Stopping Sight Distance and Horizontal Sight Line Offsets at Horizontal Curves

Criteria for the design of horizontal curves implicitly rely on design speed to produce safe and efficient designs, particularly for horizontal sight line offsets and stopping sight distances. Current design guidance provides a method for calculating minimum horizontal sight line offsets that is accurate and valid only when both driver and object are within the limits of the curve. Other methods are available to estimate minimum horizontal sight line offsets when the driver, object, or both are not within the curve limits. However, design guidance recommends using the calculated value for offsets as a conservative estimate near the ends of curves. In this study, speed prediction models and reliability theory were used to estimate the probability that drivers would not have enough sight distance to see, react to, and stop before reaching an object in the roadway if horizontal sight line offset criteria were applied when the driver or object was outside the limits of a horizontal curve. Six scenarios at the curve approach and inside the curve were analyzed. Reliability estimates (based on minimum horizontal sight line offsets from current minimum design criteria) and stopping sight distance distributions (based on individual driver characteristics) indicated that the probability of drivers not having enough stopping sight distance was much greater on the approach to than inside the horizontal curves. For improvement of design consistency, the use of calculated horizontal sight line offsets beyond the limits of the curve (approach and departure tangents) is suggested to provide extra sight distance to drivers near the curve.