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Vehicular Sensor Networks for Traffic Monitoring

In proceedings of 17th International Conference on

Computer Communications and Networks (ICCCN 2008)

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Outline Introduction

Motivation and Problem

Metric Definition

Traffic Status Estimation

Performance Evaluation

Future Work and Conclusion

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Introduction

Traffic monitoring in city urban area

Traditional approach: loop detector, camera,etc

infrastructure cost

maintenance cost

communication cost

not scalable

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Another way?

The existing vehicular sensor networks of taxi companies vehicle dispatching security purposes not special for traffic monitoring

Whether it can be used for traffic monitoring?

If “yes”,Advantage: Low infrastructure cost Low maintenance cost Cover the entire road network, scalable

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What we have…

Data basis and features: Long sampling interval due to communication cost

Sparse and incomplete information

Error, etc.

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Outline Introduction

Motivation and Problem

Metric Definition

Traffic Status Estimation

Performance Evaluation

Future Work and Conclusion

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Motivation

What sort of performance for traffic monitoring we might expect from such vehicular sensor networks providing sparse and incomplete information

Now in Shanghai, we utilize a test

bed with mobile sensors

installed in about 4000

taxis

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Problem

Whether we can demonstrate the feasibility of taxi-based sensor networks for traffic monitoring?

Whether the tradeoff between the accuracy of traffic status estimation and low communication cost can be well handled?

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Outline Introduction

Motivation and Problem

Metric Definition

Traffic Status Estimation

Performance Evaluation

Future Work and Conclusion

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Metric definition Three key characteristics in macroscopic

traffic-flow model:

flow rate

mean traffic speed

density

Public tends to consider more in terms of mean speed rather than flow rate or density in evaluating the quality of their trips

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Definitions of mean traffic speed freeway VS roads in urban area

Length: Time cost:

iL

t

link >> i

i

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Whole time cost ∆t to pass a link

=traveling time ∆t1+ intersection delay ∆t2

For a given link Li with length li, the mean traffic speed at time tk is defined as:

)(|)(|1

)(

ki tCcc

ki

iki

ttC

ltV

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Outline Introduction

Motivation and Problem

Metric Definition

Traffic Status Estimation

Performance Evaluation

Future Work and Conclusion

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A sample data from a sensor is defined by a 4-tuple D(SID, T, , ), and two consecutive data samples can construct a data pair.

A data pair from sensor s can be defined as:

p(s, t1, t2) = {s, t1, 1, t2, 2}

1 and 2 are the geographic coordinates from the consecutive data samples at t1 and t2, respectively

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The link-based algorithm (LBA)

LBA only aggregates data pairs of sensing data from link Li as well as links adjacent to either of intersection nodes of Li.

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The vehicle-based algorithm (VBA) VBA utilizes every available data pairs and

disseminates them back to all links traveled to estimate mean traffic speed.

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A vehicular mobile sensor system: Intelligent Traffic Information Service (ITIS)

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Outline Introduction

Motivation and Problem

Metric Definition

Traffic Status Estimation

Performance Evaluation

Future Work and Conclusion

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Performance Evaluation

Large-scale field testing on arterial and inferior roads

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The testing results showed VBA-based is better than LBA-based algorithms due to the data feature. More specially, the average error of VBA-Avg can be within only 17.3%

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Lessons Learned

Map-matching

Poor map-matching performance degrades the accuracy of traffic status estimation

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Traffic light

The mean speed of whole trip of 56 km is 21.1 km/h.

traffic light delays: 82 minutes

total time cost: 159 minutes

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Outline Introduction

Motivation and Problem

Metric Definition

Traffic Status Estimation

Performance Evaluation

Future Work and Conclusion

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Conclusion

A performance evaluation study has been carried out in Shanghai by utilizing the sensors installed on 4000 taxis for traffic monitoring

Two types of traffic status estimation algorithms, the link-based and the vehicle-based, are introduced based on such data basis.

The results from large-scale testing cases demonstrate the feasibility of such an application in most of cities

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thanks!