extracted by competitive learning architecture; test data.

Figure 10: Competitive layer outputs without pre-training in training data.

Summation loss of each architecture

Table content: <table><tr><td>Architecture</td><td colspan="2">Loss (training) Loss (test)</td></tr><tr><td>Baseline</td><td>335.8</td><td>2603</td></tr><tr><td>Competitive learning</td><td>13.14</td><td>816.4</td></tr></table>

Summation loss without pre-training

Table content: <table><tr><td>Architecture</td><td colspan="2">Loss (training) Loss (test)</td></tr><tr><td>Baseline</td><td>335.8</td><td>2603</td></tr><tr><td>Competitive learning</td><td>13.14</td><td>816.4</td></tr><tr><td>Competitive learning without pre-training</td><td>164.0</td><td>870.9</td></tr><tr><td>4.3 EFFECT OF PRE-TRAINING</td><td/><td/></tr></table>

Peer review (iclr2017):
Authors propose a competitive learning architecture that learn different RNN predictors independently, akin to a committee of experts which are chosen with a hard switch at run-time. This work is applied to the task of predictive different driving behaviors from human drivers, and combines behaviors at test time, often switching behaviors within seconds. Prediction loss is lower than the similar but non-competitive architecture used as a baseline. It is not very clear how to interpret the results, what is the real impact of the model. If behaviors switch very often, can this really be seen as choosing the best driving mode for a given situation? Maybe the motivation needs to be rephrased a little to be more convincing? The competitive approach presented is interesting but not really novel, thus the impact of this paper for a conference such as ICLR may be limited.