Nenagh, in County Tipperary, lies on the main N7 national road from Dublin to Limerick. In 2000, the Annual Average Daily Traffic on the N7 going through Nenagh town centre was over 10 000 vehicles per day, 12% of traffic being heavy goods vehicles (HGVs). It was projected that this volume will more than double from 2000 to 2015.
A 10.6km bypass for the town was thus constructed and opened in July 2000. As well as saving up to 30 minutes at peak times for through travellers, the bypass aimed to improve the environment of Nenagh town centre, which was previously one of the most congested towns in Ireland.
Road traffic noise can cause annoyance to residents, so reducing this noise in the town was one of the environmental improvements brought about by the bypass.
The objective of the study was to evaluate the noise levels in Nenagh town centre before and after the opening of the N7 bypass.
Noise measurements were taken at five locations in the town in June 2000 (prior to the bypass opening) and October 2000 (after the opening). Four of the locations were on the (old) N7 through the town. For three of these locations, noise measurements had also been taken in 1997, for the bypass route selection process.
Noise was measured in A-weighted decibels: dB(A). In Ireland and the UK, the most commonly used parameter for the assessment of traffic noise is the L10 dB(A) level, i.e. the noise level exceeded for 10% of the time. The criterion used is the L10 (18 hour) dB(A), which is the mean of the hourly L10 levels in the period 06:00 hours to midnight. In the UK, legislation has a limit of 68 dB(A) L10 (18 hour) for new road schemes before noise abatement is required. This level is exceeded near most National Primary Routes in Ireland.
Noise measurements made on the (former) N7 road through Nenagh indicated a significant reduction in noise levels in the town after the opening of the bypass. The most significant reduction was between the hours of 04:00 and 07:00 on weekdays: this appears to be due to the early morning heavy commercial vehicles on the Dublin-Limerick route.