Yellow warning signs are an important and abundant type of traffic control device. Improving warning signs could be a cost-effective countermeasure at hazardous locations. The use of fluorescent yellow sheeting in place of standard yellow sheeting provides an inexpensive method to increase the conspicuity of the traffic sign while conforming to the guidelines specified by the Manual of Uniform Traffic Control Devices. Although the properties of the fluorescent yellow sheeting indicate that the conspicuity of the signs is much higher than standard yellow, the increased conspicuity ultimately must prompt a change in motorist behavior for highway safety to be improved. Therefore, the purpose was this research was to evaluate the effectiveness of fluorescent yellow warning signs in improving highway safety at hazardous locations. A before and after study used surrogate measures (encroachments, conflicts and events, signal violations, stop sign observance, stopping behavior, and speed) to evaluate the safety effectiveness of replacing existing yellow warning signs with fluorescent yellow warning signs at seven hazardous locations. The results of this effort indicate that fluorescent yellow warning signs likely increased safety at four of the seven sites by providing a more conspicuous warning to motorists, with little change at the other three sites. However, since surrogate measures were used, the actual collision savings are unknown. The researchers recommend use of fluorescent yellow warning signs as an inexpensive countermeasure at sites like those tested in the study, and also recommend more research to find the collision savings and long-term effects.