The Impact of Data Source on Travel Time Reliability Assessment

Traffic congestion and associated impacts such as air pollution pose major concerns to the public. While drivers are used to the daily congestion and plan for it, the unexpected change in travel time pattern causes dissatisfaction the most. The concept of reliability is relatively new in transportation operations compared to other engineering disciplines, but is becoming increasingly important as part of the planning, traffic management and system maintenance. Among all reliability measures in transportation system, travel time reliability is an increasing concern of both travelers and authorities. The travel time reliability index is a statistical measure that is based on the underlying distribution of the travel time on a segment over time. To calculate such index effectively one needs accurate and high quality or in other words reliable data. The importance of measuring travel time reliability can be summarized as the following: (1) From an economic perspective, reliability is highly important because travelers must either build in extra time in their trips to avoid arriving late or suffer the consequences of being late. This extra time has value beyond the average travel time used in traditional economic analyses. Recent work has documented the fact that reliability has value to travelers and that their behavior is influenced by it. (2) Because of the extra time required in planning trips - and the uncertainty about what travel times will actually be for a trip - reliability influences decisions about where, when, and how travel is made. (3) Due to the extra economic cost of unreliable travel on users, transportation planners and operators need to include these costs in the project planning, programming, and selection processes. This is particularly true of strategies that deal directly with roadway events (e.g., incidents). In the past, most assessments of these types of strategies have missed this important aspect of travel. In recent years researchers and practitioners have developed several measures for travel time reliability. The Federal Highway Administration (FHA) defines travel time reliability as "the consistency or dependability in travel times, as measured from day-to-day and/or across different times of the day". Fortunately, recent technological advancements in vehicle tracking, internet enabled mobile devices and in vehicle electronic systems has dramatically improved access to travel time data. Private sector companies such as INRIX take advantage of these resources to provide real-time travel time information both on arterials and freeways mainly by capturing, consolidating and filtering global positioning (GPS) track of the probe vehicles. On the other hand with almost uniform increase in congestion levels much of the attention and resources of state and regional transportation agencies has been directed to dealing with operational issues. However, longer term solutions lie in the planning level decisions. Average travel times used to be the main performance measure used as the basis for such policy level and long-term decisions. Access to inexpensive and accurate travel time data at high resolution (for example, 5 minute intervals) has made it possible to consider travel time reliability in planning stage. Previous work indicates that reliability is determined by the variability in conditions that travelers encounter from day-to-day. Therefore, reliability metrics indicate that variability exists in the system; however they do not tell what is causing it. A recent report by Strategic Highway Research Program (SHRP)identifies the "Seven Sources of Congestion" as the factors that cause travel times to be unreliable and contribute to total congestion: incidents, inclement weather, work zones, special events, traffic control device timing, demand fluctuations, and inadequate base capacity. These categories were developed to move away from the recurring/nonrecurring nomenclature that has been in wide use. This project supports Objective 2.1 (Travel Reliability) of the State Highway Agency (SHA) Business Plan and investigates the effect of data source on freeway travel time reliability assessment. More specifically, data on a major corridor covering sections of I95 South, I495 West and I270 North will be used. The area is covered by a number of permanently installed Bluetooth sensors. The data has been constantly reported and archived since September 2011 and the University of Maryland (UMD) team has unrestricted access to the database. At the same time, SHA has procured INRIX data on the same corridor. Since 2008, the UMD team has published several validation reports on INRIX data performance on both I95 and I495 as part of the I95 Corridor Coalition Vehicle Probe Project (VPP). Validation results show that INRIX meets quality standards to be used as a source for travel time data. The UMD team also has access to weather, incident, speed and volume data through Regional Integrated Transportation Information System (RITIS). Travel time is the starting point for sound congestion measurement because it reflects the actual experience of system users. In order to improve travel time reliability, the first step is to measure it. Measures of travel time reliability better represent a commuter's experience than a simple average travel time. However reliability measures may be impacted by the data source that is being used to collect the travel time. Major tasks for this study can be summarized as: (1) Bluetooth data processing: each Bluetooth sensor reports the identification number of the devices along with a timestamp. Converting sensor data into travel time and trajectory information requires matching, filtering and aggregation. (2) INRIX data processing: INRIX reports data on segments of different length called Traffic Message Channel (TMC). In order to accurately obtain travel time of a path that consists of several TMC segments, a backtracking algorithm must be applied to generate new records. (3) Reliability assessment: this study relies on data from two independent sources for reliability assessment. A through comparison of two travel time reliability measures including planning time index (PTI) and buffer index (BI) will be conducted. The results will be presented for morning and afternoon peak as well as off-peak hours on I95 SB, I495 WB and I 270 NB. (4) Final report: findings of the impact of data source on travel time reliability assessment and analysis details will be presented as a final report.