3.3.2. Independent Variables

The key independent variable was the "type of paving design," where vehicle speed was measured (recording spots). Two criteria for classifying the paving-design types were created, using the data at the recording spots. The first measure was the extent of "visual separation (VS)" between vehicles and pedestrians, which was created by the paving patterns. Three categories of visual separation were developed on the basis of the extent of stamped asphalt and the visual designs, which influence the extent of a sense of a barrier between vehicular and pedestrian zones. The three categories were as follows.


• VS-C: Stamped asphalt pavement covered just a part of the street, which implied that pedestrians should walk within the paved area. This was interpreted as not pursuing coexistence.

We expected that VS-A and VS-B would improve pedestrian safety more e ffectively than VS-C, because VS-C restricted the pedestrian area and reinforced the idea that street use was exclusively for driving. However, assessing the di fferences between VS-A and VS-B was complex. Although VS-A more closely adhered than VS-B to the integrative design principle of shared space, VS-B might be more e ffective under certain conditions. Kaparias et al. suggested that introducing a "safe zone" at the roadside, just for pedestrians, might play an important part in the successful operation of a shared space by increasing pedestrians' mobility and walking freedom [24] (p. 20).

The second measure of the type of paving design was the extent of visually interrupting "driving continuity (DC)," which was based on variation in the transverse and diagonal line designs. The nine study sites were categorized into three groups based on the expected e ffects of the transverse lines or surface designs as visual interference on consistent driving speed: Specifically, to cause drivers to decelerate. The three DC types are described below.


We expected that DC-A and DC-B would induce more speed deceleration than DC-C. However, similarly to VS, DC had problems in the comparisons between DC-A and DC-B. Kim and Shim argued that drivers cognize the entire change created by a PPS, and the surface design details do not significantly influence their behaviors [58]. Considering that argument, there might be little di fference in the e ffects of DC-A and DC-B on speed.

The method used to categorize the VS and the DC design types was a focus group interview with three experts on 11 January 2019, to eliminate researcher bias. The three interviewees were highly qualified professionals with PhD degrees in the field, who teach urban design as a full-time faculty. We outlined the project and the PPS designs' goals to them, and we showed them pictures of the paving status at each site.
