**2. Literature Review**

### *2.1. The Shared Space Concept and PPS Project*

The shared space concept aims to ensure self-regulating streets, where various users, particularly pedestrians and vehicles, interact without physical segregation, tra ffic regulations, or control devices [19,22]. First proposed in the 1970s, by Hans Monderman, a tra ffic engineer from the Netherlands, the idea has spread throughout the world in response to the negative e ffects of motorization [20]. Other terms have been coined to define this concept, such as "simplified streets," "naked streets", and "shared streets"; although di fferent, they all share certain schematic aspects [23]. The fundamental purpose of shared space is to improve pedestrian safety and mobility by reducing the tra ffic control features that tend to encourage drivers to assume their dominance on the street [23,24]. A core feature is to create some uncertainty in terms of priority for motorists by breaking away from segregating pedestrians from vehicles using barriers.

According to Engwicht, *mental speed bumps* encourage drivers to be more attentive to their surroundings and to slow down [25]. This is similar to John Adams' risk compensation theory, which is applicable to a shared street environment [26–28]. According to this theory, street users can be encouraged to be careful on the street, by preventing them from relying on safety devices and regulations. Presumably, responsibility and conscientiousness occur only in states of uncertainty [26]. Hamilton-Baillie likened the shared space to an ice rink, where users negotiate their activities with "an intricate and unspoken set of protocols" [19] (p. 169). Ultimately, the shared space becomes a self-regulating street, creating a safe and e fficient tra ffic environment that enhances public life [22].

To achieve the shared space goals, the Seoul PPS introduced various paving designs. The Seoul city governmen<sup>t</sup> aimed to minimize the negative e ffects of excessive vehicle speed, inappropriate parking, and other reckless behaviors by preventing street users from perceiving the space as typical streets for vehicles. Accordingly, the government-run Architecture and Urban Research Institute (AURI) applied the following design principles when they drew up the PPS design alternatives. First, they used the pedestrian-friendly paving approach, which usually has been limited to sidewalks, for the entire street to encourage pedestrians and drivers to think about the entire street as a pedestrian-priority space. In addition to eliminating barriers such as curbs, fences, and street signs, integrated paving designs also helped to blur the boundary between the pedestrian and vehicular zones. However, the average 2014 budget for the projects in this study was about USD \$90,000 including planning and construction costs [29]. The Seoul city governmen<sup>t</sup> encouraged lower level governments to used stamped asphalt, rather than block-type pavements, because it quickly accepts the desired diverse colors and complicated patterns at relatively low cost. Second, the PPS actively used lined patterns, which occasionally cross the streets at right or diagonal angles and, sometimes, section it to break up the driving space continuity. The lines on the colorful surfaces were intended to create visual impacts that cause deceleration. Although most of the PPS generally observed these principles, the final designs di ffered from each other. Thus, it is reasonable to expect that the e ffects of the PPS would vary by design type.

### *2.2. Optical Illusions to Induce Deceleration*

Drivers are highly influenced by the various stimuli they see while driving. According to Cohen [30,31], drivers obtain about 90% of information visually. Because the PPS expected behavioral changes by using indirect approaches, it is important to understand the relationship between the visual elements on surfaces, driving behaviors, and travel speeds. In most shared spaces, pedestrian-friendly pavement materials such as square granite setts, bricks, and concrete blocks were used instead of conventional materials, such as asphalt. The di fferences in surface texture and color were used to encourage street users to visually distinguish the street from streets in general [32–34]. In addition, shared spaces extensively embrace flush surfaces and street furniture (e.g., benches), and minimize tra ffic control devices (e.g., signals, lane markings) to create seamless and abundant walking experiences. These elements, including visual impacts, create a combined effect of deceleration, leading to enhanced walking environments [24,32–44].

Previous studies have also examined the influence of visual disturbances caused just by street surfaces. First, a concrete block pavement, the most widely used method to calm traffic [45], might lead to cautious driving and fewer traffic accidents by making drivers perceive specificity of the streetscape [46,47]. Second, regardless of the type of street markings available (e.g., center lines, peripheral transverse lines, or chevron patterns), previous studies revealed that a series of horizontal lines increased peripheral visual stimulation and caused drivers to instinctively slow down [48–53]. Street markings have been extensively used to slow down vehicles by distorting drivers' perceptions of their speed on a highway, particularly one that is curved [52,54]. Thaler and Sustein described this phenomenon as a representative example of the "nudge effect" in their book, *Nudge* [55].

However, the results might happen not only via an immediate intuition; they might be an alerting mechanism [52]. Zaidel et al. [56] and Chrysler and Schrock [57] suggested the drivers interpret painted stripes on street surfaces as warning signs and, therefore, make conscious decisions to drive slower and sharpen their attention. In other words, they might decide to ignore the stripes after the initial novelty of the lines has faded. On the basis of their research on the PPS, Kim and Shim determined that the visual elements were not sufficient to cause drivers to make decisive behavioral changes, although they contributed to creating a feeling of unfamiliarity [58]. Thus, although they might induce some extent of deceleration, the indirect and visual aspects of the PPS might not be effective in the long term - unless they consolidate their symbolic meanings at an early stage.

In short, previous studies have found that visual differentiation on the street surfaces influences instinctive driving behaviors. However, most of the previous studies about the block pavement and the transverse line markings were conducted on spaces exclusively designated for driving, such as highways, which are different from the narrow *i-myeon-do-ro* where the PPS is implemented. Therefore, we examined the visual impacts of these street surfaces on pedestrian safety. Unlike the previous studies, focusing on single sites, this study comprised all the sites transformed by the PPS in 2014 to strengthen interpretive generalization.
