Patent Description:
Travel time reliability is one of the key indicators for the performance of transportation systems. It has been increasingly recognized as an important measure for assessing the operation efficiency of road facilities, evaluating alternative traffic management strategies and providing travelers with timely and reliable route guidance.

Travel time reliability is defined as the consistency or dependability in travel times, as measured from day to day and/or across different times of the day. The analysis of travel time reliability is as important, if not even more important, than the traditional analysis of average travel time. In order to assess travel time reliability, travel time distribution needs to be determined first. Travel time distribution is necessary for measuring the probability of arriving on time and finding a reliable path for risk-averse travelers.

Prior art document "<NPL>et al. )" discloses that path TTD estimation is the preliminary preparation for TTR analysis. In general, there are two approaches for estimating path TTD. One is to identify the best statistical model for fitting travel time observations, the other is to a model travel time observations for path TTD prediction.

Prior art document "Dependent Discrete Convolution Based Probabilistic Load Flow for the Active Distribution System" discloses the calculation of the joint probability density distribution by means of a dependent discrete convolution.

It is therefore an object of the present invention to provide a computer implemented method for calculating a travel time distribution of a vehicle path for a driver assistance function of the vehicle capable of calculating said travel time distribution accurately and in reasonable time, making it feasible for real world application.

The object is solved by the computer implemented method for calculating a travel time distribution of a vehicle path for a driver assistance function of the vehicle of claim <NUM>, the system for calculating a travel time distribution of a vehicle path for a driver assistance function of the vehicle of claim <NUM>, the computer program of claim <NUM> and the computer-readable data carrier of claim <NUM>. Further developments and advantageous embodiments are defined in the dependent claims.

The computer implemented method for calculating a travel time distribution of a vehicle path for a driver assistance function of the vehicle comprises receiving, in particular by a server via a wireless connection, travel time data of a plurality of other vehicles for a plurality of road segments along the selected path of the vehicle.

The method further comprises selecting travel time data of at least a neighboring first road segment and second road segment along the selected path of the vehicle. In addition, the method comprises discretizing travel time distributions for the neighboring at least first road segment and second road segment along the selected path of the vehicle.

Moreover, the method comprises calculating a dependence-factor of a dependency between the first road segment and the second road segment, wherein the dependence-factor incorporates a copula-based correlation between the first road segment and the second road segment, and performing a discrete convolution dependent upon the dependence-factor to calculate the travel time distribution of the vehicle path.

The system for calculating a travel time distribution of a vehicle path for a driver assistance function of the vehicle comprises means for receiving, in particular by a server via a wireless connection, travel time data of a plurality of other vehicles for a plurality of road segments along the selected path of the vehicle.

The system further comprises means for selecting travel time data of at least a neighboring first road segment and second road segment along the selected path of the vehicle and means for discretizing travel time distributions for the neighboring at least first road segment and second road segment along the selected path of the vehicle.

In addition, the system comprises means for calculating a dependence-factor of a dependency between the first road segment and second road segment, wherein the dependence-factor incorporates a copula-based correlation between the first road segment and the second road segment, and means for performing a discrete convolution dependent upon the dependence-factor to calculate the travel time distribution of the vehicle path.

In addition, the computer program comprises program code to perform the method according to the present invention when the computer program is executed on a computer.

Moreover, the computer-readable data carrier contains program code of a computer program for performing the method according to the present invention when the computer program is executed on a computer.

It is an idea of the present invention to use dependent discrete convolution for estimating a path travel time distribution from individual segment travel time distributions by using copulas to characterize the correlation between segments. The path travel time distribution estimated by the dependent discrete convolution is accurate and allows for the calculation of the path travel time distribution in a reasonable time thus making it feasible for real world application.

According to a further aspect of the invention, the dependence-factor of the dependency between the first road segment and the second road segment is calculated using a copula function and a rank-based correlation between the first road segment and the second road segment. This combination advantageously allows for a fast and accurate calculation of the path travel time distribution of the vehicle path.

According to a further aspect of the invention, the copula function is estimated by estimating segment travel time distributions from the received travel time data of the plurality of other vehicles for the plurality of road segments along the selected path of the vehicle. By using historical travel time data of other vehicles of a vehicle fleet sufficient data can advantageously be provided for the copula function to estimate segment travel time distributions.

In accordance with a further aspect of the invention, copula parameters of the copula function are estimated using a finite Gaussian Mixture Model. Said copula parameters are thus advantageously estimated using non-parametric estimators.

According to a further aspect of the invention, the rank-based correlation between the first road segment and the second road segment is calculated using rank-based correlation coefficient Kendall's tau. Said calculation is thus performed advantageously using scale-invariant measures of association which are more flexible in characterizing the dependence of different road segments.

According to a further aspect of the invention, the dependence factor is given by <MAT> wherein c is the copula density, Pα is an approximation of the probability density function of the first road segment and Pβ is an approximation of the probability density function of the second road segment.

According to a further aspect of the invention, a probability density function of the path travel time distribution of the vehicle is calculated from a joint probability density function with a cumulative distribution function and the probability density function of the first road segment and second road segment along the selected path of the vehicle. In doing so, the dependency and correlation of different segment travel time distributions can be effectively modeled. Furthermore, a conditional mean and variants of a particular segment travel time given the observation of the other can be estimated.

The here described features of the computer implemented method for calculating a travel time distribution of the vehicle path for the driver assistance function of the vehicle are also disclosed for the system for calculating the travel time distribution of the vehicle path for the driver assistance function of the vehicle and vice versa.

For a more complete understanding of the present invention and advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings. The invention is explained in more detail below using exemplary embodiments, which are specified in the schematic figures of the drawings, in which:.

Unless indicated otherwise, like reference numbers or signs to the figures indicate like elements.

<FIG> shows a flowchart of a computer implemented method for calculating a travel time distribution of a vehicle path for a driver assistance function of the vehicle according to an embodiment of the invention.

The method comprises the step of receiving S1, in particular by a server <NUM> via a wireless connection, travel time data of a plurality of other vehicles for a plurality of road segments along the selected path of the vehicle <NUM>.

The method further comprises the steps of selecting S2 travel time data of at least a neighboring first road segment α and second road segment β along the selected path of the vehicle <NUM> and discretizing S3 travel time distributions TTD for the neighboring at least first road segment α and second road segment β along the selected path of the vehicle <NUM>.

In addition, the method comprises the steps of calculating S4 a dependence-factor of a dependency between the first road segment α and the second road segment β, wherein the dependence-factor incorporates a copula-based correlation between the first road segment α and the second road segment β, and performing S5 a discrete convolution dependent upon the dependence-factor to calculate the travel time distribution TTD of the vehicle path <NUM>.

The dependence-factor of the dependency between the first road segment α and second road segment β is calculated using a copula function and a rank-based correlation between the first road segment α and the second road segment β.

The copula function is estimated by estimating segment travel time distributions TTD from the received travel time data of the plurality of other vehicles for the plurality of road segments along the selected path of the vehicle <NUM>. Copula parameters of the copula function are estimated using a finite Gaussian Mixture Model.

The rank-based correlation between the first road segment α and the second road segment β is calculated using rank-based correlation coefficient Kendall's tau. A probability density function of the path travel time distribution TTD of the vehicle <NUM> is calculated from a joint probability density function with a cumulative distribution function and the probability density function of the first road segment α and second road segment β along the selected path of the vehicle <NUM>.

<FIG> shows a system for calculating the travel time distribution of the vehicle path for a driver assistance function of the vehicle according to the embodiment of the invention.

The system comprises means <NUM> for receiving, in particular by a server <NUM> via a wireless connection, travel time data of a plurality of other vehicles for a plurality of road segments along the selected path of the vehicle <NUM>.

The system further comprises means <NUM> for selecting travel time data of at least a neighboring first road segment α and second road segment β along the selected path of the vehicle <NUM> and means <NUM> for discretizing travel time distributions TTD for the neighboring at least first road segment α and second road segment β along the selected path of the vehicle <NUM>.

The system moreover comprises means <NUM> for calculating a dependence-factor of a dependency between the first road segment α and second road segment β, wherein the dependence-factor incorporates a copula-based correlation between the first road segment α and the second road segment β, and means <NUM> for performing a discrete convolution dependent upon the dependence-factor to calculate the travel time distribution TTD of the vehicle path <NUM>.

<FIG> shows a graph of a discretization of the two-dimensional plane for dependent discrete convolution.

A road network can be represented as a directed graph G = (V,E) which is an ordered pair of a (finite) set of vertices V and a set of edges E, representing geolocations and road segments connecting these locations, respectively. An edge e ∈ E comprises a pair of two vertices v<NUM>,v<NUM> E V. Besides we have o,d ∈ V, representing the origin and the destination, respectively. The travel time for each segment is represented as a random variable x, which is derived from historical data. Thus, empirical segment TTD is discrete.

For characterizing segment TTD as continuous probability density function f(x) with distribution function F(x), both parametric and nonparametric estimators can be used. Examples for parametric estimators are Normal, Lognormal, Gamma, and Weibull, while Kernel Density Estimation and Gaussian Mixture Model are examples for nonparametric estimators.

A path from o to d is comprised of several successive segments. As historical data for entire paths are not available, path TTD is obtained by aggregating segment TTD, which is explained below.

Copulas are functions that relate multivariate distribution functions of random variables to their one-dimensional marginal distribution functions. According to Sklar's theorem, for an n-variate distribution function F(x<NUM>,. ,xn) with marginal distribution functions F<NUM>(x<NUM>),. ,Fn(xn), there exists a certain copula function C which meets the relationship <MAT>.

If marginal distributions are all continuous, C is unique. Based on Sklar's theorem the concept of copula provides an efficient way of modeling dependent variables. The joint distribution f(x<NUM>,. ,xn) of probability density functions f<NUM>(x<NUM>),. ,fn(xn) can be obtained by <MAT> with copula density <MAT>.

There are different measures of dependence. The most commonly used one for two random variables X, Y, is Pearson's linear correlation coefficient, defined by <MAT> where cov (X,Y) is the covariance of X and Y, and σ denotes the standard deviation of X and Y, respectively.

Pearson's linear correlation coefficient has the deficiency that it is not invariant under nonlinear strictly increasing transformations. Scale-invariant measures of association are more flexible in characterizing the dependence, such as rank-based correlation coefficient Kendall's tau. For two independent and identically distributed random vectors (X<NUM>,Y<NUM>) and (X<NUM>,Y<NUM>) Kendall's tau is defined as the probability of concordance minus the probability of discordance, <MAT>.

Kendall's tau produces values ranging from -<NUM> to <NUM>, with a positive correlation indicating that the ranks of both variables increase at the same time while a negative correlation indicates that as the rank of one variable increases, that of the other one decreases.

Archimedean copulas are very popular because they are easily derived and they are capable of capturing wide ranges of dependence. The definition of the Archimedean copula is based on the generator function ϕ. Archimedean copulas take the form <MAT> where ui := Fi(xi) for i ∈ {<NUM>,. The generator function ϕ is a continuous and strictly decreasing function which meets ϕ(<NUM>) = ∞, ϕ(∞) = <NUM>. The pseudo-inverse of ϕ is the function ϕ(-<NUM>), which also is a continuous and strictly decreasing function meeting ϕ(-<NUM>)(<NUM>) = ∞, ϕ(-<NUM>)(∞) = <NUM> and is defined as <MAT>.

If the generator function is definite, its corresponding copula function is also explicit. Hence, different generator functions result in different types of copulas. One frequently used archimedian copula is the Clayton copula, which has the generation function <MAT> where ϑ is the copula parameter, describing the correlation. Kendall's tau is related to ϑ by <MAT>.

The bivariate Clayton copula density for two marginals u<NUM> and u<NUM> is <MAT>.

A copula model for estimating path TTD is estimated in two steps. First segment TTDs are estimated from empirical GPS data. Next, the copula parameters are estimated. Segment TTDs need to be characterized using continuous marginal distributions.

A finite Gaussian Mixture Model (GMM) with three components can be used as it showed an accurate fit. The finite GMM with k components is represented as <MAT> where µj are the means, sj are the inverse variances, πj are the mixture weights satisfying <MAT>, and N is a normalized Gaussian with specified mean and variance.

The parameters of the GMM are obtained by the Expectation-Maximization algorithm. For the second stage of the estimation process, the copula parameters are estimated using: <MAT>.

As there is no analytical method for obtaining the distribution of the sum of travel times using the copula model, samples need be generated from the copula model to estimate path TTD.

As path TTD estimation using convolution has a strong precondition that the segment TTDs being convolved must be independent, we propose a formulation of convolution for dependent segment TTDs. DDC is illustrated for calculating path TTD comprised of two segment TTDs, where travel time for segment <NUM> is denoted as a random variable α, and travel time for segment <NUM> is denoted as β. The PDF for path TTD denoted as fα+β can be calculated from the joint PDF fα,β with CDFs Fα and Fβ, and PDFs fα and fβ, respectively: <MAT> where [α,α] is the domain of α and [β,β] is the domain of <NUM>. If α and β are independent, fα,β(x,y) can be obtained by the product of its marginal PDFs: <MAT> and it becomes <MAT> which is the convolution equation. For computerized calculation, the discrete form of convolution is used: <MAT> where Pα(i), Pβ(i) and Pα+β(i) are the discrete approximations of fα(·), fβ(·) and fα+β(α). For that purpose, an N-dimensional space is discretized with a step of Δp, as shown in <FIG>. For each dimension Δp may differ, however, for simplicity reasons the same Δp was chosen for every dimension. Then the discrete approximation of fα(·) is defined as follows: <MAT>.

The discrete approximation Pβ(i) of fβ(·) is defined analogously.

Now we consider the case, where α and β are dependent.

Then <MAT> does not hold. However, based on Sklar's theorem we can use <MAT> and obtain <MAT>.

Using <MAT> and <MAT> the dependent convolution becomes <MAT>.

If Δp is small enough, c(Fα(x),Fβ(y)) can be approximated as a constant over the <NUM>-dimensional interval of [jΔp - Δp/<NUM>,jΔp + Δp/<NUM>] × [(i - j)Δp - Δp/<NUM>,(i - j)Δp + Δp/<NUM>] and is denoted as cjΔp,(i-j)Δp.

Compared to the traditional convolution the DDC has similar form as <MAT> except that there is a an extra multiplier cjΔp,(i-j)Δp determined by the dependency between α and β.

When α and β are independent, cjΔp,(i-j)Δp := <NUM> and equation <MAT> degenerates into the traditional convolution in equation <MAT>.

In case of dependent segments, this factor incorporates the copula based correlation between the segments. Compared to the copula framework this approach allows for a simplified and fast computation of the TTD.

<FIG> shows a schematic illustration of a freeway arterial on which the vehicle is traveling according to the embodiment of the invention.

Said freeway arterial comprises three lanes in each direction, an on-ramp and an off-ramp as well as road segments α, β and γ. The vehicle <NUM> is shown to travel along the vehicle path <NUM> leading through segments α, β and γ.

The travel time data is collected from probe vehicles. The setup includes a fleet of probe vehicles, which has a module that reports GPS data and a central server, which collects all data in a database. Each vehicle samples the current GPS positions in intervals of <NUM> to <NUM>, which are stored in in the local memory of the vehicle together with the according timestamp. After sampling a few positions, a filter mechanism decides whether sampled GPS positions are transmitted to the central server.

This filter continuously compares the velocity of the vehicle with the velocity given by a traffic provider. If the velocity at one of the sampled data points deviates more than <NUM>% to <NUM>% (depending on the software version on the module) from the provided velocity, the recently samples positions and according timestamps are transmitted to the central server.

Each transmitted position is linked to an alias, which is randomly generated by the vehicle and changes over time due to protection of driver's privacy. At the server, single transmitted positions of the same alias can be connected in order to reconstruct vehicle trajectories.

The collected raw data is then matched to the segments of the road network. Velocities are derived from the difference of time and location, respectively, between two GPS points. Travel times are then obtained using the velocity and the length of the segment.

<FIG> and <FIG> show a freeway segment travel time STT distribution and Gaussian Mixture Model for segment α, β and γ respectively according to the embodiment of the invention.

In order to understand the correlation between segments we investigate the individual segment TTDs. For the DDC no estimation of marginal distribution is necessary, as it can directly work with discrete histogram data. For the copula model marginal distribution needs to be estimated. Freeway travel time are comprised of four states. The first state is the free flow state, where median travel times are low and the spreads of the distribution is small. The distribution is approximately symmetric.

The second state is the congestion onset, where median travel times are still low. The third phase is congested traffic, where median travel times are high, while the distribution is wide. In these periods congestion can be expected in different degrees of severity, resulting in a wide range of possible travel times.

The fourth phase is the congestion dissolve, where median travel times are low, but the distribution is skewed to the left, resembling that in most cases congestion has dissolved but in a decreasing number of cases still congestion occurred. <FIG>, and <FIG> show segment TTD for Segment <NUM>, Segment <NUM>, and Segment <NUM> of the freeway arterial, respectively.

For estimation of marginal distribution, a GMM with five components was found as best fitting. The parameters of the GMM can be found in table <NUM>. The five components can be interpreted as follows The first GMM component resembles the free flow phase. The second component reflects the phase of congestion onset, as well as the congestion dissolve. The third and fourth components reflect the congested traffic with different levels of congestion, respectively. The fifth component denotes the range of extremely heavy congestion with a large standard deviation resembling the tail of the distribution which happens rarely, thus, the very small mixture weight.

<FIG> shows a scatter diagram for freeway segment travel time distributions of segment α and segment β according to the embodiment of the invention and <FIG> shows a scatter diagram for freeway segment travel time distributions of segment β and segment γ according to the embodiment of the invention. This shows segment correlation.

In addition, Kendall's tau <MAT> as a correlation measure is given for each segment pair, with a values of <NUM> and <NUM> for the first and second segment pair, respectively. We can observe lower tail dependence for each scatter diagram. Thus, segment correlation exists and needs to be taken into account when aggregating successive segment TTDs.

A three-dimensional Clayton Copula with copula parameter <NUM> showed the best fit. Therefore, it was used for path TTD estimation and comparison with the DDC.

<FIG> shows various probability density functions for the freeway arterial according to the embodiment of the invention and <FIG> shows a cumulative density function for the freeway travel path according to the embodiment of the invention.

For estimating path TTD we compare the performance of the proposed DDC methodology with the copula model, convolution, and the empirical distribution. The number of samples generated from the Copula Model was set to the same number as original data samples. For the DDC a discretization step of one second was found suitable, and, thus, is chosen.

<FIG> and <FIG> show PDF and CDF for the travel time of a path comprised of the three freeway segments estimated by the various models and the empirical data, respectively. Kolmogorov-Smirnov and Cramer-von-Mises statistics can be found in table <NUM>. A lower value for the KS and CVM statistic, respectively, indicates a better fit.

As one could expect, the shape of path TTD is analogous to the ones of segment TTDs with the same states, which were defined above. The convolution treats the successive segments as independent. Path TTD obtained by the convolution does not resemble any states. Instead we obtain a unimodal distribution. The DDC and the Copula Model account for segment correlation. Path TTD obtained by DDC and Copula Model show an accurate resemblance of the TTD states. While there is only a small underestimation for the free flow peak, and a small overestimation for the state denoting both congestion onset and congestion dissolve, path TTD obtained the two models accounting for segment correlation fit the empirical distribution accurately. The Copula Model performs slightly better than the DDC.

In order to test the eligibility for real world applications, we estimate path TTD for an increasing number of segments and compare computational times in the next section. Compared to the uninterrupted travel flow on freeway facilities, the interrupted nature of travel flows on urban arterials make TTD estimation much more challenging. For that reason we use a data set of an urban arterial with several road features resulting in an interrupted travel flow, in order to investigate the potential for real world application of the proposed DDC methodology and the Copula Model.

<FIG> shows a schematic illustration of an urban arterial comprising a travel path of a vehicle in accordance with the embodiment of the invention.

The urban arterial comprises a plurality of roads including a main road comprising road segments <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>. In addition, a plurality of streets <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> meet the main road at right angles at either intersections <NUM>, <NUM> or between respective intersections. In addition, said urban arterial comprises a number of traffic lights <NUM>, <NUM>, <NUM>, <NUM>. The vehicle <NUM> is shown traveling along travel path <NUM> on the main road of the urban arterial.

The calculation of the travel time distribution TTD for the urban arterial is analogous to the afore-mentioned calculation of the travel time distribution of the freeway arterial. The individual segments <NUM> to <NUM> are analogous to the segments α, β and γ of the freeway arterial. Thus, the method for calculating a travel time distribution TTD of a vehicle path <NUM> for the driver assistance function of the vehicle in the context of the urban arterial can thus be performed in the same way as described in the context of the freeway arterial.

The parameters of the GGM are listed in table <NUM>. The first component of GMM denotes the situation when vehicle travel with free flow speed. The second component implies partially delayed vehicles, while the third component denotes the congested situation. For the longest segment, Segment <NUM>, with a length of <NUM>, three peaks can be observed. For the segments with a shorter length, the observable distinction is not that clear, however GMM still shows an accurate fit.

In addition, Kendall's tau for each segment pair is given ranging from values between <NUM> and <NUM>, showing that segment correlation exists. However, as there are several factors affecting the travel time of each segment and the correlation in between segments, such as signal control, parking lane, turning lane, congestion, and weather, the correlation structure is complex and differs for each segment pair. The values of individual travel times for each segment scatter in a wide range of the joint distribution space.

Only segments <NUM>, <NUM> and <NUM>, which are relatively short and do not have signal control nor turning lanes show a strong lower tail dependence. This is also resembled by a high value for Kendall's tau, with <NUM>, <NUM>, and <NUM>, respectively. The line through the origin is caused by the estimation of the velocities of probe vehicle described above. If one GPS point is located before Segment i and the next GPS point is located behind Segment j, the velocity for both segments is equal. Therefore, travel time of Segment i and Segment j are proportional, causing the line through origin, where the corresponding samples lie.

For estimating path TTD we compare the performance of the proposed DDC methodology with the copula model, convolution, and the empirical distribution. For the copula model, the two-stage estimation procedure described above was used. The number of samples generated from the Copula Model was set to the same number as original data samples. For the DDC a discretization step of one second was used.

A lower value for the KS and CVM statistic, respectively, indicates a better fit. both DDC and Copula Model perform better than the convolution due to their ability to incorporate segment correlation. DDC and Copula Model model the tails as well as the bulk of the distribution more accurately than the convolution. Goodness of fit tests show that the Copula Model estimates path TTD more accurately than the DDC, while the path TTD estimate of the DDC is more accurate than convolution by far.

However, when comparing computing time, the power of the DDC is unveiled. For application in route guidance systems, a path comprised of <NUM> segments is realistic. While the DDC takes approximately <NUM> seconds, which is feasible for applications, the Copula Model takes approximately <NUM> seconds, which is unfeasible for real world applications.

Claim 1:
Computer implemented method for calculating a travel time distribution (TTD) of a vehicle path (<NUM>) for a driver assistance function of the vehicle (<NUM>), comprising the steps of:
Receiving (S1) by a server (<NUM>) via a wireless connection, travel time data of a plurality of other vehicles for a plurality of road segments along the selected path of the vehicle (<NUM>), said travel time data comprising a current GPS position of each vehicle and a timestamp, wherein the travel time data is based on a filter continuously comparing a velocity of each vehicle with the velocity given by a traffic provider, wherein if the velocity at one of sampled data points deviates more than <NUM>% to <NUM>% from the provided velocity, recently sampled positions and according timestamps are transmitted to the central server (<NUM>);
selecting (S2) travel time data of at least a neighboring first road segment (α) and second road segment (β) along the selected path of the vehicle (<NUM>);
discretizing (S3) travel time distributions (TTD) for the neighboring at least first road segment (α) and second road segment (β) along the selected path of the vehicle (<NUM>);
calculating (S4) a dependence-factor of a dependency between the first road segment (α) and the second road segment (β), wherein the dependence-factor incorporates a copula-based correlation between the first road segment (α) and the second road segment (β), wherein the dependence-factor of the dependency between the first road segment (α) and second road segment (β) is calculated using a copula function and a rank-based correlation between the first road segment (α) and the second road segment (β); and
performing (S5) a discrete convolution dependent upon the dependence-factor to calculate the travel time distribution (TTD) of the vehicle path (<NUM>), wherein a probability density function of the path travel time distribution (TTD) of the vehicle (<NUM>) is calculated from a joint probability density function with a cumulative distribution function and the probability density function of the first road segment (α) and second road segment (β) along the selected path of the vehicle (<NUM>).