Due to the rising needs in highway maintenance and construction, the number of work zones is increasing throughout the United States. Highway work zones disrupt normal traffic flow and create safety problems. With a total of 1,010 fatalities and more than 40,000 injuries in the work zones in 2006, more improvements in safety are needed. The objective of this research project was to measure the effectiveness of a Portable Changeable Message Sign (PCMS) in reducing vehicle speeds in the upstream of the one-lane, two-way work zones on rural highways. The effectiveness of the PCMS was evaluated under three different conditions during field experiments: (1) PCMS switched on; (2) PCMS switched off, but still visible; (3) PCMS removed from the road and out of sight. Based on the data analysis results, the authors concluded that the presence of PCMS was effective in reducing the vehicle speeds in these work zones. The major contribution of this research project was to quantify the effectiveness of PCMS in rural one-lane, two-way work zones which had previously not been studied in detail.