This study sought to assess and mitigate the effects of dynamic message signs (DMSs) on highway traffic. A review of traffic data found that many motorists slowed as they approached an active DMS and then increased their speed after passing the active DMS. The researchers identified that this speed variation could pose safety hazards to other motorists. The study identifies the effects of active DMSs on highway traffic using a sample from Rhode Island and identifies measures to improve the design and display of DMS messages to help mitigate speed variations of approaching traffic through the use of a questionnaire and driving simulation experiment.
The study consisted of a three pronged approach: a traffic data analysis, a driver questionnaire, and driving simulation experiment.
- Traffic data analysis: traffic data, including mean volumes and speeds in 30-minute increments, were collected by several mobility technology units (MTUs) near DMSs in Rhode Island between June 1 and June 14, 2007 when messages were displayed. 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 public locations in Rhode Island in order to obtain a representative sample of the state driving population,150 licensed drivers participated.
- Driving simulator experiment: video-based driving simulation experiment 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. The message categories refer to the content: danger warning, informative, and regulatory. Message type refers to how the message is displayed: graphic-aided message with full text (GFT), a graphic-aided message with partial text (GPT), and a text-only message (T). Each of three message categories was included with each of the three message types, creating a total of nine test messages. Additionally, a few practice trials were given to ensure participants' readiness prior to the actual simulation. A total of 36 drivers participated in the driving simulation test. Participants sat in a fix-base vehicle and were asked to respond by pressing one of four response buttons mounted on the steering wheel as soon as they comprehended the messages ("1" if a roadwork message was observed, "2" for a crash message, "3" for a slippery road message, and "4" if other messages were observed).
- Traffic data analysis: average speed was significantly lower when DMSs were active. Speed variations in traffic were not noticeable when DMS messages were displayed for less than 30 minutes or when DMS messages were displayed during rush hours in the initiating stage analysis. Speed variations were observed in all cases of the ending stage analysis. As for DMS message displays active for more than 30 minutes, drivers decreased speeds and headway distances as they approached active DMSs in the initiating stage analysis and increased their speeds and headway distances in the ending stage analysis. This was also observed when DMS messages were active during non-rush hours. The decrease in both speeds and headway distances in the "initiating stage analysis" and the increase in both speeds and headway distances in the "ending stage analysis" provided evidence of the impacts of active DMSs on highway traffic.
- Driver questionnaire: most drivers indicated they would reduce speed when approaching active DMSs. Lengthy, complexly worded, and abbreviated messages could also cause drivers to slow down. Drivers preferred text-only messages over graphic-aided messages and single frame over two-frame messages if they displayed the same content. Finally, 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 much shorter response time compared to GFT messages. This might be due to the fact that GFTs had more information on them and thus required longer response time. 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 to other message categories.