Robert L. Bertini

Possible explanations of phase transitions in highway traffic

It is shown that all the phase transitions in and out of freely flowing traffic reported earlier for a German site could be caused by bottlenecks, as are all the transitions observed at two other sites examined here. The evidence suggests that bottlenecks cause these transitions in a predictable way, and does not suggest that stoppages (jams) appear spontaneously in free flow traffic for no apparent reason. It is also shown that many of the complicated instability phenomena observed at all locations can be explained qualitatively in terms of a simple Markovian theory specific to traffic that does not necssarily include spontaneous transitions into the queued state as a feature.

Using Travel Time Reliability Measures to Improve Regional Transportation Planning and Operations

Estimation of travel time is of increasing importance to travelers and transportation professionals alike as congestion worsens in major urban areas. In fact, the reliability of travel time estimates on a given corridor may be more important for travelers, shippers, and transport managers than the travel time itself. This paper examines the uses of measured travel time reliability indices for improving real-time transportation management and traveler information with the use of archived intelligent transportation system data. A literature review of travel time reliability and its value as a congestion measure is followed by a description of a content analysis of 20 regional transportation plans from across the nation. Results from the content analysis indicate that travel time reliability is not currently used as a congestion measure and that the most common measures of congestion are the volume-to-capacity ratio, vehicle hours of delay, and mean speed. As a case study using data from Portland, Oregon, several reliability measures are then tested including travel time, 95th percentile travel time, travel time index, buffer index, planning time index, and congestion frequency. The buffer index is used to prioritize freeway corridors according to travel time reliability. Metropolitan planning organizations should use travel time reliability in the following ways: (a) incorporate it as a systemwide goal, (b) evaluate roadway segments according to travel time reliability measures, and (c) prioritize roadway segments using those measures.

GENERATING TRANSIT PERFORMANCE MEASURES WITH ARCHIVED DATA

Measuring the performance of a transit system is the first step toward efficient and proactive management. Since 1990, the use of performance measures for transportation planning and operations has gained a great deal of attention, particularly as transportation agencies are required to provide service with diminishing resources. In the past, it was very difficult and costly to collect comprehensive performance data. Thus, until recently, the transit industry has relied on limited, general, and aggregate measures for reporting performance to external funding and regulatory agencies. In Portland, Oregon, the local transit provider (TriMet) has developed a bus dispatch system (BDS) consisting of automatic vehicle location, communications, automatic passenger counters, and a central dispatch center. Most significantly, TriMet had the foresight to develop a system to archive all of its stop-level data, which are then available for conversion to performance indicators. It is demonstrated that there are powerful ways in which the data collected by the BDS can be converted into potentially valuable Transit Performance Measures (TPMs). These TPMs have been proposed in the past but were not implemented because of data limitations. It is envisioned that systematic use of TPMs can assist a transit agency in improving the quality and reliability of its service, leading to improvements for customers and operators alike.

Analysis of transit signal priority using archived trimet bus dispatch system data

Findings are presented on changes in bus running times, on-time performance, and excess passenger wait times following implementation of transit signal priority (TSP) in select bus corridors in the Portland metropolitan region. Analysis of the effectiveness of TSP is often undertaken by using simulation techniques or empirical studies that are limited in either scope or data availability, or both. The current research uses an abundance of trip-level data collected from TriMets Bus Dispatch System, in Portland, Oregon. The study focuses on the most common performance measures of interest to both transit operators and passengers and shows that the expected benefits of TSP are not consistent across routes and time periods, nor are they consistent across the various performance measures. The authors believe that benefits of TSP will accrue only as the result of extensive evaluation and adjustment after initial deployment. In most cases, an ongoing performance monitoring and adjustment program should be imple...

Transit Buses as Traffic Probes: Use of Geolocation Data for Empirical Evaluation

With the growing availability of data because of the deployment of intelligent transportation systems, methods for assessing and reporting traffic characteristics and conditions have begun to shift. Although previous level-of-service methods were developed for use with limited data, actual performance measures can now be developed and tested. On freeways, performance measures often are estimated directly by using data from inductive loop detectors (e.g., speed, occupancy, vehicle counts). For arterials with numerous signalized intersections, performance measures are more challenging because of more complicated traffic control and many origins and destinations. However, within signalized networks, travel time, speed, and other key performance measures can be obtained both directly and indirectly from sources such as automatic vehicle location (AVL) data. The use of AVL data for characterizing the performance of an arterial is demonstrated. First, data are extracted from the bus dispatch system of the Tri-County Metropolitan Transit District (TriMet), the transit provider for Portland, Oregon. Then, the performance characteristics as described by bus travel on an arterial are compared to ground truth data collected by probe vehicles equipped with Global Positioning System sensors traveling with normal (nontransit) traffic on the same arterial on the same days. Comparisons are made between the two methods, and some conclusions are drawn regarding the utility of the transit AVL data.

I-880 FIELD EXPERIMENT: ANALYSIS OF INCIDENT DATA

The I-880 field experiment has produced one of the largest data bases on incidents and freeway traffic-flow characteristics ever compiled. Field data on incidents were collected through observations of probe-vehicle drivers before and after the implementation of freeway service patrols (FSPs) over a freeway section. Supplementary information was collected from the California Highway Patrol’s computer-aided dispatch system, FSPs, and tow-truck company logs. The incident patterns are described and the major factors affecting incident frequency and duration are identified. FSPs significantly reduced the response times but did not have a significant effect on the duration of all incidents.

Dynamics of Variable Speed Limit System Surrounding Bottleneck on German Autobahn

This paper presents findings of an ongoing empirical study focusing on identification and examination of several recurring freeway bottlenecks. It integrates the fusion of several traffic management and driver information data sources along an 18-km (11.2-mi) section of Autobahn 9 near Munich, Germany. These combined data sources further understanding of traffic dynamics and driver behavior before, during, and after bottleneck activation. The primary focus of this paper has been the investigation of variable speed limit and traveler information systems provided by means of overhead dynamic message signs so as to improve understanding of how these systems affect driver behavior and bottleneck formation and location. Toward that end, speed limit and information messages have been compared with actual traffic dynamics on the segment of Autobahn 9, and the analysis has found a strong correlation. It has been found that when drivers were warned of approaching congested conditions, the speed limit was reduced b...

Some observed queue discharge features at a freeway bottleneck downstream of a merge

Details of traffic evolution were studied upstream and downstream of a freeway bottleneck located near a busy on-ramp. It is shown that on certain days the bottleneck became active upon dissipation of a queue emanating from somewhere further downstream. On such occasions, the bottleneck occurred at a fixed location, approximately one kilometer downstream of the merge. Notably, even after the dissipation of a downstream queue, the discharge flows in the active bottleneck were nearly constant, since the cumulative counts never deviated much from a linear trend. The average bottleneck discharge flows were also reproducible from day to day. The diagnostic tools used in this study were curves of cumulative vehicle arrival number versus time and cumulative occupancy versus time constructed from data measured at neighboring freeway loop detectors. Once suitably transformed, these cumulative curves provided the measurement resolution necessary to observe the transitions between freely flowing and queued conditions and to identify some important traffic features.

Experience Implementing a User Service for Archived Intelligent Transportation Systems Data

In cooperation with the Oregon Department of Transportation (ODOT) and other regional partners, the Portland regional intelligent transportation systems (ITSs) data archive was recently inaugurated via a direct fiber-optic connection between ODOT and Portland State University (PSU). In July 2004, the Portland Regional Transportation Archive Listing was activated; it received 20-s data from the 436 inductive loop detectors composing the Portland areas advanced traffic management system. PSU is designated as the regions official data archiving entity, consistent with the ITS architecture being developed. This paper discusses the steps taken for successful implementation of the Portland regions functional ITS data archive and plans for development and expansion. Included is a discussion of the archive structure, data storage, data processing, and user interface. An experiment involving Metro, the Portland regions metropolitan planning organization, demonstrates that archived loop detector data can be use...

Empirical Measurement of Freeway Oscillation Characteristics: An International Comparison

A country-specific analysis of freeway traffic oscillations was conducted. Loop detector data from sites in the United States, Germany, and the United Kingdom were analyzed. With the use of a method applied in previous work, traffic oscillations were identified in all three countries. Calculation of the cross-correlation coefficient revealed that they traveled upstream at speeds of about 19 to 20 km/h at the U.S. site, 16 km/h at the German site, and 14 km/h on the U.K. freeway. Similar magnitudes were found in the literature verifying the hypothesis that they propagated faster in the United States than in Germany. Furthermore, an oscillation frequency was identified by calculation of the datas autocorrelation. However, since the oscillation frequency was likely to be site specific, conclusions regarding general differences between the frequencies measured in different countries cannot yet be made. For the sites analyzed, it was found that oscillations appeared every 8 to 12 min on the M4 (U.K. site), every 10 to 30 min on the A9 (German site), and every 3 to 6 min on OR-217 (U.S. site). Although the magnitudes of the latter two countries were supported by the literature, further empirical research on several different sites should be pursued to draw final conclusions.