Agenda item

(Report by Director, Kent Highway Services)

(1)     The report set out a methodology for prioritising the County Council’s investment in quiet surfacing schemes and put forward a robust and transparent process for determining the priority of re-surfacing quiet surfacing schemes within Kent.  The prioritisation methodology considered both the potential noise benefits associated with re-surfacing a given section of road, the impact that would have on local residents and the cost of undertaking the resurfacing scheme; thus providing an indication of the cost-benefit of each scheme.

(2)     Road traffic noise was widely considered to be a genuine nuisance to those people who lived, worked and attended school or college in the vicinity of a culpable road(s).  In the most prominent cases, constant or regular exposure to high levels of traffic noise could have a detrimental impact on the quality of life experienced by local people and could potentially result in a significant proportion of the population being bothered by it.  Although KHS had no influence over vehicle tyre types or engine manufacturing, it could take steps to reduce vehicle type noise by specifying the type of road surface installed at a particular location and thereby seek to improve people’s quality of life.

(3)     Noise was measured in units called decibels (dB).  As traffic noise fluctuated continually, it was necessary to define it in a manner that could be related to the subjective response of those experiencing it.  Attitude surveys had indicated a relationship between the annoyance caused by traffic and the sound level exceeded for 10% of the time during an 18 hour period between 0600 and 0000 (midnight).  Road traffic noise was generated as vehicles travelled along a road and was a major contributor to environmental noise exposure to premises, including residential properties, located nearby.  The main sources of road traffic noise were:-

·               Engine and Powertrain Noise; engine and vehicle exhaust noise tended to be the dominant source of noise when traffic was moving slowly, particularly when traffic comprised a large proportion of heavy goods vehicles.

·               Tyre Rolling Noise:  tyre noise was likely to be experienced when traffic was free flowing, especially at moderate to high speeds.

·               Body Rattle Noise: this noise occurred when a vehicle, particularly a heavy goods vehicle, passed over a traffic hump, pothole or sunken trench in the carriageway; thus causing the vehicle body, and sometimes its goods, to rattle.

·               Other Factors, these included; vehicle speed, volume of traffic, heavy goods vehicle composition, gradient of the road and surface type.

(4)     The proposed methodology for predicting road traffic noise was taken from an existing approach provided by the Department for Transport’s (DfT) Calculation of Road Traffic Noise (CRTN).  The method was based on acquiring accurate traffic information relating to the traffic composition and vehicle speeds over a set period.  Additional information was also collected and included; road gradient and surface type and could be acquired using the JCAM (carriageway condition survey) technology.  The CRTN deemed that valid noise prediction levels could be made within 300m of a road; therefore this determined that properties within this distance of a targeted road would be included.

(5)     The proposed methodology also provided an ability to measure the influence of the road surface on noise generation and allowed for a comparison of different surfaces and standard measurement conditions.  The approach had been incorporated into the noise test provided by the Highways Authorities Product Approval System (HAPAS) and was currently used in the UK for the approval and certification of road surfacing products for use on public roads.  Therefore, a comparison could be made for given road surface type against that for a standard Hot Rolled Asphalt (HRA) surface. 

(6)     Studies had shown that surfaces with a smaller aggregate size were quieter.  This was as a result of the different vibrating frequency as the tyres passed over the surface.  For example, a surface dressing with a 6mm aggregate had been shown to be approximately 2dB(A) quieter than one with a 10mm aggregate and approximately 2.5dB(A) quieter  than one with 14 mm aggregate.  It should be noted, however, that in some cases of surface dressing, by reducing the aggregate size, a loss of durability might occur.  Modern TSCS (also know as Quiet Surfacing) could be significantly quieter than dense HRA and surface dressed surfacing of the past.  The influence of a road surface on noise would also be determined by vehicle speed; generally speaking, the greater the vehicle speed, the greater the level of noise reduction.

(7)     TSCS provided a surface that was safe under wet skidding conditions as water on the road could escape from beneath the tyre patch through interstices in the surface as well as the texture depth and type tread.   There were no known incidences of higher rates of pedestrian crashes as a result of quieter surfacing being applied.

(8)     The proposed methodology also sought to quantify the proportion of people “bothered” by road noise.  The Highways Agency’s Design Manual for Roads and Bridges (DMRB) provided industry standard noise nuisance levels for human response to noise.  It provided the percentage of people annoyed by road traffic noise, defined as ‘bothered very much or quite a lot’.  The relationship between nuisance and noise was based upon research undertaken by TRL.  Using this relationship, the total number of people bothered by road traffic noise in a given scenario could be predicted.

(9)     Information relating to the existing road surface could be acquired via the JCAM carriageway condition survey and integrated with the noise reduction level at the analysis stage in order to determine the scope of reducing road traffic noise.   The key stages of the assessment methodology comprised:-

·               Based upon the predicted noise level, noise nuisance levels for each receptor would be calculated using the tables contained within DMRB.  For residential properties an average occupancy of 2.36 would be assumed.  Where schools were present the approximate number of pupils would be confirmed.

·               The total number of people bothered by noise would be calculated for the two scenarios  and compared. 

·               The cost of resurfacing the road would be calculated and divided by the reduction in people bothered by road traffic noise for each scheme.

·               All sites proposed for resurfacing were then ranked with those providing greatest noise benefits and lowest cost being preferable to those with least noise benefits and highest costs.

·               The priority list would be presented to the Board on an annual basis for their recommendation to approve the programme of works.

(10)   In order to rigorously test and calibrate the proposed methodology and the noise prediction model component, a brief Pilot Study was recommended.  The study was planned for the Summer 2008 and would identify two suitable roads and seek to quantify potential noise reduction levels at each site and prioritise where investment should be made based on the reduction in people bothered and the cost-benefit at each scheme.  If supported, the results of the survey would be presented to the Board.

(11)   The Board supported:-

(a)     the proposed methodology for prioritising quiet surfacing schemes; and

(b)     a Pilot Study, which would run during the Summer 2008.