Mobile Century

The Mobile Millennium team is pleased to announce that data from the Mobile Century field test IS NOW available or our colleagues to download and study. Potential downloaders will be authenticated as having a genuine research agenda.

In addition to the cell phone data collected during the experiment, the download includes data extracted from video recordings of the experiment, and data from in-road sensors (loop detectors) that the project used to validate the cell-phone-generated traffic information.

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Traffic monitoring is most commonly accomplished with government-deployed, dedicated equipment.  Adopting new technology in this paradigm can be costly and slow.  However, recent advances in the mobile internet, cell phone technology, and location-based services may be leveraged to transcend the old paradigm.  Doing so will reduce costs, increase coverage and yield a wealth of new data that will empower the traveling public with real-time access to current traffic conditions.  Furthermore, transportation operators will gain access to an unprecedented wealth of information to help them better manage road networks.

Nonetheless, significant technical barriers and privacy concerns may impede widespread acceptance of a new paradigm.  To understand and overcome these barriers, the Mobile Century experiment was conceived as a proof-of-concept demonstration of a traffic monitoring system based on probe vehicles equipped with GPS-enabled mobile phones.

On February 8, 2008, CCIT, Caltrans, Nokia, and UC Berkeley's Department of Civil and Environmental Engineering collaborated to conduct an unprecedented experiment in the area of traffic monitoring. Mobile Century was intended as a proof of concept. The event was enormously successful, both technically and logistically.

The goal of this controlled field experiment was to test traffic data collection from GPS-equipped cell phones driving on a stretch of a highway located in the San Francisco Bay Area. One hundred vehicles carrying the GPS-enabled Nokia N95 drove along a 10-mile stretch of I-880 from 9:30am to 6:30pm.

The principal objectives for this experiment were to feature

  • online, real-time data processing;
  • privacy-preservation;
  • data efficiency, i.e. not requiring excessive cellular network load.

The sheer scale of the experiment required significant logistical effort.  A base station was erected at Union Landing, to house a temporary control center.  Over one hundred graduate students from UC Berkeley were employed to circulate in loops along Interstate 880 between Hayward and Fremont, California, for an entire day.  During the experimental deployment, an average penetration rate of probe vehicles was sustained near 2% (a significant logistical feat), which is viewed as realistic in the near future considering the increasing penetration of GPS-enabled cellular devices.

Classical methods of traffic modeling operate in the density domain, and use data such as occupancies and flows from inductive loop detectors.  Understanding how to use velocity measurements instead was a significant technical contribution.  In this work, the classical model was converted to the velocity domain, and GPS-based measurements were directly fed into the model.

Mobile Century proved that data from GPS-enabled mobile phones alone were sufficient to infer traffic features, i.e., to construct an accurate velocity map over time and space.  The methods employed were able to function properly during both congested and free flow traffic conditions, and to detect correctly a traffic incident that occurred during the deployment.

Another important contribution from this work was that ground-truth travel times were recovered by re-identifying vehicles captured on videotape.  Therefore all results in this report can be asserted with high confidence.  We conclude that the quality of data obtainable from present-day smartphones is adequate for useful, real-time traffic applications, such as calculating travel times.

The architecture of the traffic monitoring system was designed such that identity information is encrypted and handled separately from traffic information, with no single entity having access to both.  The spatial sampling strategy is based on the use of virtual trip lines that can be re-configured on-the-fly.  This feature builds-in guaranteed flexibility for future monitoring needs.

The new paradigm demonstrated in Mobile Century yet requires substantial effort to bring to fruition.  Any industrial-grade, real-time system will require partnerships between government, academia, and industry.  Business cases for future deployment must address incentives for public participation

In conclusion, Mobile Century was the first to demonstrate the near-term potential for using velocity data from GPS cell phones to reconstruct traffic state with precision.  This opens the door for further research in this area to scale up the solution and to deliver considerable value to Caltrans and the traveling public.