Gudelines fort he assesment and evaluation of env. impacts of road projects

7/30/2019 Gudelines fort he assesment and evaluation of env. impacts of road projects

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GUIDELINES FOR THE ASSESSMENT AND EVALUATION OF

ENVIRONMENTAL IMPACTS OF ROAD PROJECTS

Spyros PapagrigoriouENVECO S.A.

ABSTRACT: Environmental Impact Studies of road projects must include specific information

about: the technical and operational characteristics of the project, the description of theexisting situation of the environment within the project area as well as the area that will bepotentially influenced, the environmental pressures on the natural and man-madeenvironment, including the use of atmospheric emissions and noise emissions models, theconsideration and environmental assessment of alternatives, including the reasons behindthe selection of the proposed solution and finally, the implementation of technically feasibleand economically viable mitigation measures of likely environmental impacts

Key words: Road projects, environmental impacts, alternatives, mitigation measures


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1. INTRODUCTION

Road construction projects include the construction,realignment and / or widening of motorways,express roads, regional, urban, or peripheral roadsas well as the construction of tunnels or bridges.

In particular the proposed specifications of

Environmental Impact Studies of projects mustinclude information about the following:

The technical and operational characteristics ofthe project

The description of the existing situation of theenvironment within the project area as well asthe area that will be potentially influenced

The environmental pressures on the natural andman-made environment, including the use ofatmospheric emissions and noise emissionsmodels

The consideration and environmentalassessment of alternatives, including thereasons behind the selection of the proposedsolution

The implementation of technically feasible andeconomically viable mitigation measures of likelyenvironmental impacts

Environmental Impact Studies in the EC areundertaken according to the specifications of ECDirective 97/11 on the assessment of the effects ofcertain public and private projects on theenvironment. The specifications of the Directive97/11 are quite general and sometimes vagueconcerning the information that should be includedin an EIS.

In addition, each type of project and activityrequires a different methodological approachconcerning the issues that should be investigated.As a result many EU countries have developedspecific guidelines which address the information tobe included in an EIS of various categories ofprojects and activities.

In Greece, Directive 97/11 was enforced byLaw 3010/2002 whereby projects are grouped inthree categories (A, B, C), according to their scaleand likely impacts on the environment. JointMinisterial Decision 15393/2332/202 provides a

classification of projects belonging in categories Aand B into ten (10) categories, one of them beingroad projects.

Greece has recently prepared specificguidelines for the preparation of EIS documents foreach specific category. These guidelines will soonbe included in the legislative framework of EIA. Thispaper present a summary of the above mentionedguidelines concerning the part of assessment andevaluation of environmental impacts of roadprojects.

2. ASSESSMENT AND EVALUATION OFENVIRONMENTAL IMPACTS OF ROADPROJECTS

Environmental impact analysis for road projectsconsists of comparing the expected changes in thebiophysical and socioeconomic environment with

and without the project. For each type of potentialimpact or environmental concern, the analysisshould predict the nature and significance of theexpected impacts (some may be quantitative, othersqualitative), or explain why no significant impact isanticipated. Analysis of potential environmentalimpacts should include specific discussion of themagnitude and duration of impacts.

The predicted impacts of a road project fallinto three categories:i) direct;ii) indirect; andiii) cumulative.

These three groups can be further brokendown according to their nature, into

positive and negative impacts

random and predictable impacts

local and widespread impacts

temporary and permanent impacts

short- and long-term impactsIn general, the anticipated impacts on the

environmental parameters that are mentioned in thebaseline condition must be discussed. However,emphasis should be given on the impacts on soil,water resources, fauna and flora, atmosphericenvironment and acoustic environment.

2.1 Soil2.1.1 Impacts and settingSoil is an important component of the naturalenvironment, and is a primary medium for manybiological and human activities, including agriculture.Its protection in relation to road developmentdeserves considerable attention. The costs ofcorrecting problems are often many times greaterthan the costs of simple preventive measures.

Soil-related impacts of road development

include: Loss of productive soil

Erosion (destabilization of slopes, side-tipping ofspoil materials, waterflow diversions)

Contamination of soil

2.1.2 Nature and scale of impactsWhenever there are indications of potential soilproblems at the outset of the assessment, an initialreconnaissance survey of the site with roadengineers and soil scientists should be carried out.

Soil surveys are a valuable tool in assessingsoil erodibility, which itself is affected by soil

properties such as particle size, structural stability,organic matter content, clay content, and chemicalmakeup.

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Consideration should be given to thewatershed as a whole. Potential erosion problemscan often be predicted by observing local streamwater flow during and immediately after stormevents.

Evidence of soil problems on previous orsimilar works is a useful indicator for anticipating the

impacts of the current project. Consultation with thelocal population is also a vital part of assessingexisting and imminent erosion problems.

Much of the information necessary for theidentification of potential impacts can be obtainedfrom maps (geological, hydrological, pedological andtopographic) as well as aerial photographs.Geographic information systems can be extremelyuseful in bringing together, and considering spatially,information from a variety of areas. For example,thematic maps of soil type distribution, rainfall,vegetative cover, slope aspect, and drainage basinboundaries could all be overlaid on a representation

of the projected road alignment to highlight potentialproblem areas for soil erosion.

2.1.3 MitigationThere is a wide range of techniques designed toreduce the risk of damaging the soil and to fit theproject into its environment with minimal adverseeffects. Simple techniques such as replanting will beeffective in many situations, whereas moresophisticated techniques, such as retaining walls,are used only in the most difficult cases.

2.1.4 Reducing impacts on soils: An action

checklistThe more important steps of the EA process relativeto the incorporation of soil considerations into theroad development and planning process are asfollows:

Determine baseline conditionsCollect relevant data to identify the sensitivity of theroad and surrounding areas works to erosion,instability and contamination. Soil type, vegetationand land use are particularly important points toconsider.

Examine alternative routesMake informed route choices, thereby avoidingareas with a high risk of erosion or slope instability,or which are particularly sensitive to contamination,where feasible.

Select preferred designChoose the design which best limits potential forimpacts and gives priority to mitigative measuresthat are easy to implement and require only localmaterials. Consider form and incline of slopes,design of the drainage network, replanting methodsand schedule, as well as ease of futuremaintenance.

Prepare mitigation plansSoil-sensitive mitigation plans should includemeasures to maintain and repair soil protectiveplantings, as well as drainage and slope-retainingstructures.

Environmental specifications for contractors

Contractors responsibilities regarding constructionand maintenance activities should cover such issuesas erosion control, prevention of fuel spills duringconstruction, and planting as well as timely wateringof plantings.

Consider legislationLegislation dealing with soil conservation should beconsidered. This might include limits on land usealong the road (for example, restrictions on grazingand harvesting of vegetation) and requirements forerosion control (especially in mountainous regions).

2.2 Water Resources2.2.1 Impacts and settingNo matter where a proposed road may lie, it mustintersect a drainage basin, and where thisintersection occurs, alteration of the local hydrologyis inevitable. Road development can lead to threetypes of modification to the natural hydrologicalenvironment; these are discussed below.

Surface water flow modificationRoads that intersect drainage basins generallymodify the natural flow of surface water by

concentrating flows at certain points and, in manycases, increasing the speed of flow. Depending onlocal conditions, these changes can contribute toflooding, soil erosion, channel modification, andsiltation of streams. These effects are often felt wellbeyond the immediate vicinity of the road.

Groundwater flow modificationRoad drainage and excavation can lower the watertable in surrounding areas, while embankments andstructures can raise the water table by restrictingflow. The potential effects include deterioration ofvegetation, increased susceptibility to erosion, loss

of water for drinking as well as agricultural use, andhabitat changes for fish and wildlife.

Water quality degradation (surface andgroundwater)Sedimentation, changes in biological activity instreams and on their banks, uncontrolledconstruction activities, and spills of chemicals andpollutants can all have adverse effects on roadsidewater quality. Chronic pollution of surface runofffrom exhaust emissions, pavement and tire wear,petroleum product drippage, and corrosion of metalsmay be issues on some very busy roads. Where oilor lignin is applied to gravel roads to keep dustdown, the likelihood of contamination is quite high.

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increase flood risks. Factors to be consideredinclude horizontal and vertical alignment, crosssection, slopes, drainage of subgrade andsurrounding area, as well as restoration of naturalsurface and underground flow.

Undertake careful planning and design at water

crossingsPlanning and construction of water crossings needsto be coordinated with local aquatic conditions suchas flow regimes, fish movement, and human use.Lack of planning can have severe long-term effects;for example, an improperly sized bridge culvertcould cause flooding, leading, in turn, to interruptionof fish migration, erosion, and siltation.

Water resources-sensitive mitigation plansMitigation plans should include proposals for specifictechnical measures, such as planting of clearedareas, installation of flow-speed dissipaters in

drains, and recommendations for subsequentmaintenance.

Water resources management specifications incontract documentsEnvironmental specifications for contractors shouldcover road design, bridge and tunnel construction,drainage installation, and any work-site plans whichmay affect water flows and quality.

Invoke laws, regulations and guidelines on waterquantity and qualityLegislation and regulations should be considered as

they affect drinking water intakes, irrigation watersupply, and wildlife habitat, possibly with thedefinition of protection perimeters; constructions infloodable zones; and discharge capable of changingthe quality of surface water or ground water.

2.3 Air QualityAir pollution from road traffic should be consideredfor all projects in which a new road or a change incapacity of an existing road is proposed.Furthermore, construction-related air pollution needsto be evaluated for every project undertaken.

Project-level assessment should identify thepollutants of greatest concern as well as thelocations where pollutants might exceed acceptablelevels. The causes of pollution within the trafficstream should be clearly identified, since these havea large effect, especially at the regional level, on thechoice of mitigation strategy. It is important todetermine whether pollution arises mainly fromgasoline or diesel vehicles, whether it can be tracedlargely to a specific vehicle type, and whether it isproduced by all vehicles of a given type ordisproportionately from a small percentage of badlymaintained vehicles.

2.3.1 Impacts and settingAirmass ContaminantsThe main products of the combustion of motor fuelsare carbon dioxide and water, but inefficiencies andhigh temperatures inherent in engine operationencourage the production of many other pollutantsof varying effect. The major pollutants of significance

to roadside air quality in vehicle emissions arediscussed below.

Nitrogen oxides (NOx).

Hydrocarbons (HC).

Carbon monoxide (CO).

Sulfur dioxide (SO2).

Particulates

Lead (Pb).

Aldehydes.

Secondary pollutants.In addition to emissions from vehicle

exhaust, dust can also have major impacts on

roadside air quality. This is especially true in thecase of unpaved roads, which make up a largeproportion of roads in less-developed regions.

Human healthThe health impacts of motor vehicle air pollution aredifficult to quantify and, hence, difficult to value ineconomic terms. In many cases, establishment ofdirect cause-and-effect linkages between localizedautomotive air pollution and specific illnesses isproblematic. However, evidence does stronglysuggest that exposure to several of the majoremission constituents is responsible for certain

health conditions.

FloraPlants, domesticated and wild alike, are affectedboth physically and chemically by air pollutants. Dustsettles on leaves and can interfere with pollinationand photosynthetic function if the accumulation issignificant. Acidification of surface water caninterfere with nutrient uptake by roots, thus affectinggrowth. Ethylene, a hydrocarbon, has a detrimentalhormonal influence on plant growth, while NOx, SO2and ozone can all cause localized death of leaftissue (leaf necrosis). Finally, plants can absorb

toxic pollutants such as lead from the air, making theconsumption of these plants hazardous.

FaunaAlthough most of the research efforts concerningpollutants effects on animals have focused onhuman health, some faunal health problems havebeen connected to air pollution. As in humans, theproblems are mostly respiratory in nature.Acidification of aquatic ecosystems has definiteimplications for the health of aquatic species.

Built environment

Objects in use by humans are vulnerable to airpollution on two fronts: staining and corrosion.Particulates are responsible for dirtying all manner of

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structures, including modern buildings, monuments,and cultural heritage sites.

Acid deposition associated with NOx andSO2 is especially destructive of limestone, marble, orlime mortar structures. Acidity originating in vehicleemissions is also blamed for deterioration of paintsand accelerated corrosion of metals.

2.3.2 Nature and scale of impactsAssessment at the project levelAssessment of the potential air pollution impacts of aproposed road development requires thedetermination of pollution levels:a) before the proposed development, i.e. for theexisting conditionsb) in the future, assuming the project does not goahead, andc) in the future, assuming the project does go ahead.

In all three cases, the assessment relies onthe following information, current or projected:

Traffic volume. The key factor in air emissions isthe traffic volume (measured as vehicle-kilometres per hour by vehicle type). Often anunderstanding of traffic peaks and their durationwill be required in order to make meaningfulprojections of emissions levels

Traffic composition. A percentage breakdown ofthe number of vehicles by type. Heavy trucksand buses are distinguished from lightpassenger vehicles, newer vehicles from olderones, and diesel-powered vehicles from thosethat are gasoline powered

Speed of traffic.Average speed of vehicles, withsome indication of the consistency of speed(degree of traffic congestion)

Dispersion dynamics

Vehicle emission levels, by major pollutant.Useful indicators might be mean annualemissions; hourly concentration peaks, and dailyvalue exceeded once a year.

Road surface. Whether the road is paved or notmakes a difference to the amount of dustgenerated.

Once the current and projected pollutionlevels have been determined, comparisons can be

made with industrial, regional, and nationalstandards for air quality.

Measurement of roadside pollutantsActual measurement of current emission levels isaccomplished at the roadside with the help ofexhaust gas analyzers; these measure CO,hydrocarbons, NOx and dust and particulate matter.

2.3.3 MitigationProject-specific design improvements to limit motorvehicle air pollution impacts include

selecting road alignments which avoid passing

close to housing, schools, and workplaces providing sufficient capacity to avoid traffic

congestion, even with projected increases in

traffic flow. Traffic management provisionsshould ensure that vehicles operate at peakefficiency in populated areas

avoiding placement of busy intersections, andtunnel vents and openings near housing,schools or workplaces

taking account of prevailing wind direction when

siting roads and road features, includingrefuelling stations, near population centres

avoiding steep grades and sharp curves whichwould promote deceleration, acceleration andshifting wherever possible

sealing high-use dirt roads, where they passthrough populated areas, to control dust

planting tall, leafy, and dense vegetationbetween roads and human settlements to filterpollutants

National and regional strategic andregulatory measures related to air pollution mayform part of an environmental action plan or an airquality strategy for a major city. They can havesome influence on the baseline conditions at theproject level, and so are relevant to individualprojects.

Measures could include policies,regulations, charges, and enforcement programscovering

vehicle emissions standards as well asinspection and maintenance requirements

retirement or retrofitting of high-consumptionand high-polluting vehicles

fuel technology and quality

pricing of motor vehicle purchase and use management of demand for motor vehicle travel

management of traffic efficiency, and

investment in better mass transport, such asbuses and trains.

2.3.4 Minimizing impacts on air quality: Anaction checklistRoad development has tremendous potential fordegrading the quality of nearby airsheds if properplanning is not implemented. The more importantsteps in the EA process, relative to considering andincorporating air quality concerns into road planning

and development, are highlighted below.

EstabIish baseline conditionsFive key steps are recommended:i) define study area according to the size of theairshedii) determine existing air quality levelsiii) identify the contributors (and their nature) to airquality degradationiv) identify the characteristics of the existing trafficstreams, andv) describe the sensitive valued ecosystemcomponents and the most applicable indicators of

emissions impacts.

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Predict future air quality conditionsPredict future air quality conditions for both no-project and project-completion scenarios,considering a number of alternative designs.

Select preferred alignment anddesignSelect a preferred alignment and design by

assessing air emissions for the no-project option inrelation to the net emissions (i.e. emissions withmitigative measures factored in) for each proposedalignment and design alternative. Such a processshould help to avoid serious impacts to sensitivevalued ecosystem components and should precludethe need for elaborate and costly mitigative andcompensatory measures, while still retaining aproject that fully addresses the original need.

Define mitigative measuresDefine air quality mitigative measures for inclusion inthe environmental management plan if the project

requires a full EA, or as part of a mitigation plan ifthe projects impacts are well understood and agood body of knowledge exists on the effectivenessof standard mitigative measures.

Prepare environmental specifications for contractorsand operatorsPrepare environmental specifications for contractorsand operators which incorporate the proposedmitigative measures into the project as distinct taskslinked to a timetable. These clauses should addresshow environmentally sensitive construction workshould be undertaken, and which follow-up

measures will be required. A similar format shouldbe used for the operators or proponents, but with afocus on maintenance, enforcement, and reportingto the regulatory agency.

Invoke legislationInvoke existing laws, regulations and guidelines onvehicle emissions or propose some to fill identifiedgaps. Legislation and regulations are an importantcomponent of an integrated air quality strategy andshould, in time, become the principal means used tocontrol air pollution.

2.4 Flora and FaunaRoad development continues to be a major player inthe overall reduction of biodiversity, and properplanning at the project level can go a long way inlimiting the loss, while still serving the transportneed.

2.4.1 Direct ImpactsHabitat lossThe consumption of land, and the consequent lossof natural habitat, is inherent in road development.Where new roads intersect habitat, the areaoccupied by the road itself, borrow pits, and quarries

is subtracted from the total habitat area available toflora and fauna,

Habitat FragmentationWhen a road cuts through an ecosystem, the sum ofthe two parts created by the cut is less than thevalue of the initial whole, even when the habitat loss

is ignored. Ecosystems are characterized bycomplex, interdependent relations betweencomponent species and their physical environment,and the integrity of the ecosystem relies on themaintenance of those interactions. By slicingthrough habitat, roads affect an ecosystems stabilityand health.

Roads tend to fragment an area into weakerecological sub-units, thus making the whole morevulnerable to invasions and degradation.Nevertheless, roads and natural ecosystems can co-exist if the relationship is built on careful p1anning.r

Corridor restrictionsMost animal species tend to follow establishedpatterns in their daily and seasonal movements. Theareas through which they travel on their way to andfrom feeding, breeding and birthing grounds, andbetween their seasonal ranges, are known ascorridors. When a road intersects or blocks a wildlifecorridor, the result is either cessation of use of thecorridor because animals are reluctant to cross theroad, an increase in mortality because of collisionswith vehicles, or a delay in migration which mayresult in the weakening or disappearance of anentire generation of the population. Unfortunately,

some animals are attracted to roads for variousreasons, including protection from predators, goodfood supplies, better travel conditions, and so forth.This often leads to accidental death and poaching.On busy roads, the death rate for the localamphibian or other slow-moving animal populationscan be as high as one in ten.

Aquatic habitat damageRoad development has perhaps its most seriouseffects on aquatic ecosystems. Erosion from poorlyconstructed and rehabilitated sites can lead todownstream siltation, ruining spawning beds for fish.

Constriction of flows at water crossing can make thecurrent too fast for some species. Alterations of floodcycles, tidal flows, and water levels can upsettrophic dynamics by affecting the life cycle ofplankton, and have corresponding effects on the restof the food chain.

Rechanneling of waterways is oftenundertaken as part of road construction to avoidflooding and make crossing structures simpler. Inthe process, natural streambeds are dug up anduseful obstructions, including large boulders, areremoved. The same applies to shade trees on thebanks. Frequently, the result is a straight,featureless channel, which may be an efficientevacuator of water, but has little in the way of the

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Rapid appraisalRapid appraisal is a useful tool for assessments inwhich the complexity of the environment is so great,or the time available so limited, that a full-scalestudy is not feasible. Rapid appraisal allows a greatdeal of varied information to be brought together and

synthesized in a relatively short period of time. Themethod blends modern scientific with traditionalknowledge instead of relying solely on quantitativeresearch and empirical results.

Useful sources of informationEvery assessment should make use of the variousdocuments available, including

topographic, pedological (soil), vegetation, andclimate maps

scientific and technical studies

research reports

aerial or satellite photographs

biological inventories and

computer databasesThis information may be available from

research centres, specialized institutions,universities, government departments, and otherproject offices. Geographic information systems mayalso be very useful in visualizing the spatialrelationships between ecosystems, the distributionsof their component species, and a proposed roadalignment. They may also have applications inmonitoring the effects of road development.

2.4.4 MitigationRe-engineering road cross-section designsRoad cross-section can be modified to reduce theimpact on the environment, for example, by usingnarrower widths, lower vertical alignments, smallercuts and fills, flatter side slopes, and less clearing ofexisting vegetation. Narrower rights-of-way andlower vertical alignment may make crossing easierfor animals that find roads a physical orpsychological barrier. Also, providing longer sightlines for drivers can reduce collisions with animalsby allowing more reaction time.

PlantingPlanting in road rights-of-way and adjacent areascan help to support local flora and fauna. In somecases, planting may provide additional habitats andmigration routes for local animals, while alsoguarding against erosion. Border plant species mayneed to be chosen for resistance to wind or fire insome areas.

Planting should be done wherever possiblewith native species, which are likely to require littlemaintenance and may prove beneficial inmaintaining ecosystem integrity. In cases wherenon-native species are deemed essential, careful

monitoring should be planned, to ensure that they donot compete too successfully with native speciesand spread uncontrollably.

Animal crossingsAnimal crossings can be used to assist the migrationof animals. At important crossing points, animaltunnels or bridges have sometimes been used toreduce collision rates, especially for protected orendangered species. Tunnels are sometimes

combined with culverts or other hydraulic structures.These measures are expensive and used

only at a few locations where they are both justified(by the importance of the animal population and thecrossing route) and affordable (relative to the cost ofthe project and the funds available).

FencingFencing or plant barriers can reduce the risk ofcollisions between animals and vehicles. In somecases, semi-permeable fencing is used, whichexcludes species that are more likely to be involvedin collisions while letting less problematic species

through.

Water crossingsAquatic ecosystems are particularly sensitive to roaddevelopment, and there are a number of ways inwhich the impacts can be lessened. Standing watercan be bridged instead of filled. Stream rechannelingshould be avoided as much as possible, but where itmust be done, efforts should be made to recreatelost channel diversity. Careful attention should bepaid to erosion control techniques nearwatercourses.

Culverted crossings should be designed with

the needs of migratory aquatic species in mind.Baffles might be installed to slow the flow enough toallow fish and others to swim against the current,and culvert bottoms should be set below the level ofthe stream bed. Pre-development streambedgradients should be maintained wherever possible.

Traffic control measuresReduction of the speed limit may reduce the rate ofcollisions between vehicles and animals. Somejurisdictions apply lower speed limits, particularly atnight and in areas of frequent animal crossings.Signs warning motorists of the presence of animals

in places where animal corridors cross the road mayalso help to reduce collisions. Roadside reflectorsmay be used to scare animals away from theroadway when vehicles approach at night.

2.4.5 Minimizing impacts on flora and fauna: Anaction checklistThe more important steps in the EA process relativeto the incorporation of concerns about conservingbiodiversity in the road development process arehighlighted below.

Collect relevant dataBaseline data should identify areas of ecologicalinterest within the study area. The identification

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specially installed for noise control or naturallyoccurring, can lower the impact of road noise.Vehicles tend to produce the most noise whileascending and descending steep slopes and whilerounding sharp corners; this means that roads whichincorporate these features will tend to be noisier atthose points.

Environmental factorsWeather conditions such as temperature, humidity,wind speed, and prevailing wind direction can play arole in determining how individual sites are affectedby road noise. Temperature and humidity determineair density, which in turn affects the propagation ofsound waves. Downwind sites are generallyexposed to greater noise levels than are sitesupwind of roads.

Ambient noise levels, associated withindustrial and other human activity, affect theperception of the magnitude of the road noise

impact. In areas with low ambient noise levels, thenoise from a new road development will generally bemore noticeable than a similar noise level would bein an environment with higher ambient noise levels.New roads in quiet areas or noisy trucks at night areoften perceived as worse than higher levels of noisein a busy area during the workday. On the otherhand, measured noise levels and potential healthimpacts are highest where traffic noise combineswith noise from other sources, possibly producing anunacceptable overall noise level.

Topography can also help determine noiseimpact. For instance, noise from roads occurring in

mountain valleys or canyons tends to be morenoticeable than that from a similar road on a flatplain, because noise is reflected off valley walls. Bythe same token, hills and knolls can act as naturalbarriers to noise if they occur between the road andreceptors. Above-grade roads, which are oftennecessary in flood-prone areas, tend to broadcastnoise over greater distances.

Spatial relationshipsPerhaps the greatest determinant of noise impacts isthe spatial relationship of the road to potential noisereceptors. The closer the road to receptors, the

greater the impact. The higher the populationdensity in roadside areas, the greater the number ofpeople likely to be receptors, and, consequently, thegreater the impact.

Traffic streamThe noise production of a particular traffic stream isdetermined by a number of factors:

the type of vehicles in the stream and their levelof maintenance

the number of vehicles passing per unit time

the constancy of flow -vehicles tend to be noisier

in stop-and-go traffic, and the speed of traffic flow -noisiest at high speeds

The relationship between traffic streamcycles and ambient noise is also important; ambientnoise levels are generally lowest at night, and iftraffic noise peaks at night, the impact will be great.Conversely, if traffic noise peaks at the same timethat ambient noise levels do, the effects will be lessnoticeable.

2.5.3 Assessing noise impactsAssessment of potential road noise impacts relies onan accurate evaluation of the baseline conditions, athorough knowledge of the characteristics of theproposed road development, and a goodunderstanding of how roadside activity will evolve inthe medium and long term.

Noise measurementNoise measurement specifications require definitionof the period of measurement, the noise parameterto be recorded, and the position of the recording

instrument relative to the road and adjacentproperties.

Noise level standardsNational standards may specify one noise level notto be exceeded for all types of zones (such asLeql2hr under 70 dBA) or, more realistically,different noise levels for different zones, such asindustrial, urban, residential, or rural areas. Lowerlimits are sometimes specified for nocturnal noise.Details of road noise standards are usually availablefrom national transportation agencies.

2.5.4 MitigationVehicular measuresMotor vehicle noise can be reduced at source, forexample through vehicle construction, selection oftires and exhaust systems, as well as vehiclemaintenance. Control of vehicle noise emissions canbe attempted using vehicle design rules and in-usenoise regulations and enforcement, subjects beyondthe project-level scope of this handbook.

Surface design and maintenanceThe application of a bituminous surface layer overworn concrete roadways is effective in reducing

frictional noise. The use of open-graded asphalt andthe avoidance of surface dressings may also beeffective in reducing frictional noise in sensitiveareas. Generally, smooth, well-maintained surfacessuch as freshly laid asphalt without grooves andcracks will keep noise to a minimum.

Road geometryRoad design should avoid steep grades and sharpcorners to reduce noise resulting from acceleration,braking, gear changes, and the use of engine brakesby heavy trucks at critical locations.

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Gudelines fort he assesment and evaluation of env. impacts of road projects

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