Driver Understanding of Sequential Portable Changeable Message Signs in Work Zones

Ullman, Brooke R.; Ullman, Gerald L.; Dudek, Conrad L.; Williams, Alicia A. · 2007 · Crossref

DOI: 10.3141/2015-04

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Summary

This study investigates driver comprehension of messages displayed on sequential portable changeable message signs (PCMSs) in work zones. The research was motivated by the Manual on Uniform Traffic Control Devices (MUTCD) recommendation to use two PCMSs in sequence when information exceeds the two-phase limit of a single sign. Prior research indicated that drivers often struggle to process multiple phases of information, raising concerns about whether motorists could effectively integrate split messages into a cohesive understanding. The primary objective was to assess whether drivers could piece together information from two sequential PCMSs and whether repeating a key information unit across both signs (redundancy) improved comprehension. The researchers conducted a driving simulator study using the Texas Transportation Institute’s simulator, which provides a realistic interactive driving environment. Thirty-two participants, demographically representative of the Texas driving population, drove through simulated environments containing either sequential PCMSs or large dynamic message signs (DMSs) displaying identical information. The study compared two message formats: one with four units of information where one unit was repeated on both PCMSs, and another with five units of information with no redundancy. Participants were queried via intercom after passing each sign to measure recall and comprehension. The experimental design counterbalanced message types and locations to control for learning effects, ensuring that total legible reading times were comparable between the sequential PCMSs and the single-location DMSs. The results demonstrated that message length and redundancy significantly impacted driver comprehension. For messages containing four units of information with one repeated unit, comprehension rates for sequential PCMSs were comparable to those for single-phase DMSs, with overall correct response rates of 78% and 77%, respectively. Statistical analysis showed no significant difference between the two formats. However, when messages contained five units of information without redundancy, comprehension rates dropped substantially. Drivers recalled only 54% of the information from sequential PCMSs compared to 63% from two-phase DMSs. Specifically, recall for the problem location was significantly lower for the first PCMS (31%) than for the DMS (59%). Even the DMS format failed to meet the desired 85% comprehension threshold for five-unit messages. The study concludes that sequential PCMSs are effective only if the total message length is kept at or below four units of information. Presenting five units of information, even across two signs, results in unacceptable comprehension rates and should be avoided to maintain the credibility and safety of work zone traffic control. While redundancy (repeating one unit) was tested, the high recall of non-repeated information in the four-unit condition suggests that keeping the message concise is the critical factor. These findings reinforce existing human factors guidelines regarding message load and provide empirical evidence that splitting longer messages across sequential signs does not mitigate the cognitive difficulties drivers face with high information loads.

Key finding

Presenting five units of information on sequential portable changeable message signs results in substantially lower driver comprehension rates compared to four-unit messages, which perform comparably to single-location dynamic message signs.

Methodology

simulator

Sample size: 32

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

Ranked by relevance to this paper. Hover a topic for its definition.

• signage environment
• work zones
• emergency work zone conspicuity
• external distraction