Superelevation Calculator

Superelevation is the cross-slope, or banking, built into a highway curve so a vehicle's weight helps counteract the outward pull of cornering at speed. This calculator applies the AASHTO Green Book's standard side-friction formula, e = V^2/(15R) - f, where V is design speed in mph, R is curve radius in feet, and f is the maximum side-friction factor the pavement is assumed to provide at that speed (friction demand falls as speed rises, since AASHTO's tables assign lower f values to higher-speed curves). The result is capped between 0 (a curve so gentle or slow that no banking is needed) and 0.12 ft/ft (12%, a common practical ceiling for rural highways), so an aggressive combination of high speed and tight radius simply reports the maximum allowed rate rather than an unbuildable one. Two companion outputs answer the inverse questions independently of the entered radius and speed: Min Radius is the tightest curve a given design speed can use while staying within the 8%-superelevation ceiling this calculator assumes as typical practice, and Max Speed is the fastest a given radius can safely be driven under that same ceiling.

Because Min Radius is derived purely from design speed and side-friction factor, and Max Speed purely from curve radius and side-friction factor, neither one depends on the specific curve radius or design speed you entered for the main superelevation-rate calculation -- they describe the limiting case for the ceiling rate, not a property of your specific inputs. The Side Friction Factor itself follows AASHTO's Method 5: a fixed maximum side-friction value looked up by design speed alone (Table 3-7), the approach AASHTO recommends for open-highway and rural high-speed design. That's different from AASHTO Method 2, which develops side friction first (in proportion to curvature) up to that same maximum before adding any superelevation at all, and is typically applied to low-speed urban streets where superelevation is often limited or impractical -- this calculator does not model Method 2's friction-first threshold behavior. Both methods trace back to Chapter 3 of the AASHTO Green Book, 7th Edition (2018) -- the same edition the New York State DOT Highway Design Manual (Chapter 2, Section 2.6.5) cites by name when it directs designers to Method 5 for standard curves and Method 2 for low-speed urban streets.