A comparative empirical assessment of roundabouts operations in rural areas: impacts on congestion-specific vehicle speed profiles, pollutant and noise emissions

Abstract

Increasing concern about global warming and air quality has meant an increasing use of energetic and environmental indicators in roundabout design. The main objective of this paper is to compare different rural roundabouts in terms of traffic performance, pollutant and noise emissions through an integrated empirical assessment. Field measurements were carried out with a light duty vehicle in single-lane (SL), compact two-lane (CTL) and multi-lane (ML) roundabouts using Portable Emission Measurements Systems, OBD scan tool and Sound Level Meter, to measure real-world exhaust emissions, engine activity and acoustic data, respectively. Road traffic scene at roundabouts were obtained from video recordings. Afterwards, predictive discrete models that correlate the probability of speed profiles (no stop, stop once and multiple stops) with roundabout operational parameters were developed. This research tested the hypothesis that carbon dioxide (CO2), nitrogen oxides (NOx) and equivalent continuous A-weighted sound level (LAeq) were impacted by the differences in: 1) the approaching, conflicting and exiting traffic volumes; 2) the volume-to-capacity ratio (V/C); and 3) the roundabout layout. Although SL yielded the lowest CO2 per vehicle, its implementation resulted in high LAeq at low traffic volumes because vehicles drove at moderate speeds in the approach compared to other layouts. CTL was the worst option in terms of both CO2 and NOX. This paper shows a methodology that can be used in other roundabouts with similar layouts by simply identifying their traffic volumes and representative speed profiles; this can help researchers, traffic planners or practitioners on the optimization of roundabout designs.