When Should Ghost Islands Be Provided at Priority Junctions, and the Application of DMRB Standards on Local Roads in the UK



When Should Ghost Islands Be Provided at Priority Junctions, and the Application of DMRB Standards on Local Roads in the UK

Authors

Mr Steven Windass BSc(Hons) MSc(Eng) MCIHT MIHE, Local Transport Projects Ltd

Description

Research into the provision of ghost islands at priority junctions on local roads in the UK, with an appraisal of the existing DMRB guidance, and also consideration of the suitability of applying other DMRB standards on local roads.

Abstract

The ‘Design Manual for Roads & Bridges’ (DMRB) outlines the mandatory requirements for a ghost island right-turn lane at priority T-junctions on trunk roads in the UK, based on a threshold for minor road traffic flow of 500 vehicles per day (TD 42/95). However, more recent guidance for local roads (Manual for Streets 2) acknowledges that junctions without ghost island provision "will often be able to cater for higher levels of turning traffic without resulting in significant congestion". This paper provides an assessment of this junction feature, appraising the original DMRB guidance, and testing the assertion in Manual for Streets 2.

Utilising research undertaken as part of an MSc dissertation, this paper seeks to establish more applicable guidance for local roads in the UK, specifically targeting urban junctions with a 30mph speed limit and built up environs. Building upon a review of previous research projects, this study includes a modelling assessment of the two key design considerations relating to the provision of ghost islands: junction operation and road safety.

The operation of the two junction types (simple and ghost island) has been assessed utilising the empirical traffic capacity model developed by TRRL’s Kimber & Coombe (1980), as well as the vehicle delay model developed by Kimber & Hollis (1979). These models are incorporated into the PICADY software programme, which has been utilised to undertake detailed junction operation assessments.
The road safety performance of the two junction types has been analysed through use of the collision prediction model developed empirically by Summersgill et al. (1996).
The results of the research appear to indicate that ghost islands can provide capacity and delay benefits with respect to non-priority major road traffic, relative to a simple junction. However, the overall operation of a junction can be primarily influenced by the level of capacity and delay for the non-priority minor road traffic, and the results show that ghost islands can increase delays for this stream during the majority of the assessed scenarios. So ghost islands would be preferred to simple junctions for some traffic patterns, with the converse true for others, in terms of capacity and delay implications. These modelling assessments have been utilised to establish recommendations relating to junction operation, to provide a suitable starting point when considering whether to provide a ghost island at an urban priority T-junction on a non-trunk road in the UK.

In terms of road safety, my research indicates that ghost islands are likely to increase the number of collisions at a priority T-junction. There is, therefore, a case that ghost islands should not be provided at urban priority T-junctions on road safety grounds.

The recommendations from the paper aim to act as a guide to practitioners and early stage decision-makers to inform the decision as to whether a ghost island should be provided at a priority T-junction in an urban area, with no existing guidance for local roads in the UK. These recommendations are not intended to replace site-specific analysis, which should still be undertaken as part of the detailed junction design process to help balance the possible benefits in terms of reduced delay, against the expected disbenefits in terms of collision costs.

Given the results of the research, this paper also seeks to consider the suitability of DMRB standards on local roads, not just in the use of ghost islands, but also other design parameters.

Publisher

Association for European Transport