I’m going to talk about road noise projects. Any time a road is expanded, moved, modified, or a new one is built and there are residences, schools, parks, or churches nearby, a noise impact study needs to be conducted. The study looks to see if sound walls are justified.
Sound walls are the primary noise reduction measure used in attenuating noise. In some areas, the noise reduction from RAC (rubberized asphalt concrete) is allowed. You can move the road up, down or in some direction to reduce the noise, but many times that is not practical. In some cases were noise mitigation is warranted and sound walls are not feasible, buying properties or improving exterior of a home is done. Generally by improving the windows.
However, more likely a sound wall is the most practical option. Usually concrete tilt up panels are used because of their durability but any solid, airtight, continuous barrier could be used – such as wood, vinyl, a berm, rock, …
Road Noise Study Procedure
In conducting a noise study you start by evaluating the site to identify the representative noise sensitive receivers (homes/schools/parks/libraries/churches). If there is a group of homes, you want to make sure you evaluate the most impacted home, if there is no problem at that home, then the group should be fine. If the noise levels of one you evaluate is high, then you need to evaluate others in the group to determine the extent of the impact.
The evaluation procedure is generally as follows:
- Predict the existing noise level from the existing roads at representative homes/parks/schools during the loudest hour (usually rush hour). This needs to be done using free software provided by Federal Highway Administration (FHWA) called Traffic Noise Model (TNM).
- Measure the noise levels at the representative homes during rush hour to make sure the model is correct. Make adjustments to the model if needed.
- Predict the future worse case hour noise condition TNM.
- Compare the predicted noise levels with the criteria – to see if noise mitigation needs to be evaluated.
- If mitigation needs to be evaluated, determine the needed size of the barrier and see if it is reasonable
First you need to determine who is funding the road change. Is it a federally funded project, state funding, county or city? With that information you will know what regulations you need to follow.
Most all road noise regulations use the Federal Highway Administration’s (FHWA) Noise Abatement Criteria (NAC) as the basis for their regulation. For residences, churches, parks, hospitals, schools, etc, the FHWA noise limit is approaching 67 dBA (that is the one hour average noise level during the peak noise hour – rush hour) – so 66 dBA or less. There is a second criteria that the noise level can not increase by more than 5-15 dBA.
Each state, county, and city will have their own definition of approach and allowable increase. In Arizona, ADOT says that the limit is 64 dBA – but the county I am in says that 66 dBA is the limit. The county here says that you can give a 3 dBA reduction for using RAC (rubber asphalt concrete) the state does not allow you to take any deduction. They both consider a substantial increase as 15 dBA (for the second part of the rule). So you need to know whose guideline you need to follow.
I have tested RAC, it attenuates the wheel road interaction noise but does nothing for engine noise. It is really effective on flat roads or down hills and better if most of the vehicles are cars. It is less effective on inclines and on trucks and motorcycles which have loud motor noise.
Once you know if there are homes that will be impacted (have high noise levels), you need to see if noise mitigation (usually barriers) is feasible and reasonable.
Each jurisdiction will have different rules about this but in general:
- The barriers will have a height limit of 10-20 feet
- If only one residence/church/park/school/etc is helped, the barrier is not considered reasonable
- There will be a cost limit for a barrier. You need to calculate the height and length of the proposed barrier that will provide noise reduction to these impacted homes and multiply that area by the cost per square foot that the regulation states (such as $35/ft2). Then you determine how many homes are being helped and you calculate the cost per benefited receiver. The guideline will state what the dollar limit, such as $49k per benefited receiver.
- A benefited receiver is one that gets at least 5 dBA noise reduction from the barrier
- Some regulations will require a receiver or a percentage of receivers get at least 7 dBA noise reduction
- Mitigation is for only the first floor of multi-story residences
- No mitigation will be provided for undeveloped properties unless building permit issued prior to the final EAMR document
If the barrier is determine to be feasible and reasonable then a majority of the impacted property owners must approve the mitigation.
Traffic Noise Predictions
Here are a few rules of thumb.
- If you double the traffic volume, you get a 3 dBA increase in noise.
- If you double your distance from the road, you get about 3 dBA lower.
- If you block the line of sight between the noise source (vehicle) and receiver (person’s ear), you will get a 5 dBA noise reduction.
- If you completely block half of the road, you will get a 3 dBA noise reduction.
- Usually a break in a barrier (such as for a driveway) will render it ineffective (providing less than 5 dBA noise reduction).
The TNM program takes the road geometry, vehicle speeds, the mix of vehicles (cars, medium trucks, heavy trucks, motorcycles), topography, ground type (grass, hard soil, …) and the location of the receivers.
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