Abstract
This paper presents a reliability-based method for the design of intersection sight distance (ISD) at traffic roundabouts using the linear and nonlinear deceleration profiles of the entry vehicles. The reliability method is based on the first-order second moment method which is simple and relatively accurate compared with advanced methods. The nonlinear deceleration profile includes a shape parameter that produces the linear profile as a special case. Deterministic and reliability-based formulas for the required ISD for an approaching vehicle are developed for the entry vehicle on the left and the vehicle on the circulating roadway. Then, the design values of the ISD legs, applicable to any type of roundabout, are presented for different probabilities of non-compliance (Pnc) and different coefficients of variations. For the special case of single-lane symmetrical roundabouts, which have a well-defined geometry, the lateral clearance needs are established. The sensitivity analysis shows that ISD is very sensitive to both the mean and variance of the critical headway. The results show that the deterministic method results in ISD values that correspond to a very small Pnc, indicating that the method is very conservative. The proposed method, which provides flexibility in selecting ISD for any given Pnc, should be of interest to highway designers and practitioners to promote roundabout safety.
Highlights
Intersection sight distance (ISD) plays a crucial role in the design of roundabouts
The following comments are offered: The developed design values of the intersection sight distance (ISD) legs are applicable to any type of roundabout
The lateral clearance needs for other complex roundabouts can be established graphically using the developed ISD values
Summary
Intersection sight distance (ISD) plays a crucial role in the design of roundabouts. Adequate ISD is required for a driver at each entry of the roundabout to identify safe gaps from conflicting traffic streams. ISD should be checked at two locations of the approach vehicle at each entry: (1) a certain distance before the crosswalk and (2) a stationary vehicle at the yield line. Drivers at these locations should clearly see the two conflicting vehicles: circulating and entering from the immediate left. The current methods for calculating ISD are deterministic Such methods are presented in the roundabout information and design guides by Rodegerdts et al [1]; the Transportation Association of Canada [2]; the American Association of State Highway and Transportation Officials (AASHTO) [3]; the Federal Highway Administration (FHWA) [4]; the Government of Queensland (GQ) [5]; and the Department for Regional Development, Northern Ireland (DRDNI) [6]. The ISD design parameters, such as design speeds, deceleration rate, and critical headway for the conflicting vehicles are random variables in nature instead of fixed values, and they may be correlated with one another
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