Horizontal alignment design shall follow AASHTO Green Book criteria for design speed, radius, superelevation, and sight distance. Minimum curve radius is determined by the design speed and maximum superelevation rate. MassDOT uses a maximum superelevation rate of 8% for rural highways and 6% for urban facilities. Compound curves require a ratio of long radius to short radius not exceeding 1.5:1. Broken-back curves (closely spaced curves in the same direction) are discouraged per MA design policy.
References: AASHTO current edition, MA Standard Specifications, FHWA Technical Advisories, and applicable federal regulations (23 CFR). Contractors shall maintain current copies of all referenced standards on the project site.
Source: MassDOT Highway Design Manual, 2024 Edition. Pages 5–6 of 37.