Guidelines for the Use of Diagrammatic Guide Signs and Their Alternatives

This project summary report summarizes the research performed in Texas Department of Transportation (TxDOT) Project 0-5147. While text-based guide signs are commonly seen on freeways in Texas, diagrammatic signs are used by other agencies and exist as an option in the Texas Manual on Uniform Traffic Control Devices (MUTCD). The MUTCD recommends diagrammatic signs for guiding drivers through uncommon freeway interchange geometries. Recent research has proposed other designs using graphics as well as words, referred to as "modified diagrammatic signs." This two-year project tested standard text signs, MUTCD-style diagrammatic signs, and modified diagrammatic signs for four interchange types for three-lane roads. The geometries tested were left exits, left lane drops, two-lane right exits with optional exit lane, and freeway-to-freeway splits. These are the four situations listed in the MUTCD as candidates for diagrammatic signs and represent unusual configurations that may violate driver expectations. Researchers chose candidate sign designs to be tested. In five locations around the state, participants viewed slides of advance guide signs and responded by choosing which lane or lanes would lead them to a desired destination. The signs performing best were then used in a driving simulator. After the drive, participants completed a short computer-based questionnaire. In general, this project found that the current standard text-only sign sequences are effective at guiding drivers to their desired destinations. While diagrammatic signs may offer benefits to some drivers, no evidence was found that they perform significantly better across the entire population. When comparing standard diagrammatic signs to modified diagrammatic signs, standard diagrammatic signs were preferred by drivers. The current guidance of the Texas MUTCD and accompanying standards sheets are supported by this research. No changes to current standards or practice are recommended.