Passenger car-induced lateral aerodynamic loads on cyclists during overtaking
Graphical abstract
Introduction
The overtaking of a cyclist by a road vehicle is a common process in everyday traffic situations on streets in the built environment and on country roads. Since a driving vehicle displaces the air around it and thereby induces a flow field of varying velocity and static pressure, a nearby cyclist is subjected to a transient aerodynamic load. The cyclist itself experiences the transient aerodynamic load as a sequence of pressure and suction load phases with major components in the direction of travel and lateral to it. Depending on the strength and suddenness of the loading procedure the cyclist may feel frightened or even threatened and its traffic safety is impaired. Problematic overtaking maneuvers, usually because of too close clearance, belong to the most common incident type which is judged as ‘very scary’ by cyclists (Aldred, 2016). Based on (Aldred and Crosweller, 2015) it is concluded that for regular cyclists these ‘very scary’ incidents are on average a monthly experience. Regarding fatalities, too close overtaking is implicated as contributing factor in approximately 20% of all cyclist deaths (Knowles et al., 2009; McCarthy and Gilbert, 1996).
In terms of legal frameworks, however, road traffic regulations mostly stay qualitative instead of specifying numbers with respect to overtaking distances so far. It was in April 2020 that with the amendment to the German road traffic regulations (StVO, 2013), minimum clearances became statutory in Germany for the first time. Motorists have to ensure a minimum clearance of 1.5 m in town and 2.0 m out of town when overtaking cyclists. Here, clearance is defined as the distance between the outer edge of the vehicle to the outer edge of the cyclist.
Information about actual observed overtaking distances in everyday traffic situations is provided by an extensive street measurement campaign which was conducted in Berlin/Germany (Lehmann and Rauscher, 2019; Tagesspiegel, 2019). The campaign comprised 100 cyclists travelling in total 13.300 km on roads of various classes in inner city and suburban districts of Berlin within a period of two months. From the total of 16.700 captured overtaking maneuvers, 56% had a clearance of less than 1.5 m. In 18% of all cases the clearance was less than 1.0 m and in 1% less than 0.5 m. An analysis for the different road classes revealed that overtaking maneuvers were less frequent on side roads compared to main roads, however, the fraction of smaller clearances was larger for side roads. Nevertheless, main roads were regarded to be more critical when considering both together, the frequency of and clearances in overtaking maneuvers.
A study by García et al. (2015) reports noncompliance to the 1.5 m clearance in around 50% of all overtaking maneuvers on narrow roads in Spain, with the tendency of increasing compliance levels for increasingly wider roads. Love et al. (2012) observed violations of the three feet (~0.9 m) legal overtaking clearance, as statutory in Maryland US, in 23% on roads with shared-lane marking and in 17% in standard lanes in the city of Baltimore. In a study in UK, Walker et al. (2014) found between 1 and 2% of all overtaking maneuvers to take place with distances of less than 0.5 m. They further note that the visual appearance of the cyclist, e.g. by clothing portraying different levels of riding skills like a sport rider’s outfit or a vest with ‘novice cyclist’ printed on the back, did not affect the closest overtaking distances markedly. The cyclist’s perception of an overtaking maneuver was studied by Llorca et al. (2017). They show that next to the lateral clearance, the vehicle speed and type significantly affect the cyclist’s subjective perception of risk during an overtaking maneuver.
Despite its relevance and increasing topicality, the subject has rarely been considered from an aerodynamic perspective so far and knowledge about road vehicle-induced loads acting on cyclists while overtaking is largely lacking. Former studies on vehicle-induced aerodynamic loads have instead dealt with loads on other road vehicles during overtaking or passing (e.g. Clarke and Filippone, 2007; Corin et al., 2008; Gillieron and Noger, 2004; Howell et al., 2014; Kremheller, 2015; Liu et al., 2018, 2017; Noger et al., 2005; Uystepruyst and Krajnović, 2013). A work by Blocken and Toparlar (2015) for example, studied the altered drag conditions of a racing cyclist by a following car - a situation often encountered in individual time trials in elite cycling, however, does not consider overtaking maneuvers. A report by the Federal Highway Administration (FHWA, 1977) indicates side loads on cyclists by passing heavy vehicles and states an estimated tolerance limit for the load of 17 N. However, the origin of the data is not traceable and they seem to be empirical and not fully physically based.
A survey on vehicle-induced aerodynamic loads on pedestrians shows that past studies have rather addressed train-induced pressure and suction loads and their relevance for the safety of persons or stability of objects at platforms (Baker et al., 2014; Gerhardt and Krüger, 1998; Izadi et al., 2019; Khayrullina et al., 2015; Lee, 2009; Sanz-Andrés et al., 2004b; Sanz-Andrés and Santiago-Prowald, 2002; Sterling et al., 2008; Zhou et al., 2014). These studies were stimulated because trains may pass platforms at a relatively high speed and, in addition, platforms are often in an environment where the space is confined to one or more sides and, hence, higher aerodynamic loads may be expected than in unconfined spaces.
Strauss et al. (2007) measured vehicle-induced flow velocities originating from road vehicles passing a standing life-sized mannequin. Their experiment series comprised different vehicle types passing the mannequin with speeds between 30 and 80 km/h at distances ranging from 0.6 m to 1.8 m. The peak air speed increased with increasing vehicle speed but did not show a pronounced dependency on the passing distance. Overall, the peak air speed occurred when the vehicle was in front of the mannequin and was found to depend on vehicle type. The maximum peak air speed measured was 11.3 m/s at 1.10 m above ground for a cab-over tractor with semi-trailer when passing the mannequin with 75 km/h at a distance of 1.14 m. However, the involved vehicle-induced loads on the mannequin were not measured.
Studies on road vehicle-induced aerodynamic loads on objects other than road vehicles are limited to pressure and suction loads on roadside pedestrian barriers, traffic panels, flat plates and walls (Cali and Covert, 2000; Lichtneger and Ruck, 2015, 2018; Quinn et al., 2001; Sanz-Andrés et al, 2003, 2004a; Wang et al., 2013a, 2013b). Cali and Covert (2000) investigated loads on an overhead highway sign due to vehicle-induced gusts at a scaled model (1:30). They observed that the peak load occurred when the vehicle leading edge was close to the sign. This load was pointing in the opposite driving direction and its magnitude was approximately one fifth of the dynamic pressure based on the vehicle speed times the sign area. Upon this, a sudden flip over to a load pointing in driving direction with magnitude of approximately one eighth of the same product was observed. Quinn et al. (2001) performed full-scale measurements of vehicle-induced loads on different shaped and sized road signs and pedestrian barriers. For signs oriented perpendicular to the driving direction, they also observed a peak load acting in opposite driving direction when the front of the car was passing. Based on the theory of unsteady potential flow, Sanz-Andrés et al. (2003) and Sanz-Andrés et al. (2004b, 2004a) developed models for vehicle-induced aerodynamic loads on traffic sign panels, pedestrian barriers and pedestrians. The driving vehicle and its effects were modelled by a superposition of source terms which were designed to reflect a simplified and approximated vehicle-induced flow field. The models were able to qualitatively represent the main characteristics of the load time series associated with the passing of the vehicle front. Extensive full-scale measurements on vehicle-induced pressure and suction loads on differently positioned and oriented traffic panels and street parallel walls were performed by Lichtneger and Ruck (2018, 2015) and compiled in the internet database (VIPAS, 2013). They also observed a characteristic sudden flip over in load from pressure to suction when the vehicles were passing street parallel objects. Their experiments delivered a broad data base for the quantification of vehicle induced loads on flat elements in dependency on the vehicle type, its passing speed and distance.
The frequent violations of the safety clearance required by regulations and the partly extremely small overtaking distances - statutory minimum distances of 1.5 m and 3 feet were violated in around 50% and 20%, respectively, and in 1–2% of the cases the overtaking distance was less than 0.5 m (Lehmann and Rauscher, 2019; Tagesspiegel, 2019; Walker et al., 2014) - pose an enormous risk potential for cyclists in everyday road traffic. The risk potential in combination with the lack of knowledge regarding the associated loads indicate the need for the quantification of actual vehicle-induced aerodynamic loads on cyclists during overtaking maneuvers. The present study makes a contribution at this point in that it provides and analyzes measurement data obtained from full-scale field measurements of these loads on various types of cyclist dummies in dependency on vehicle overtaking velocity and distance.
Section snippets
Experiment setup and measurement technique
Lateral pressure and suction loads exerted by an overtaking station wagon (type Audi A4 Avant) on various types of cyclist dummies were acquired in full-scale field measurements. A station wagon was chosen because its general shape is, visually judged, in between that of a sedan and a SUV, hence, it is considered to be the best compromise to represent the shape spectrum of passenger cars. Moreover, the share of station wagons in the vehicle type ‘passenger car’ is around 20% for Northern
General characteristics of the load time series
Fig. 2 shows as an example of the ensemble-average time series of the resultant lateral vehicle-induced load Fres,y acting on the adult dummy on the touring bike without saddlebags (TB). The data were obtained from 5 overtaking maneuvers with the station wagon with nominal speed and overtaking distance of Vveh = 80 km/h and dy = 1.0 m, respectively. The actual values of the individual runs ranged from 78.5 km/h to 78.7 km/h and from 1.09 m to 1.17 m. At the time of the measurements,
Comparison and classification to train-induced flip over loads
For a general classification of the vehicle-induced flip over loads as presented in Fig. 3, they are compared to data obtained in a study with high-speed trains passing a circular cylindrical instrumented dummy (CID) performed by the Federal Railroad Administration (Lee, 2009). The circular cylindrical instrumented dummy (CID) is a representation of the generic geometry of the human body. It consists of a cylinder with a diameter of 0.39 m and axial length of 0.92 m supported on a post with
Summary and conclusions
In the present study, lateral loads on various types of cyclist dummies induced by an overtaking station wagon measured in a field measurement campaign were presented and analyzed. The cyclist types were represented by full-size person dummies of an adult and a child on a touring bike - with and without saddlebags, a racing bike, and a children’s bike, respectively. Overtaking maneuvers with different vehicle speeds ranging from 40 to 100 km/h and overtaking distances between 0.5 and 2.0 m
Credit author statement
No remarks.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft DFG (German Research Foundation) under grant Ru 345/32-3.
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