Researchers evaluated the effectiveness of traffic control device (TCD) treatments for changes in horizontal alignment through two studies. The first study evaluated the behavior of 103 unfamiliar drivers who navigated through 167 individual changes in alignment on two-lane highways at three test sites in three states. Driver behavior instrumentation included a global positioning system to measure speed and an accelerometer to measure longitudinal and lateral acceleration. The second study evaluated the safety effects of various TCDs used within, and in advance of, changes in horizontal alignment at 541 sites across four states. The safety evaluation included crash, roadway, and traffic information. The findings from the two evaluations included regression models and crash modification functions that identified how TCDs affected behavior and safety. The unfamiliar drivers were observed to drive more conservatively when they approached and navigated alignment changes treated with TCDs. TCDs also led to reduced crash frequencies at curves. These results were used to develop guidelines for the selection of TCDs for changes in horizontal alignment on the basis of volume, approach speed, and the advisory speed. The guidelines define conditions in which pavement markings alone are appropriate, where an advance warning sign should be added, and where either delineators or chevrons should be used in addition to the advance warning sign and markings.