8 Work Zone Safety
8.2 Dynamic Speed Advisories
8.2.1 The ADAPTIRTM System- Nebraska
The information on the ADAPTIRTM System was obtained from the Midwest States Smart Work Zone Deployment Initiative Year One Report.22
Impetus for the Activity
The States of Iowa, Kansas, Missouri and Nebraska formed the Midwest States Smart Work Zone Deployment Initiative to test and evaluate technologies for improving safety and efficiency of traffic operations in work zones. By working together, the agencies avoid unnecessary duplication of research through sharing of research results.
Criteria for System Selection (Hardware and Software)
The group held a series of three workshops to identify work zone problems, to define solutions and to select the technologies for evaluation. Potential solutions were presented by technology providers, and these solutions were matched with work zones on specific construction projects. The ADAPTIRTM System, developed by The Scientex Corporation, through a cooperative agreement with FHWA and Maryland State Highway Administration, was matched with a work zone in Nebraska.
Description of the System
ADAPTIRTM was deployed and evaluated during the 1999 construction season on a rural I-80 work zone, on a segment with an ADT of 38,000 vehicles per day. The system is a condition-responsive, portable work zone traffic control system. It can provide drivers with real-time information on work zone traffic conditions via variable message signs and highway advisory radio. The objective of the system is to improve the safety and efficiency of traffic operations in advance of the work zone. Safety is addressed by advising drivers of slower speeds ahead.
The ADAPTIRTM system is made up of the following components:
- Variable message signs;
- Highway Advisory Radio;
- Central system controller;
- Radar sensors;
- Roadside remote stations.
The radar sensors continually measure speeds at multiple locations. This data is continuously fed to the roadside remote stations.
For this application of the system, information was provided to the driver by four variable message signs. Message types included speed advisory messages, delay messages and diversion messages. Highway advisory radio was not used in this application. Because this study is interested in safety, the remaining information presented will focus on the speed advisory messages.
Speed advisory messages were presented to the driver in a two-stage format. The first phase provided the location (I-80 E), the word ADVISORY, and the time of the message. The second phase stated, “REDUCED SPD AHD xx MPH.”
The logic used to select whether a message should be displayed and the speed to be displayed is as follows. The central systems controller analyzed data from the roadside remote stations at 8-minute intervals, except between 1 and 8 PM, when the interval was 4 minutes. If the speed of the traffic measured downstream of a variable message sign was no more than 16 km/h below the speed at the variable message sign, the sign remained blank. If the speed difference was greater than 16 km/h, a speed advisory message was displayed. The displayed speed was the speed downstream to the nearest 8 km/h.
System Performance
No problems with system performance were noted in the report.
System Effectiveness Evaluation
The effectiveness of the ADAPTIRTM system was formally evaluated. Safety-related measures of effectiveness included:
- Traffic speed and lane distribution in advance of the lane closure taper;
- Number of forced merges in advance of the lane closure taper; and,
- Driver compliance with the advisory speed messages.
The evaluation team found that the system had no effect on the speed and lane distribution of traffic within 60 m of the lane closure taper. The system also had no effect on the numbers and rates of forced merges in advance of the lane closure taper. It was noted that the data were collected during periods of uncongested flow, where messages are seldom displayed on the variable message signs.
The evaluation team found that during uncongested flow, speed advisory messages were not effective in reducing speeds. When traffic flow approached congested levels, the team found that the speed advisory messages were effective in reducing speeds where the drivers were aware of the work zone ahead and were likely to perceive the need to reduce their speed. The team recommended further research on spacing of the variable message signs.