Description

This research report presents the theoretical formulation and field calibration of roadside friction and speed-flow models within the HDM-4 framework under the ISOHDM project. Roadside friction refers to the speed-reducing interference caused by marginal activities along a highway corridor, such as roadside trading, bus stops, unregulated parking, non-motorized transport (pedestrians, bicycles, animal-drawn carts), and frequent access points to adjacent commercial developments. The document establishes quantitative methodologies for categorizing roadside friction intensity (low, medium, high, very high) and incorporating its effects into free-flow speed reduction equations and speed-capacity relationships for single-carriageway and multi-lane highways. Field survey data collected in developing nations—particularly in Asia and Africa—are analyzed to determine empirical speed reduction factors and increase in speed variability (acceleration noise). The report details how roadside friction reduces nominal highway capacity, increases travel time delays, elevates fuel consumption, and accentuates vehicle operating costs. By incorporating these roadside friction metrics into HDM-4, road agencies can evaluate economic benefits of access management, bypass construction, shoulder paving, service road implementation, and roadside space regulation in both peri-urban and rural highway projects. Overall, the report emphasizes rigorous empirical validation, standardized data acquisition protocols, and modular integration into the broader Highway Development and Management analytical engine. By combining physical principles with statistical calibration across international trial sites, this working paper provides a robust theoretical and practical baseline for highway authorities to evaluate long-term infrastructure policy options, allocate maintenance budgets efficiently, and maximize economic returns across regional and national road networks.