Practice Note 14/1 : Part 1 - Land Compensation Act 1973 - Background to the 'physical factors'.
The Valuation Office Agency`s technical manual covering all aspects of compulsory purchase and compensation.
1) This Practice Note seeks to summarise the various physical factors the adverse effect of which can give rise to compensation under Pt I LCA 1973. It is written largely in terms of physical factors arising from the construction or improvement of roads as this is likely to give rise to the main volume of cases under Pt I to be referred to valuers. The underlying principles will however be equally applicable to other major public works which give rise to claims under Pt I.
2) In terms of the assessment of compensation payable under Pt I the assistance which can be obtained from scientific data which may be available as to the effect of the physical factors on adjacent property has to be kept in perspective. It must always be accepted that whilst such scientific evidence may be of assistance to the valuer in reaching conclusions as to the impact of works it must take second place to the over-riding opinion of the valuer as to what the reaction of the market would be to the circumstances existing at the date compensation is to be assessed. Evidence provided by sales, settled claims and decisions in the rating and council tax context will ultimately provide an aggregation of evidence against which claims will have to be considered.
3) The commonly accepted definition of noise is ‘sound that is undesired by the recipient’. Sound is a periodic fluctuation of air pressure. The amount by which the pressure changes is termed the sound pressure and the rate at which the fluctuations occur is their frequency. The pressure fluctuations propagate through the air rather like the ripples widening over the surface of a pond when a stone is dropped into the water. Practically all sounds have components at many frequencies though some are mostly high frequency (screeches, whistles) and others are mostly low frequency (rumbles, booms).
4) The incidence of noise particularly in terms of traffic noise can be complex and constantly changing. It can vary in loudness, pitch and incidence over time, and in the case of major roads it will at any one point in time be the combination of numerous different noises from various sources. Except where traffic is continuous, noise and relative silence will be interspersed at irregular intervals, which may in itself be a source of annoyance.
5) The range of sound pressure for noise levels that are normally met is very great; that at the kerbside of a busy London street in the rush hour is ten-thousand times greater than that of a quiet whisper. The ear can encompass such a wide range because it does not respond in simple proportion in that the ears’ sensitivity decreases as the sound grows stronger recognising a multiplication of sound intensity rather than an addition. The decibel (dB) scale which is used to measure sound pressure reflects this type of response so that a tenfold increase of sound pressure corresponds to a range of 20dB on the scale. Thus taking the pressure of just audible sound and giving this a value of 1dB, a sound of 10 times that pressure has a level of 20 dB whilst a sound of a little more than three times the pressure of the just audible one has a level of 10 dB. The sound pressure at which a sensation of pain begins in the ear is about one million times greater than that of the quietest sound discernible and this has a level of 120dB.
6) The ear is not however equally sensitive to all parts of the sound frequency range. Sound level meters used to measure noise are designed so that they can differentiate between frequencies in a similar way to the ear (referred to as frequency weighting) and the scale most commonly used is the A-scale. Noise measured with such weighting is referred to as decibels on the A-scale (dB(A)). It has been found that this scale reflects fairly accurately average human perception over the frequency distribution normally found with road traffic noise. The dB(A) scale is logarithmic, so that loudness is doubled or halved by differences of 10 dB(A). On this scale differences of 2 to 3 dB(A) will seldom be significant but differences of 5 dB(A) or more are clearly noticeable.
7) Most road traffic noise at any given time falls between 40 and 90 dB(A). Other typical noise levels in ascertaining order are:
|Watch ticking:||20 dB(A)|
|A quiet dwelling late at night:||35 dB(A)|
|Quiet conversation at one metre||50 dB(A)|
|A car at 10 metres:||60 dB(A)|
|Telephone bell at 11/2 metres:||70 dB(A)|
|An unsilenced pneumatic drill at 71/2 metres:||95 dB(A)|
8) Noise recordings are normally made over short periods of about four to five minutes, repeated at hourly intervals where the aim is to establish the 24-hour pattern. Recorded noise is then ‘filtered’ in the laboratory to ascertain one or more of three quantities:
L10: the level of loudness in dB(A) not exceeded for more than 10% of the time; an indicator of noise levels close to the maximum;
L50: the same for 50% of the time; the mean level of loudness;
L90: the same for 90% of the time; an indicator of noise levels close to the minimum. The term ‘noise climate’ embraces the range of sound between L10 and L90 over any given period and thus represents the sound heard for 80% of the time. Most of the information available derived from field measurements in the case of road traffic are in terms of L10 because it has been found that the dissatisfaction experienced by occupants of dwellings varies in accordance with the peak noise level. For the measurement and prediction of noise from traffic the average L10 values for each hour between 6 am and 12 midnight on a normal weekday has been recommended by the Noise Advisory Council and adopted by the Government as giving satisfactory correlation with dissatisfaction. This is known as L10 (18 hour).
9) The methodology and measurement of traffic noise was set out in the Department of Environment London Survey (1965) known as the Wilson report. This report is still the recognised reference basis for the measurement of traffic noise.
Applying the criteria of the Wilson report to residential property, noise problems would be expected if the following noise levels are encountered:
a) an L10 noise level of 30 dB(A) is exceeded indoors during periods of sleep
b) a noise level of 40 dB(A) is exceeded during daytime hours
c) if noise levels are in excess of 45 dB(A) family activities will be aggravated and
d) in relation to the facade of a property if noise levels exceed 42 dB(A) at night and 68 dB(A) during the daytime problems can be expected.
It is well accepted that continuous noise level at a higher level can be less annoying than a continuously varying noise with a lower average noise level. It is important therefore to consider not only absolute noise levels but also the extent to which they vary.
10) There is one important source of noise for which dB(A) are not normally used, and that is aircraft. The sound generated by aircraft engines has predominant components in particular frequency bands and these can have a significant subjective effect for which A-weighted sound levels do not adequately account. Measurements are therefore made (in dB) in each of a number of restricted frequency bands; from these a total level is calculated giving due emphasis to the predominant components. The calculated total is called the ‘perceived noise level’, the units of which are PNdB.
Values of PNdB are generally higher than values of dB(A) would be for the same sound, and as a rough guide a difference of 13 can be assumed.
11) The term ‘source noise’ is used to indicate the additional noise arising as a result of the introduction of a new noise source into a locality which would be the case where a new road is constructed or an existing road is improved enabling it to carry substantially heavier traffic.
In the case of roads for equal traffic volumes source noise will generally be less acute in free-flow conditions than noise on congested routes regularly interrupted by signals or uncontrolled junctions. Noise will normally increase with gradient, but mean levels also vary appreciably with mean speeds. Since tyre noise becomes relatively more important as speeds rise the choice of road surface can have a significant effect on noise levels. Volume for volume there is also a substantial variation both in maximum loudness and annoyance with traffic ‘mix’. Heavy lorries are well known to be the major source of difficulty and commercial traffic on principal roads or motorways would generally be considerable. It has been found that the likely ‘kerbside’ peak daytime noise levels on such roads will be in the range of 78 to 83 dB(A) in terms of L10.
12) There is only a limited amount of information available as to the 24 hour distribution of noise of major urban roads though it is clear that with present traffic volumes noise falls to some extent in the evening and often very sharply between midnight and 6am. However as traffic volumes grow, differences between daytime, evening and night-time levels will probably diminish. This is because source noise rises rapidly with traffic volume up to a point which varies with mean speeds, but only very slowly thereafter. However, present levels of late evening and night noise on major urban roads still fall into the band where noise is highly sensitive to traffic volume.
13) The term ‘ambient noise’ is used to indicate the existing noise climate in a locality before the introduction of a new noise source. In modern society exposure to noise of all kinds is very common but traffic noise contributes more to this than any other factor.
Against this background of noise from all sources a new “source noise” will not be heard in isolation and in some cases will not differ substantially from what is already experienced. This will be particularly true with improvements to roads already very busy as noise only grows slowly after traffic volumes have reached given levels. In such cases there could occasionally be diminution of nuisance where traffic flows more freely and smoothly than before.
In considering the effect of a new source noise on adjoining property the principal question is not therefore how bad are the new effects, but rather, how much worse are the effects from the new source than those which previously existed in terms of ambient noise, and over what areas is it possible to distinguish the two clearly.
14) Noise diminishes as it spreads. Distances are a major factor but obstacles with unbroken surfaces such as banks, walls and solid fences can play an important part and the nature of the ground also has some effect. Houses constitute very effective barriers particularly when terraced or built close together. Property in a second row away from the noise source will seldom be exposed to noise levels that are unacceptable. Contrary to widespread belief however, tree belts are not much use as noise screens unless they are very wide and dense. A width of at least 50 metres is necessary to give a reduction of about 10 dB(A) and a high proportion of the trees and shrubs must be evergreen if the protection is to be maintained throughout the year. Because of the time taken for new planting to mature it is seldom realistic to regard new tree belts as a solution to the noise problem of motorways near to housing, although in some cases trees give substantial benefit by obscuring the view of the traffic which seems to render the noise less objectionable even though it is not actually abated by the trees.
It has been found that L10 attenuates by 4 dB(A) for every doubling of distance over unobstructed but non-absorbent ground. Patterns of unobstructed attenuation by distance are thus logarithmic so that where it takes 50 metres to achieve the first 10 dB(A), reduction the reduction over the next 50 metres and the one after that are only 5 and 21 /2 dB(A) respectively. Attenuation is slightly faster when sound travels over soft ground and an opposing wind can reduce sound levels by as much as 10 dB(A).
Internal noise levels
15) What is said about attenuation takes no account of internal noise levels. Whilst it is rarely possible to provide a wholly effective noise screen out of doors buildings themselves reduce the noise level within the building. Most walls and roofs do so by about 35dB(A) but doors and windows are far more vulnerable and these, particularly the latter, account for most of the external noise which enters buildings. A part-open window will normally reduce external noise by 10 to 15 dB(A) and a window fully closed by about 20 dB(A). Additional attenuations of at least 30 dB(A) can be achieved by various systems for sealing and double-glazing windows but alternative ventilation will normally be needed to achieve attenuations above 25 dB(A). With normal windows however attenuations of 10 to 15 dB(A) greater than would otherwise obtain can be achieved by substantially cheaper insulation systems not involving special machinery for ventilation (see also paragraph 23).
Prediction of noise levels
16) Where it can be expected that there will be a given flow of traffic on a new road, noise at source can be predicted with some confidence and a major practical point is that noise rises very little after traffic volumes reach a given level. A prediction of traffic noise in the period midnight to 6am however provides particular difficulties.
A fair amount is known about the way noise spreads with distance and about the extent to which it is reduced by the various obstacles it may encounter. Predictive systems have been based on this knowledge but none of these cover every situation, particularly in the case of road interchanges where there are other multiple noise sources, and in other unusual situations. The only practical way of predicting noise in the exceptional situation is by close analogy with existing situations of a similar type.
17) There are no statutory noise standards that are required to be met where development is carried out. However on the basis of research it has been established that at least half the occupants of dwellings affected by noise from free-flowing traffic will normally be dissatisfied when L10 externally exceeds 70 dB(A). On the basis of this evidence local authorities were advised that there must be a strong presumption against permitting residential development in areas which are or are expected to become subject to noise levels in excess of 70 dB(A) on L10 (18 hour) scale (DOE Circular 10/73 ‘Planning and Noise’ (January 1973) – subsequently replaced by PPG 24 ‘Planning and Noise’ (October 1994) in turn replaced by the National Planning Policy Framework (March 2012)). This level is regarded as constituting the limit of the acceptable rather than a standard of what is desirable and authorities are encouraged to adopt substantially more stringent noise standards for new development particularly in the case of noise sensitive development such as schools, hospitals, housing etc. In this context it will be noted that the noise level prescribed by the Noise Insulation Regulations 1975 SI 1975/1763 (as amended by the Noise Insulation (Amendment) Regulations 1988 (SI 1988/2000)) above which an entitlement to insulation against noise under section 20 LCA 1973 can arise is 68 dB(A) of L10 (18 hours).
18) As noted in paragraph 14 normal roofs and walls have a resistance of about 35 dB(A) and only rarely will the need to increase their resistance arise. However in practice the overall noise insulation value of a structure is heavily dependent upon the insulation value of the weakest points, that is airbricks, chimneys and flues and external doors and windows.
Except in the case of aircraft noise, chimneys and flues are generally shielded from the direct noise path and for this reason do not pose problems. Airbricks however can have a significant effect on noise resistance, a single airbrick in a normal domestic room being capable of halving the decibel resistance of the complete wall. Normally constructed entrance doors and windows have a similar resistance, that is about 5 dB(A) when wide open, 10-15 dB(A) when partly open and 18-20 dB(A) when closed. Doors are seldom open for long periods of time and can therefore, in practice, be considered to be about 5dB(A) more effective than windows. However it is extremely difficult to increase the resistance of external doors except by creating double-door entrances by constructing an enclosed porch or lobby or by screening the door from the noise source. Where the thickness of glass in windows is increased and they are sealed and a powered extractor fan is installed their resistance can be increased to 30 dB(A) but whilst this method of insulation may be acceptable in kitchens and bathrooms it is not usually acceptable in living rooms where the noise of the fan can be as annoying as the external noise source. Similarly, while fixed glazing may be acceptable on ground floors it is rarely so at any other level because of cleaning difficulties. Forms of double windows with staggered opening lights can increase insulation from 20 to 40 dB(A).
19) Two of the main pollutants in the air are smoke and sulphur dioxide. Smoke is considered to be offensive when it becomes noticeable to the eye and nose. Sulphur dioxide in the concentration normally found in this country is invisible and rarely noticeable to the nose but with other pollutants that may not be instantly recognisable may still be potentially harmful.
20) Most reactions to air pollution arise in situations in which it creates annoyance, apprehension and fear, when people start worrying about their health or because it is seen as a threat to their property and possessions. In the past the degree of pollution tolerated was more than at present but economic and technical considerations will continue to make some degree of pollution inevitable.
Pollution by motor vehicles
21) Road traffic pollutes the atmosphere in two main ways
- through the emission of chemically active particles and
- through the spreading of dust and dirt
The first of these factors has caused considerable concern and has been widely studied; the second has attracted less attention but can still be the subject of widespread complaint by occupiers of residential property.
Chemical pollutants from traffic
22) Chemical pollutants from vehicles come almost wholly from exhaust gases; petrol and diesel engines emit similar products but in different quantities and proportions. Properly maintained and used diesel engines pollute the atmosphere much less than petrol engines; when used incorrectly they are liable to produce dense smoke which offends the sense of smell as well as sometimes spreading chemically active particles.
Over 100 chemical compounds have been identified in vehicle exhaust gases of which carbon monoxide, oxides of nitrogen, lead compounds, carbon particles (smoke) and organic compounds produced from petrol are regarded as the most important from an atmospheric pollution point of view. Most occur in minute quantities but carbon monoxide, lead compounds, oxides of nitrogen and the effects of smoke are all relatively more significant.
23) Most obviously poisonous is carbon monoxide which has serious and sometimes lethal effects in high concentration. This comes almost entirely from petrol engines which emit appreciable quantities, even at steady cruising speeds. Earlier evidence did suggest that to produce any physical symptoms however, carbon monoxide must be present in concentrations of well over 50 parts per million, which is a good deal higher than is found even in congested city streets. A recent survey of busy city streets in Britain seldom produced carbon monoxide concentrations of over 30 parts per million, and even in the worst situations dispersion may usually be expected to keep concentrations around houses well below city street level.
Oxides of nitrogen
24) There are no controls at present on emission levels of oxides of nitrogen and little is known about their effects on health. The two commonest compounds are nitric oxide and nitrogen dioxide, the latter being several times more toxic than the former. The former is produced in far greater quantities from vehicle engines but it oxidises into the latter, so that the ratio of the latter to the former in the air is about 2:1. However the normal levels found in city streets are only about 1% of the limits of concentration allowed for 8 hours industrial exposure and no more than 10% of that known to have any effect on animals in laboratory experiments.
25) Vehicle exhaust gases are the main source of lead in the atmosphere in urban areas. This comes mainly from the “anti-knock” constituents of petrol which to a large extent pass straight through engines into the atmosphere. The lead concentration in the air of city streets is some 20 times greater than that found in remote rural areas but is still only one to two per cent of the limit set for 8 hours’ industrial exposure.
26) Smoke formed from carbon is not thought seriously toxic although it is obnoxious and being clearly visible often forms the centre of complaint. In Britain the Construction and Use Regulations control excessive emissions. Dispersion of smoke is very rapid but other emissions themselves toxic can be carried by smoke particles and possibly adversely affect some respiratory conditions though knowledge of these processes is at present small.
Dust and dirt
27) The second main source of pollution from road traffic is that arising from the spreading of dust and dirt but little research has been undertaken in this field. Three social surveys made by consultants on behalf of the Urban Motorway Committee which included questions on dust and dirt from major roads produced varied results. Two of these were conducted in places with ambient atmospheres far from clean and in each of these cases dirt and dust were the most frequent cause of complaints about the effect of the road. The third, however, which was conducted in an area with a much cleaner atmosphere produced relatively few complaints on this account.
The main sources of road dust seem likely to be:
a) direct fall-out from the atmosphere
b) spreading from soft verges and central reserves, and from gardens or round-a-bouts
c) from derelict sites blown by wind
d) from lorries, particularly those serving construction sites
Samples of road dust subjected to analysis contained virtually no rubber or asbestos. It is concluded therefore that abrasion of tyres contributes little to dust on roads, and abrasion of clutch and brake linings less still. The abrasion of the road surface is also thought insignificant in this connection and the major sources are therefore held to be the “natural” deposits described above.
28) Vibration is experienced in many ways and, for example, in industry or with bus and rail traffic, it is expected and largely accepted; where induced by road traffic or other construction works, however, it is increasingly resented. Concern often focuses on damage to ancient buildings which may be identified in the short term or may not be identified but arises as a result of long term exposure to vibration, however, other grounds for complaints arising from the effects of traffic and such in the home are now fairly common. Vibration may be either ground-borne or airborne. A good deal is known about the former but relatively little is known about airborne vibration.
29) Vibration is always present in moving vehicles; the tyres and suspension help prevent this being transmitted into the ground. Ground vibrations arise from loaded wheels passing over irregularities in the road surface and when these are more than 2.5cm deep the resulting vibration is likely to be felt in the vicinity. Vibration increases with vehicle speed and axle load though not in a simple fashion.
The manner in which vibration is transmitted to the ground is very complex and there is no clear understanding as to the degree to which attenuation of vibration takes place over given distances. Experiments suggest there could be as much as 85% attenuation of vibration at 50 metres from the source.
30) Buildings respond to vibration in different ways and it should be remembered that vibrations similar in kind and extent to those excited externally may be caused by normal internal usage. Little authenticated direct data exists on vibration damage to property due to traffic, and judgements on likely damage threshold from traffic induced by vibration therefore rest almost wholly on analogies with vibration from other sources, notably blast damage. Little is known about fatigue effects arising from repeated and prolonged exposure to induced vibration nor about the extent to which vibration when superimposed on an already high stress level may trigger off physical chain reactions leading to structural or other damage.
New buildings may be insulated from vibration through the ground by mounting on flexible blocks but this process though very well-established is expensive and likely to be used only exceptionally.
31) The nature of airborne vibration is not yet well understood. It is thought to arise from low-frequency sound waves coming almost wholly from vehicle engines, and in particular from diesel engines which have a much higher proportion of low sound frequencies than those fuelled by petrol. Engine size and design may be significant factors though models of the same power vary widely in their effects and confident generalisations about causes cannot be made.
The effects of airborne vibrations on building are unlikely to be damaging. The main effect is to make windows and doors rattle and shelves vibrate. Pulsations can also be set up inside buildings sufficient to cause floors and partitions to vibrate to a perceptible extent. These vibrations will obey the usual laws of sound but because they involve low frequencies and long wavelengths they attenuate relatively little with distance insulation or screening.