This study assessed effects of dynamic message signs (DMSs) on highway traffic. Traffic data from a sample collected in Rhode Island found many motorists slowed while approaching an active DMS and then increased speed after passing the active DMS. This speed variation could pose a safety hazard to other motorists. The study identified effects of active DMSs on highway traffic and learned lessons about how drivers read and interact with different types of messages displayed on a DMS.
The study took a three pronged approach:
- Traffic data analysis: traffic data, including volume and speeds, were collected near DMSs in Rhode Island for a two week period. The study compared "pre-display" and "first 30-min" traffic conditions (initiating stage analysis) and "last 30-min" and "post-display" traffic conditions (ending stage analysis).
- Driver questionnaire: a 24 multiple-choice question survey collecting drivers' opinions on slow-downs and DMS use. Surveys were administered at multiple locations in Rhode Island to obtain a representative sample of the driving population, 150 drivers participated.
- Driving simulator experiment: video-based driving simulation developed and conducted to evaluate various DMS message designs in a simulated driving environment. Two main factors, message category and message type, were considered in a factorial experiment with three repetitions. Message categories were: danger warning, informative, and regulatory. Message types were: graphic-aided message with full text (GFT), graphic-aided message with partial text (GPT), and text-only message (T). Each message category was included with each of the message types, yielding nine test messages. A few practice trials were given to ensure participant readiness prior to the actual simulation. Thirty-six (36) drivers participated. Participants sat in a fix-base vehicle and were asked to respond by pressing one of four buttons mounted on the steering wheel once they comprehended the messages.
- Traffic data analysis. Decrease in both speeds and headway distances in the "initiating stage analysis" and increase in both speeds and headway distances in the "ending stage analysis" provided evidence of impacts of active DMSs on highway traffic.
- Driver questionnaire. Most drivers indicated they reduced speed when approaching an active DMS. Lengthy, complex, and abbreviated messages could cause drivers to slow down. Drivers preferred text-only messages over graphic-aided messages and single frame over two-frame messages when displaying the same content. Two-frame messages with more information were preferred over single-frame messages with less information and abbreviations.
- Driving simulator experiment. GPT messages had similar response time when compared with T messages but shorter response time compared to GFT messages. Overall, young participants (age 18-40) responded the fastest. Female participants responded faster than males except for the oldest age group. Participants responded faster to danger warning messages than other message categories.
Graphics may not be well accepted by drivers for use in DMS messages as indicated by the survey, yet graphic-aided messages with partial text showed promising results by producing quick response and higher response accuracy in the driving simulation experiment.
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