Urban Traffic Operations/SMART-SIGNAL System Overview
The SMART-SIGNAL (Systematic Monitoring of Arterial Road Traffic and SIGNAL) system, developed by the University of Minnesota, offers an easily implementable approach to collect and archive continuous and high-resolution traffic data on signalized arterials (Liu et al., 2008). In this system, a complete history of traffic signal control, including all vehicle actuation events and signal phase change events, is archived and stored. Based on the event-based data, arterial performance measures, such as arterial travel time, intersection queue length, and level of service, are produced. The SMART-SIGNAL system has been installed on 11 intersections along France Avenue in Hennepin County, Minnesota, and 6 intersections along Trunk Highway 55, Minnesota, since February 2007. Event-based traffic data are being collected in a 24/7 mode and then archived in a database system, thus yielding a tremendous amount of field data available for research. This chapter will briefly introduce the system architecture, data collection hardware, and data processing procedure of the SMART-SIGNAL.
As illustrated in the following figure, the SMART-SIGNAL system has three major components, data collection, performance measurements, and performance presentation through user interfaces. The data collection component collects high resolution raw data directly from the field on an event-by-event basis. Signal phase change events and vehicle-detector actuation events are acquired separately from data collection units located in traffic signal cabinets. The data are recorded in daily log files and sent to a data server at the master cabinet by serial port communication. The daily log files are finally transmitted to a database located at the Minnesota Transportation Observatory (MTO) lab at the University of Minnesota through the Digital Subscriber Line (DSL) or wireless communication. The second component of the SMART-SIGNAL system is performance measure calculation using the field-collected data. Analysis of the stored data log files yields a set of performance measures, one covers intersection level measures (e.g. queue length) and a second related to arterial level measures (e.g. travel time). Once all performance measures have been derived from the raw data, the results are made accessible to a variety of users.
Data Collection Unit
The key element of the SMART-SIGNAL is the data collection unit, which consists of an industrial PC and a data acquisition card. At each intersection, an industrial PC with a data acquisition card is installed, and event data collected at each intersection is transmitted to the data server in the master controller cabinet through the existing communication line (in this case, spare twisted pair) between signalized intersections. The data acquisition cards (PCI-6511 from National Instruments (2006)) used in the SMART-SIGNAL system, as shown in Figure 2 2a, have 64 input channels. If the total number of detector inputs and signal phases for one intersection exceed 64, an additional data acquisition card needs to be installed. A terminal box is used in order to limit the input direct current (DC) to a safe range and establish the connection between the data acquisition card and back panel of the traffic cabinet, as shown in Figure 2 2b. The terminal box allows digital voltage changes on the back panel, which indicate different traffic events in the field, to be captured by the data acquisition card installed in the industrial computer. A traffic event recorder software program, developed using the Microsoft Visual C# program, runs on the industrial computer in the field to record the events (for example, a phase 1 green change from “ON” to “OFF”) into a log file.
Data communication between two controller cabinets is done using the existing twisted pair communication lines. A protocol of RS-485 is used to transmit data and synchronize time between cabinets (B&B Electronics, 2007). After the data in the local cabinets is transferred to the master cabinet, DSL or a wireless unit installed in the master cabinet is used to send the data back to the database located in the MTO.
A sample of data is shown in the figure. Each logged event starts with a time stamp that includes the date, hour, minute, second and millisecond based on the computer system time, followed by different types of event data including phase changes, detector actuation and pedestrian calls. A complete history of traffic signal events is thus recorded.