Can tranquillity measure be a tool to push greenery?

Researchers develop formula linking tranquillity to benefits of green infrastructure

Trees: numerous studies link tranquil environments with stress reduction, well-being, longevity and pain relief - image: Garry Knight
Trees: numerous studies link tranquil environments with stress reduction, well-being, longevity and pain relief - image: Garry Knight

The level of "tranquillity" that people feel in urban areas has been found to be related in a predictable way to the amount of greenery and the level of man-made noise around them, and this has been captured in a formula developed by British researchers.

While green infrastructure is known to bring a range of environmental, social, economic and health benefits, its effect on perceived tranquillity "has received little detailed attention", according to Professor Greg Watts of the University of Bradford's Centre for Sustainable Environments (BCSE). Yet this matters because "numerous studies have shown a link between tranquil environments and stress reduction, well-being, longevity [and] pain relief", he says.

As well as having low overall sound levels, such spaces "are characterised by a soundscape dominated by natural sounds", including water, rather than man-made noise. A number of BCSE studies have already examined both these effects and the visible presence of natural features on participants' ratings of tranquillity.

This has led to the coining of a prediction equation, TRAPT (Tranquillity Rating Prediction Tool). "This can provide planners, environmentalists, civic leaders and concerned citizens with a tool to guide improvements in the urban environment by greening measures and noise reduction of various kinds and to help counter threats such as over-development, tree removal or traffic densification that might threaten existing benefits," says Watts.

Key to TRAPT are the level of man-made (usually traffic) noise in the soundscape and the percentage of "natural and contextual features" in the visual scene. Other elements known to impact on tranquillity ratings, such as the presence of litter and graffiti, are also factored in, leading to a "tranquillity rating" (TR) from zero to 10, with ratings of below five being considered "unacceptable".

Tranquillity trade-offs

The equation highlights "tranquillity trade-offs", with a 50% rise in natural features having a similar effect to decreasing noise by 14 decibels. Watts modelled a range of urban scenarios - a city square, a park by a main road with and without a natural or manufactured barrier, and a residential tree-lined street, showing what measures would be required to bring each an "acceptable" TR.

In this, he assumed that screening vegetation "has an insignificant effect on noise levels", given that "usually wide belts of dense vegetation (30m) are required to achieve significant reductions in traffic noise".

Noting that tranquillity tends to be higher in affluent areas with larger gardens, he concludes: "Policies to green public spaces where there is little scope in private residences could bring well-being and perhaps health benefits to the inhabitants. Local government policies of planting avenues of trees is one obvious means of redressing this imbalance." Watts' most recent study is published in Urban Forestry & Urban Greening.

Calculation - tranquility formula

TR = 10.55 + 0.041 NCF - 0.146 Lday + MF 

Above is the formula for the tranquillity rating (TR) of a site, where NCF is the percentage of "natural and contextual features" in view, Lday is a weighted constant level of daytime man-made noise in decibels and MF is a "minor" moderating factor to take account of the presence of litter, graffiti or natural water sounds. This value is "highly correlated" with people’s actual perceptions. For a park with a TR value of five around 50% of visitors report that they are "more relaxed" after visiting it, while at a value of eight approximately 80% report being "more relaxed".

Walking - tranquility benefits

Taking walks along relatively quiet paths and lanes can combine relaxation and lower stress with physical exercise, and TRAPT has already been used to design "tranquillity trails" for this purpose. Feedback from users of one such trail shows that "well over half" of those completing the route reported greater relaxation and reduced anxiety, according to Watts. Already Handheld Tours has produced a free smartphone app-based 4.5km circular tranquillity trail of Guildford, Surrey. "There is also interest in tranquillity trails in Ireland, US and Hong Kong,"  says Watts, and it "would be great to collaborate" on further such trails. Email

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