The guidance supplied in this document is based on established international and U.S. practices and is supplemented by recent research. The guide is comprehensive in recognition of the diverse needs of transportation professionals and the public for introductory material through design detail, as well as the wide range of potential applications of roundabout intersections. The following topics are addressed: definition of a roundabout and what distinguishes roundabouts from traffic circles; public acceptance and legal issues associated with roundabouts; consideration of all user modes, including heavy vehicles, buses, transit, bicycles, and pedestrians; a methodology for identifying appropriate sites for roundabouts and the range of conditions for which roundabouts offer optimal performance; methodologies for estimating roundabout capacity, delays, and queues with reference to the "Highway Capacity Manual"; design principles and guidance on safety and geometric design, with reference to applicable national standards such as the AASHTO "Policy on Geometric Design of Highways and Streets"; guidelines for control features such as signing and pavement markings, with reference to the "Manual on Uniform Traffic Control Devices"; illumination; and landscaping.

https://rosap.ntl.bts.gov/view/dot/15382/dot_15382_DS1.pdf

Roundabouts : an informational guide

Older drivers are currently over-represented in severe injury crashes at intersections due in part to increases in frailty and functional disabilities that occur with age. Moreover, this rate is expected to increase as older people drive more and the population ages. Road design plays a major role in road safety and is likely to contribute to the driving difficulties of the elderly because of the general lack of consideration of the needs of older road users. Intersections, in particular, stand out as a major problem for older road users. This paper reviews age-related performance deficits that affect driving and describes a crash ‘black-spot’ site analysis that examined the relationship between intersection design features (believed to influence the safety of older drivers) and the older driver crash experience in Australasia. A number of intersection design problems for older drivers were identified and recommendations for changes in road design features and traffic management practices that have the potential to reduce crash and injury risk for older drivers at intersections are made.

Intersection design for older drivers

The original "Older Driver Highway Design Handbook" was published by the Federal Highway Administration (FHWA) in 1998 (FHWA-RD-97-135). The 2nd edition, titled "Highway Design Handbook for Older Drivers and Pedestrians" (FHWA-RD-01-103) was published in 2001. This 3rd edition, under a new title, incorporates new research findings and treatments to improve the safety of the transportation system for the aging population. The Handbook is divided into three sections. The first section explains how to use the Handbook to select treatments to address problems for aging drivers and pedestrians. The second section includes treatments for 51 proven and promising traffic control and design elements distributed among five categories: Intersections, Interchanges, Roadway Segments, Construction/Work Zones, and Highway-Rail Grade Crossings. The final section of the Handbook includes the rationale and supporting evidence for the treatments. A website including all of the content of the Handbook is available at http://safety.fhwa.dot.gov/older_users/#training.

Handbook for designing roadways for the aging population.

Critical gap is an engineering concept generally used for determination of capacity of individual movements at unsignalised intersections. Majority of studies on critical gap estimation are reported from homogeneous traffic conditions where lane discipline and the rule of priority are truly respected. Vehicular interactions and drivers’ behavior at unsignalised intersections under mixed traffic are very complex. In this study critical gap is estimated by some of the existing methods like lag, Harder, logit, probit, modified Raff and Hewitt methods, at two T-intersections located in the southern part of India. The results show that values of critical gap estimated are as low as 1.60 s and there is a significant variation (12%–38%) among the values estimated by different methods. This highlights the incapability of the existing methods to address the mixed traffic conditions. An alternate procedure of estimation of critical gap making use of clearing behavior of vehicles in conjunction with gap acceptance data is proposed. Incorporating clearing time aspect gives more realistic estimation of critical gap and helps in accurately estimating the entry capacity.

Critical gap through clearing behavior of drivers at unsignalised intersections

The goal of the AASHTO Strategic Highway Safety Plan is to reduce annual highway fatalities by 5,000 to 7,000. This goal can be achieved through the widespread application of low-cost, proven countermeasures that reduce the number of crashes on the nation's highways. This twelfth volume of NCHRP Report 500, "Guidance for Implementation of the AASHTO Strategic Highway Safety Plan," provides strategies that can be employed to reduce the number of collisions at signalized intersections. The report will be of particular interest to safety practitioners with responsibility for implementing programs to reduce injuries and fatalities on the highway system.

Guidance for implementation of the aashto strategic highway safety plan. volume 12: a guide for reducing collisions at signalized intersections

Intersection sight distance (ISD) is intended to provide drivers at or approaching intersections with an unobstructed view of the entire intersection and of sufficient lengths of the intersecting highways to permit the approaching drivers to anticipate and avoid collisions. This requires unobstructed sight distance along each approach to the intersection, as well as across clear sight triangles between the approaches. The research obtained field data on driver behavior at intersections, particularly with respect to speed selection, acceleration and deceleration rates, gap acceptance behavior, and vehicle stopping positions. These data were used to formulate recommended geometric design policies for sight distance at intersections with no control, with Stop control on the minor road, with Yield control on the minor road, with traffic signal control, and with all-way Stop control. The recommended sight distance policies for approaches to uncontrolled and Yield-controlled intersections are based on a stopping sight distance model, modified to incorporate field results on the actual speed profiles used by drivers approaching such intersections. The recommended sight distance policies for locations at which stopped vehicles must enter or cross a major road are based on the actual gap acceptance behavior of drivers observed in the field.

G L Gittings

Papers

Intersection sight distance