This document provides guidance on conducting site investigations for new highway construction and widening, rehabilitation, and reconstruction of existing roads. Site investigation is an important part of the pavement design process. This guide has been developed as an integral part of the mechanistic-empirical pavement design procedures for new pavement and rehabilitation and reconstruction of existing pavement. An investigation requires four steps: (1) the preliminary site investigation and desktop data collection, (2) the detailed site investigation and data collection for design, (3) the site investigation data analysis, and (4) the project investigation report.