If 30 kilometres per hour speed limit signs are really observed, this can produce a better environment than if the same speed reduction is achieved through speed reduction measures. A smooth driving pattern reduces the levels of exhaust emissions and noise, while a jerky driving pattern has the opposite effect. Properly executed speed reduction measures also decrease the noise level. This decrease is however greatly dependent on what kind of driving pattern is achieved after these measures have been taken. The decrease will be less if the speed reduction measures give rise to an uneven driving pattern.
This report describes a project the aim of which was to increase knowledge of the overall environmental effects which a lowering of the speed limit from 50 to 30 kilometres per hour would produce in different street types and entire urban areas in Sweden. The results of this preliminary study may form the basis for planning future full scale tests. The report is based on an examination of the literature and the results of experiments conducted with the VTI survey car; driving patterns were observed and fuel consumption measured in some street environments incorporating speed reduction measures in the form of speed humps, elevated pedestrian crossings and narrowings. Even though it has not been possible within the framework of this project to give a full description of the environmental effects due to a reduction in speed limit from 50 to 30 kilometres per hour in urban areas, the following conclusions may be drawn:
• Several studies indicate relatively unambiguously that a smooth driving pattern reduces the levels of exhaust emissions and noise, while a jerky driving pattern has the opposite effect. However, driving pattern is influenced in a very complex manner by the properties of the street network and cannot be referred to a single stretch of road, with individual properties, in the street network. It is therefore important to analyse the effects of 30 kilometres per hour in urban areas with reference to a holistic perspective which takes the total traffic environment into account. There is agreement that, in relative terms, large geographically contiguous areas (e.g. housing estates, entire town districts) in which speed reduction measures have been introduced achieve greater reduction in exhaust emissions than measures in smaller areas of local character (single streets).
• If the effects of the speed levels 50/30 and 30/30 are to be assessed separately, it is a reasonable hypothesis that 50/30 generates more exhaust emissions than 30/30, one of the reasons being that 50/30 produces greater speed variation than 30/30.
• On the assumption that 30 kilometres per hour speed limit signs really have the effect of reducing speed, this should produce a better environment than if the same reduction in speed is achieved through speed reduction measures since excess emissions due to these measures are avoided.
• The results of several studies in which driving patterns were recorded in the test area itself indicate unambiguously that emissions of NOX, HC and CO are reduced when the speed limit is lowered from 50 to 30 kilometres per hour. This is particularly true in networks comprising both links and nodes, i.e. contiguous areas. What appears to hold throughout is that the greatest reduction occurs in emissions of NOX.
• The literature showed that properly executed speed reduction measures lower the noise level. However, this reduction is greatly dependent on what driving pattern is achieved after the measures have been introduced. The reduction will be less if these measures give rise to uneven driving patterns. Typical reductions in equivalent level are 2–4 dB for cars and 0–2 dB for heavier vehicles. The mean maximum level is generally lowered by ca 2 dB more.
• A general reduction in speed from 50 to 30 kilometres per hour on local streets by means of speed reduction measures may in the best case reduce the number of persons in Sweden who are exposed to A-weighted equivalent sound pressure levels between 61 and 65 dB from abour 350,000 to about 180,000. The corresponding figures for the interval 56–60 dB are 565,000 and 370,000 respectively. The assumption is that 80 per cent of the existing streets in the lower interval and 60 per cent of those in the higher interval have been improved in such a way that a reasonably smooth driving pattern has been achieved, and that the proportion of heavy vehicles on these streets is small. There will be no change in the noisiest streets since it is not expected that they will be affected by the speed reduction. The above decrease in the number of those affected by noise pollution is to be seen as wishful thinking. In view of the fact that, owing to various compromises, many of the speed reduction measures are not likely to be optimal, the decrease in the number of those exposed should perhaps be reduced, maybe by as much as 50 per cent.