Analysis of the impact of road noise on urban green spaces: a case study of the dendrological garden in Poznań, Poland

Traffic noise nuisance is a very real problem in the European Union. According to Directive 2002/49/EC of the European Parliament and of the Council of 25 June 2002, which relates to the assessment and management of environmental noise, ‘environmental noise shall mean all unwanted or harmful sounds caused by human activities in the open air’. Based on the source of the environmental noise, there are two basic categories: traffic noise and industrial noise. Traffic noise is divided into road, airplane, and railroad noise, with these groups of noises affecting the acoustic environment, including urban areas (Kucharski 2002; Podawca & Staniszewski 2019). The biggest threat to the acoustic environment is traffic noise, which is the dominant type of noise in the environment. The main communication routes (roads, railroads, airways) produce high noise levels and significantly degrade surrounding areas. Due to its widespread occurrence, perceived road noise is the most annoying component in this category. This component is mainly characterized by having a large number of sources: these can be transport vehicles and, in some areas, trams or trolleybuses. Of the mentioned group of vehicles, the loudest are trucks, emitting sounds exceeding 90 dB (Kołaska 2002). Several different quantifiable variables and indicators are commonly used to assess environmental acoustic conditions.

Appropriate values are utilized to prepare acoustic maps, which is required by law, to ensure the uniformity of the map’s form and content, and the comparability of results (Kucharski 2011). The amended Environmental Protection Law introduced two primary indicators for assessing environmental noise (Legal Act from 27 April 2001). These consist of short-term indicators used to determine and control the conditions of use of the environment during a single day: (1) LAeqD, equivalent sound level A for the daytime (understood as the period from 6.00 a.m. to 10.00 p.m.) and (2) LAeqN, equivalent sound level A for the night (from 10 p.m. to 6 a.m.) (Podawca & Karpiński 2021).

Stress and the pace of life forces people to seek rest in urban areas which are surrounded by plants and which offer ecosystem services. In most cities there are many parks, promenades, and botanical and dendrological gardens which provide this opportunity. Parks and other urban green spaces offer recreational environments which positively influence human health (Wilson et al. 2016). Often, these zones occupy small areas of land that are adequate for visual separation but do not always provide the expected acoustic conditions. Many authors have investigated the impact of urban noise on parks and other green areas, and the possible limiting of this impact through the use of vegetation barriers, the use of low-noise, thin asphalt layers (TALs), and a reduction in the permissible vehicle speed on adjacent roads (Rey Gozalo et al. 2020; Grangeiro et al. 2021; Petrovici et al. 2016). Good quality soundscapes in parks can only be achieved through a thorough understanding of the complex relationship between noise sources and the environment (Tse et al. 2012). Noise pollution is a challenging environmental issue in densely-built urban areas (Tashakor et al. 2023).

As urban parks are located in city centres and surrounded by roads with heavy traffic and intensive commercial activity, noise levels often exceed acceptable values (Ozdemir et al. 2014; Zannin et al. 2006). They have therefore been the subject of numerous studies (Margaritis et al. 2018; Yalili Kilic & Abus 2020). Urban parks can act as a buffer against noise pollution (Tashakkor et al. 2020), significantly reducing this negative impact (Cohen et al., 2014). Noise levels vary throughout the year within park areas, with significant differences occurring between leaf fall and foliage periods (Tashakor & Chamani 2021). The density and number of trees strongly correlates with the amount of noise being blocked. Factors such as the shape and size of the planted areas, and terrain conditions (slope and cover) influence noise distribution across park zones (Oliveira et al. 2022). In addition, there has been indications that there is a relationship between different plant species and a reduction in noise pollution levels (Yofianti & Usman 2021). The fences effectively reduce pollutants, and sound is noticeable in its shadow. At the same time, dense conifers can trap pollutants along park borders and lower concentrations in interiors, but are less efficient at reducing noise (Xing & Brimblecombe 2020). In many countries around the world, such parks very popular, some of which have a long history and contain a large number of plant and animal species (Bednorz & Urbaniak 2005; Celewicz-Gołdyn & Boryca 2012; Filimon et al. 2021; da Silva Santos et al. 2023). Another interesting issue is how to increase the rate of green space per capita in cities and how extensive this rate should be to achieve sustainable development goals (Mitincu et al. 2023).

In Poznań—one of the largest cities in Poland—the green space area covers over 500 ha, which includes 45 parks, 119 green spaces, and 42 sites with minimal human intervention (Środowisko 2024). These areas shelter inhabitants from noise and heat, and the garden under study is one such place. The studied green area is a dendrological garden situated in a busy part of the city where heavy traffic has been present since the 1990s. The area was planted with flora which constituted both an acoustic and visual barrier. The beginnings of the Dendrological Garden date back to the year 1922, when the living laboratory for students of the Faculty of Wood Science was opened (Dendrological Garden of the Poznań, University of Life Sciences 2024). The first collection included 190 species and covered an area of 0.86 ha. In 1927, the collection grew to 452 species and varieties, and in 1939 this figure nearly doubled; however, the garden was almost destroyed because of the Second World War. In the 1960s, the garden was reconstructed with an area of 4.3 ha, and currently occupies about 20 ha. There are numerous species of forest trees, including those which are deciduous: Acer campestre L., Acer pseudoplatanus L., Acer platanoides L., Alnus incana (L.) Moench, Alnus glutinosa (L.) Gaertn., Betula pendula Roth, Carpinus betulus L., Fagus sylvatica L., Fraxinus excelsior L., Populus alba L., Populus tremula L., Quercus petraea (Matt.) Liebl, Quercus robur L., Quercus rubra L., Salix alba L., Salix caprea L., Sorbus aucuparia L., Tilia cordata Mill., Tilia platyphyllos Scop., Ulmus glabra Huds., and Ulmus laevis Pall. It also contains coniferous trees: Abies alba Mill., Larix decidua Mill., Picea abies (L.) H. Karst., and Pinus sylvestris L.

This study aimed to determine the road noise distribution within the Dendrological Garden and its spatial variability. To achieve this, noise measurements were taken at 20 points within the Dendrological Garden and three points directly next to the road. In addition, available acoustic maps were used in the analysis for further comparisons.

The Dendrological Garden is a park belonging to the University of Life Sciences in Poznań, and is located in the western part of the city between a busy road and the east-northwest railway line (Fig. 1). In addition to the presence of many trees and shrubs, a small body of water in the centre enhances the attractiveness of the area, and there are bee hives located in the northern part. The park’s western side is set back from the road and is perceived by visitors as being a quiet resting place. In fact, the park is densely and almost uniformly wooded along the side of the road.

Figure 1.

Study site location with satellite and LIDAR-derived images

Source: own elaboration based on Geoportal

Figure 2.

Changes in noise ranges for the L_DWN and L_N indicators for 2012 and 2017

A - L_DWN 2012, B - L_DWN 2017, C - L_N 2012, D - L_N 2017

Source: own compilation based on acoustic maps of Poznań

Figure 3.

Changes in the course of L_DWN noise contours for 2012 and 2017

Source: own elaboration

Figure 4.

Changes in the course of L_N noise contours for 2012 and 2017

Source: own elaboration

The area selected for study measured 5.07 ha. The maximum difference in altitude is 11.6 m, and the highest part of the park is located in the northern part of the study area. The slope of the land is from north to south and from northwest to southeast. The slopes are generally uniform and concave. The primary noise source is the main neighbouring street, which consists of six lanes and has a total width of 28 m. In 2006, the average traffic volume was 70,000 vehicles per day (Administration of Municipal Roads 2024). The studies for this paper were conducted in 2014 and 2020 and were carried out only in the case of the absence of vehicle and pedestrian traffic (south and west sides) and the lack of railway and aircraft traffic (Poznań-Ławica Airport route).

As a result of the analysis of the acoustic maps (Portal of the Spatial Information System of the City of Poznań 2024), an improvement in the acoustic environment can be observed within the study area between 2012 and 2017. Analyses of the road noise maps according to the L_DWN index showed that most zones exposed to each noise range had lower noise levels in 2017. The index was within 15% of the 60–70 dB noise level for the total area, and over 3% for the 70–75 dB range’s area. Notably, in 2017, there appeared a zone exposed to a noise range of 50–55 dB, which was not present in 2012. The most significant difference occurred in the 55–60 dB range, the area of which had increased by over 33% since 2012 (Table 1, Figure 2).

In addition to the significant changes in the areas of the noise range zones, it is noteworthy that the higher noise levels in 2017 were adjacent to the road. The width of the difference between 2012 and 2017 for the 70 dB noise contour, near Niestachowska Street, was 13 m at its widest point, the width of the 65 dB noise contour was 72 m, and the width of the 60 dB noise contour was 170 m. Overall, the park’s acoustic environment improved significantly between 2012 and 2017 so that the course of the 55 dB noise contour for 2017 was similar to that of the 60 dB noise contour for 2012 (Figure 3). Considering only the ratio between 2012 and 2017 of the risk area regarding abnormal noise levels, one can discover that the acoustic environment had also improved. In 2012, 28.1% of the analysed area was within the 68 dB noise contour (the maximum limit for recreational areas), while in 2017, this area was only 18.8%. Similarly, the night noise level at 59 dB was 27.8% in 2012 but 20.9% in 2017. Due to the nature of the use of recreational areas, it is not necessary to consider the night-time requirements.

A similar situation occurred when analysing the L_N index. Most of the zones exposed to particular sound ranges decreased in area. For the 55–60 dB noise zone, it was almost 20% of the total study area and for the 60–65 dB zone, 4.5%. The area of the 50–55 dB zone remained almost unchanged. The most significant difference occurred with the enlargement in area of the quietest zone (45–50 dB) by about 23% in relation to the area of the surveyed section (Table 1, Figure 4). In addition to significant changes in area, it should be noted that in 2017, the road noise disappeared at a much closer distance from the road to the garden terrain. The course of the 60 dB noise contour, at its widest point, was closer to the street by about 13 m, while the 55 dB zone was closer by about 60–65 m and the northern strip by as much as 106 m. The 50 dB noise contour was at a distance of 100 m from the street in the year 2012. In general, the acoustic environment of the park improved significantly between the years 2012 and 2017, leading to a situation where the 2017 course of the L_DWN 55 dB noise contour was close to that of the 60 dB noise contour in 2012, while at night the L_N 50 dB noise contour moved significantly closer to the 2012 55 dB noise contour (Figure 3). The obtained results for LAeq, based on the field analyses, are presented in Table 2 for both studied years and courses of selected noise contours, as shown in Figures 5 and 6.

The L_N and L_DWN indicators are mainly used for long-term spatial policy, in order to allocate land for new functions, taking into consideration acoustic criteria. These indicators do not illustrate the actual road traffic noise exposure for a person spending, for example, an hour in a green area. Therefore, an interpretation has been made in terms of direct noise exposure based on direct measurements.

The differences detected during holidays were much smaller, and the area of the 45–65 dB noise range increased by less than two times (Table 2, Figure 6). This was in the range of 3–7% for the 45–65 dB noise levels for the total study area. A minor change in the area of the zone most exposed to noise was noted, while the section least exposed to noise (40–45 dB) significantly expanded, by almost 20% in relation to the area of the entire analysed terrain (Table 2, Figure 5). On holidays (Saturdays), the changes were similar (within 2–5% for 45–65 dB noise levels). The area of the zone with noise levels of 65–70 dB decreased by only 0.4%, while the 40–45 dB zone increased by more than 16% of the total are of the analysed site (Table 2, Figure 6). Those parts of the area which had above-normal noise levels occupied about 2.5% on weekdays, but on holidays it was only about 1.5%. The reduction in area between 2014 and 2020 for the above 65 dB noise levels was insignificant. Comparing the results obtained for Tuesdays, there was an improvement in the acoustic environment, which was evidenced by the increasing 40–45 dB area for LAeq. This may have happened due to the changing density of the park’s vegetation (deciduous and coniferous trees and shrubs). The area of the zone with LAeq above 65 dB decreased from 2.64% to 2.48%. The shrub vegetation had also become denser in the study area, especially in front of sites 1–4, 6, 10, 11, 14, 15, 17, and 18 (Figure 1).

Recent changes to every aspect of human life, including increasing exposure to noise, have been observed in most parts of the globe, both on land and in water, triggering countermeasures and new methodologies (Myrberg 1980; Mazaris et al. 2009; Farina et al. 2014; Pawłat-Zawrzykraj et al. 2021). Acoustic pollution is one of the primary factors of land degradation (Farina 2019; Can et al. 2020; Podawca et al. 2021), especially in cities and all kinds of urban green areas. Botanical and dendrological gardens are examples of quiet zones, which can play a crucial role in the well-being of local communities and tourists. Our studies were conducted in a dendrological garden, while additional data for further analyses were retrieved from documents containing acoustics maps of Poznań. An improvement in acoustic environment can be observed between 2012 and 2017 within the studied green area.

To some extent, this could be a result of the modernisation of the neighbouring road which took place in 2013 (new asphalt pavement), but may also be due to plant growth rate in the area between the road and the park, as observed from the eastern side. Analysing road noise maps using data from weekday studies shows that between 2014 and 2020, most of the areas of zones exposed to specific sound ranges decreased, excluding zones which had low noise. The area of the 45–65 dB noise zone was significantly more extensive for weekdays in 2020, while the area of the 65–70 dB zone decreased by over nine times (Table 2, Figure 5).

An observed expansion of the belt of above-normal noise to the north may be related to the increase in traffic on the neighbouring crossroads from the west (near Site A). Similar trends can be observed when analysing the differences between the results obtained on non-working days in 2014 and 2020, as in the case of Tuesdays. To some extent, the more significant reduction in the area of those zones exposed to noise exceeding the LAeqD (from 1.75% to 1.38%) may also have occurred due to the restrictions imposed during COVID-19, when vehicle traffic was lower than in 2014. The pandemic caused an unusual reduction in traffic, and a significant decrease in the noise index was to be expected. Such an anomaly is difficult to interpret, especially when associated with increased vehicle speeds. If a traffic reduction situation emerges in the future, relevant studies are planned for the presented area.

The studied area—a dendrological garden—is a valuable part of Poznań’s city green areas, both during work days and on weekends, providing shelter from road noise and contact with nature; thus, even small spaces (about 5 ha) which are located in urban areas can provide zones for resting. Dense and well-maintained vegetation can help create excellent spots with acceptable noise conditions while also offering a place for educational activities. Introducing speed limits with speed bumps could improve the resting quality in such areas. An ideal solution would be using low-noise thin surface layers of asphalt (TALs) at the same time as using the concepts mentioned above, and in the case of the studied park, a noise level range of 40–45 dB could cover most of the analysed area. Findings from presented studies could be an important contribution to urban planning practice due to the clear message presented, based on the results of the studies that have been carried out. The presence of such green zones can improve the quality of life in cities and the number of green zones should be increased if possible.