At the time of developing the latest Road Master Plan for the Madrid Region (Spain), the question came up as to what traffic volume should be established as a criterion to trigger the upgrade of a two-lane highway to a four-lane facility. Conventional wisdom suggested a maximum annual average daily traffic (AADT) of 10,000 veh/day. The main objective of this research was to determine whether 10,000 veh/day is a reasonable upgrade threshold or if this threshold is too conservative and a higher value can be adopted. Assessment was made based on actual traffic volume measured in two-lane facilities, service vehicle tables provided by different highway agencies and a statistical analysis of the traffic volume distribution of in-service facilities in the Madrid Region. A total of 36 segments exceeding an AADT of 10,000 veh/day at some point between 1998 and 2008 were found and analyzed. Results indicated that maximum observed values fit fairly well with the service volume tables provided by HCM 2010 and FDOT and an increase in the upgrade threshold is feasible. These results offer guidance as to what value can be considered for future planning applications, including impact traffic studies and local and regional planning.
Romana, M. G., & Hernando, D. (2016). Obtaining a Maximum AADT Sustained by Two-lane Roads: An Application to the Madrid Region in Spain. In Transportation Research Procedia (Vol. 14, pp. 3209–3217). Elsevier B.V. https://doi.org/10.1016/j.trpro.2016.05.263