Roundabouts V. Signalised Junctions: An Urban Case Study in Ireland Analysing Traffic Flow, Safety, Air Quality and Public Perception
Eoghan Clifford, NUI Galway, Rowan Hynes, NUI Galway, Richard Manton, NUI Galway
Examination of roundabout replacement with signalised junctions in the context of traffic flow, safety, air quality and public perception. Includes a case study in Ireland.
Roundabouts have been found to be effective in many areas and are generally recommended in urban areas where traffic speeds and congestion are low. In such circumstances, roundabouts minimise delay by limiting ‘lost time’ when compared to signalised junctions. Furthermore, through-traffic in roundabouts does not block right turns and roundabouts do not rely on technology which may malfunction.
Signalisation of junctions that are part of heavily-trafficked corridors can confer many benefits on users and traffic management units. In such corridors, delay can be minimised by sequencing ‘green times’ and prioritising legs of the junction. Using advanced traffic analysis software, layout in time and space can be optimised. Automated signal control can improve traffic flow but also be used to ensure emergency vehicles are not impeded by traffic. Furthermore, roundabouts have come under scrutiny due to danger posed to pedestrians and cyclists, and intersection design must reflect the increased policy priority placed on the promotion of these active modes.
This paper analyses the results of a major junction restructuring programme in Galway City in Ireland. Galway City has become known as a ‘roundabout city’ due to the proliferation of this roundabout junction design on its major access corridors. The city is nearing completion of a junction restructuring programme, which has been running since 2008, whereby roundabouts on major access corridors are being replaced by signalised junctions. Furthermore improved cycle, pedestrian and bus facilities have been put in place as part of this programme. Galway City,with a population of 75,000 people, is the third largest city in Ireland. Bisected by the River Corrib, the majority of residents live in the west of the city and the majority of employment is found in the east of the city. This settlement pattern has resulted in large east-west traffic flows across the city’s three bridges, with one bridge in particular (the N6 road) being a particular bottleneck.
This paper analyses roundabout replacement under four headings: (i) traffic flow and congestion, (ii) safety, (iii) emissions and air quality, and (iv) public perception. Traffic flow has been analysed using city traffic volume data for the case study junctions and surrounding ‘rat runs’. Initial results show that signalisation has resulted in junctions experiencing greater traffic volume, less delay and less congestion. Furthermore, traffic on surrounding ‘rat runs’ has been reduced. Safety implications of roundabout removal were assessed using a combination of collision data and perceived safety ratings. Signalisation has resulted in improved perceived safety and the data to date indicates a lower risk of collision; with increased safety for vulnerable road users in particular.
Air quality (PM10, heavy metals and benzopyrene)was measured over a two year period at the busiest junction along this corridorand the results are currently being analysed. These results will be combined with models of vehicle emissions stopped at traffic signals compared to vehicle moving through a roundabout. These results will have important implications for pedestrians and cyclists who are vulnerable to poor air quality at junctions. Finally, public perception of roundabout replacement was studied using a public survey. This found that support for the programme was mixed and varied according to user type – for example, bus drivers supported changes while taxi drivers opposed.
The completion of the roundabout replacement programme has been delayed in Galway City due to opposition to the removal of one roundabout in the corridor. The results indicate thatreduced congestion, improved air quality and improved safety were achieved due to the junction restructuring. Thus it is important that traffic engineers engage the public with scientific data to encourage public support for infrastructural improvements.
Association for European Transport