Cycling is considered to be a climate friendly means of transport. It also promotes the health of its users and is comparatively inexpensive to use (Umweltbundesamt, 2022). However, cycling only accounts for a small proportion of total traffic in Germany. In 2017, only 11% of all trips in Germany were made by bicycle. In 2002, the figure was 9% (Nobis, 2019). Especially in view of the ongoing climate change, it is desirable to further increase the share of cycling. This is also the goal of the German Federal Ministry of Digital Affairs and Transport (BMDV), which wants to gradually promote the use of bicycles until 2030 (BMDV, 2022).

This raises the question of how to motivate more people to cycle. Among a number of potential barriers, the issue of cyclists' safety seems particularly relevant. Compared to other road users, cyclists currently have an above-average risk of being involved in road traffic accidents (Alrutz et al., 2015). This affects the perception of cycling. Lack of safety is often cited as a reason why people do not cycle (Sinus Institut, 2021). Conflicts can always occur when motor vehicles (MV) and cyclists meet on a shared carriageway. Accidents between MVs and cyclists are most likely to occur at junctions. Overtaking manoeuvres (OM) are not among the main causes of cycling accidents. Nevertheless, the latter are perceived by cyclists as particularly dangerous and stressful (Merk et al., 2021; Richter et al., 2019).

For several years, German road traffic regulations (Straßenverkehrsordnung) have required MVs to leave a sufficient distance when overtaking cyclists. However, the amount of this lateral distance was originally not specified. This was changed in 2020. Since then, a lateral distance of at least 1.50 m in built-up areas and 2.00 m outside of built-up areas has been prescribed (Bundesgesetzblatt, 2020). In practice, many cyclists continue to report a high number of OMs in which overtaking MVs do not observe the prescribed lateral distance (Reutter, 2021).

However, it is not easy to verify this perception. Devices to measure lateral distance are not commercially available. For this reason, the topic has been the subject of increasing research in recent years. Tagesspiegel (n/d) equipped 100 test persons with ultrasonic measuring devices and analysed their daily trips. In total, 56% of all OMs were performed under 1.50 m. DHBW Stuttgart (Plavec & Schmock, 2021) used a similar approach, with over 100 test persons. Here, 74% of the OMs were below 1.50 m. Koppers et al. (2021) performed observations in traffic and determined a value of 43% for lateral distances below 1.50 m. Richter et al. (2019) performed laser measurements in real traffic and determined a value of 50% for too close OMs. Langer (2016) investigated lateral distances with one test person and camera observation. Overall, the subject was overtaken at a distance lower than 1.50 m in 75% of all OMs. In summary, the recorded lateral distances vary between different studies. However, all studies show that at least 43% of OMs are performed to close. This must be considered problematic, as it is likely to reduce bicycle safety.

Studies show some of the influencing factors of lateral distances. Cyclists are being overtaken particular close by MVs when there is oncoming traffic (Koppers et al., 2021; Langer, 2016; Merk et al., 2022). The same can be said when a road has cycle lanes (Langer, 2016; Mros, 2021; Ohm et al., 2015). The width of the carriageway seems to have an influence, with smaller lateral distances on narrower carriageways (Langer, 2016; Merk et al., 2022; Ohm et al., 2015). Cyclist behaviour also affects lateral distances. Narrow OMs occur more frequently the slower cyclists ride (Plavec & Schmock, 2021; Tagesspiegel, n/d) and the further apart they ride from the roadside (Mros, 2021; Richter et al., 2019).

Research has not yet determined the influence of MV traffic volume on OMs. In Germany, this value is a basic input variable for the selection of bicycle infrastructure for a road. The higher the MV traffic volume during peak hour, the more likely cycling and motor traffic are to be separated (FGSV, 2010). So far it is unclear whether this requirement can be derived from a correlation between MV traffic volume and the lateral distances that occur. This study therefore addresses three research questions:

1. Does cycling infrastructure have an influence on the lateral distances between MVs and cyclists?

2. Does MV traffic volume have an influence on the lateral distances between MVs and cyclists?

3. Does time of day have an influence on the lateral distances between MVs and cyclists?

Previous research shows an influence of cycle lanes on lateral distances. Therefore, research question 1 serves as a check on the methodology of this study. Research question 2 examines whether MV traffic volume is a valid basic input variable for bicycle infrastructure planning. Accordingly, research question 3 examines whether the focus on peak hours should be questioned.

This study aimed to analyse the lateral distances between bicycles and overtaking MVs, based on measurements in Stuttgart, Germany. 42.1% of the lateral distances measured were less than 1.50 m and therefore did not meet the requirements of the German road traffic regulations. All other studies analysed showed similar or higher values, ranging from 43% to 75% (Koppers et al., 2021; Langer, 2016; Merk et al., 2022; Plavec & Schmock, 2021; Richter et al., 2019; Tagesspiegel, n/d). Possible reasons for this difference are the routes chosen and the local characteristics. Riding style of the test persons could also be an explanation. In this project, the lateral distances were collected from a single test person. This made it possible to study variables without results being influenced by riding style. Projects that collect lateral distances from a large number of subjects can give a more balanced overall picture. The test person in this study chose an outwardly confident riding style. Whether cyclists with a different riding style are more likely to be closely overtaken remains to be seen in future research.

Research question 1 set out to analyse the effect of cycling infrastructure on lateral distances. Cyclists in mixed traffic experience higher mean and median values of lateral distances as well as a lower percentage of close OMs compared to cyclists in cycle lanes. Therefore, it can be said that cycle lanes lead to closer OMs than mixed traffic. These results are in line with the findings of other studies. The majority of existing research projects find higher lateral distances in mixed traffic than in cycle lanes (Merk et al., 2022).

Research question 2 aimed to analyse the influence of MV traffic volume on lateral distances. We could not confirm a relationship between those factors. Therefore, it cannot be concluded that cyclists are overtaken more closely on high traffic roads than on roads with fewer traffic. This has not been analysed in any other studies and can therefore be considered novel. The focus of German design guidelines on MV volume can therefore not be justified by a correlation with lateral distances. Although the proportion of close overtaking does not increase with MV traffic volume, high MV traffic volumes should still be seen as negative for cyclists. The higher the MV traffic volume of a road, the more OMs can be expected. This increases both the probability of accidents on the one hand and the subjective stress of cyclists on the other (Merk et al., 2021). The results of this work therefore do not imply that cycling traffic should be guided in mixed traffic at high traffic volumes without separation from MV traffic.

Research question 3 aimed to analyse the influence of time of day on lateral distances. With the available data we could not confirm an influence. It can therefore be assumed that the average lateral distance on a road is the same throughout the day. This is also a new finding. German design guidelines use the peak hour of MV traffic as input variable for the selection of bicycle infrastructure. According to the results, there is no need to question this, as lateral distances do not differ between the peak hour and other times.

The findings of this study have some limitations. All data was collected by a single test person on a few selected routes. This was done to ensure a high level of comparability between the routes analysed and to limit the influence of riding style on the results. On the other hand, this method does not give representative results for all cyclists. The use of more cyclists on selected routes could provide information on the influence of riding style and cyclist appearance on lateral distances. Analysing more routes would be expected to provide more detailed information on the influencing factors related to individual roads. The volume of cyclist traffic was not included in the analysis, as this data was not available. It would be expected that the volume of cyclists on a road could have a significant influence on lateral distances. This should be investigated in further research. Further research should also focus on road and bicycle infrastructure design. Variables such as carriageway and cycle lane width were not considered in this study and could provide deeper insights into the topic of lateral distances. In order to have a comprehensive view of the topic, it could also be helpful to look at the perspective of MV drivers. A psychological perspective could show which influences act on the behaviour of MV drivers (e.g. road environment, stress, personal attitudes), how this influences the execution of OMs and with which measures low lateral distances can be prevented.

In this study, we analysed lateral distances between MVs and cyclists during OMs. One test subject collected data from a total of 4 081 OMs in the city of Stuttgart, Germany. In total, about 42 % of the OMs were performed with a lateral distance of less than 1.50 m, which is the required lateral distance of German road traffic regulations. For further investigation, we addressed three research questions. First, we analysed the influence of bicycle infrastructure on lateral distances. We found that cyclists on cycle lanes were overtaken significantly closer, than cyclists in mixed traffic. Second, we analysed the influence of MV traffic on lateral distances. The data could not confirm a relationship between MV traffic and lateral distances. Third, we analysed the influence of time of day on lateral distances. A relationship between time of day and lateral distances could not be confirmed. The results provide valuable insights into the factors that influence lateral distances between MVs and cyclists and can be incorporated in the process of building safe cycling infrastructure and networks.

Overall, survey results indicate an urgent need for action. The fact that 42% of all OMs are carried out in contravention of traffic rules must be seen as negative for cyclists. Low lateral distances lead to a low level of subjective safety. Subjective safety is a crucial lever to strengthen cycling in the context of climate change. Using the knowledge gained through this analysis can be a first step in building safer cycling infrastructure. Further research on the influence of infrastructure design and cyclist behaviour can further facilitate cycling safety.

The authors report no competing interests.

No external funding was received for this research. The measuring device was gratuitously provided by OpenBikeSensor e.V.

This paper originated from a master's thesis by Leo Casey at Stuttgart University of Applied Sciences, Germany.

The results of the research have been orally presented at ‘Research Day’ (6 July 2023), organized by Baden-Wuerttemberg Cooperative State University (DHBW) Stuttgart.

An early version of this paper was presented at the 11^th International Cycling Safety Conference, held in the Hague, the Netherlands, on 15–17 November 2023.