Due to decreasing resources, living in urban regions focus on sustainability in many aspects, including transportation. Sustainable transportation encourages non-motorized modes of walking and cycling as well as public transit (which also relies on walking while accessing a station), as well. However, walking as a mode is still a big mystery itself that needs further attention and research effort especially in the evaluation part. So far, the planners have discussed the concepts of walking and walkability, while engineers have mostly focused on Pedestrian Level of Service (PLOS). The scope of the problem is reflected in the diversity, and consequent inconsistency, in the available PLOS methods, which is one of the problems addressed in this chapter. The second and the bigger problem is the gap between the planning and engineering approaches in evaluating PLOS and walkability producing no consensus or clear relationship between the two, even though they overlap greatly.
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Walking can be defined as one of the individual transportation modes, which is the consequence of personal preferences (Rodrigue et al., 2006). It was the dominant mode during the 1800-1890s due to small sizes, compactness and mostly circular shape of the cities offering walking distances less than 5 km (Kaiser et al., 1995). First, invention of the railways changed the urban morphology, which was further reformed by the rapid increase of automobile usage after the1930s that marked the start of “the automobile era”. Since then, the threat by the ever increasing popularity of automobile in the urban transportation not only required major investments in road infrastructure, but also in the traffic management systems to overcome its negative effects such as, congestion, air pollution and accidents. But, neither has provided a real solution, and the “vicious circle” of automobile-based transportation problems still threatens city traffic networks, today.
A more realistic solution has been sought after within the concept of “sustainability” that gained popularity as early as the 1970s. However, the easiest way to define “sustainable” transportation starts with defining what is “non-sustainable” in transportation. According to a definition proposed by Black (2010) a sustainable transportation system is,
… One that provides transport and mobility with renewable fuels while minimizing emissions detrimental to the local and global environment and preventing needless fatalities, injuries, and congestion.
This can be further supported by the recommendation of selecting focus of urban transport policy as “reducing car trips and encouraging alternative modes, such as public transport, walking and cycling” (Babalik-Sutcliffe, 2013). Furthermore, walkability and biking are encouraged due to their positive impacts on human health (Brown et al, 2009; Rosenberg et al, 2009; Van Dyck et al, 2011).
Though easy to recommend, it is much harder to get people to choose “walking” as a mode in most cities, today. Because, despite its great capacity in sustainability, walking has the major disadvantage stemming from physical limitations of human body, in terms of walking distance and speed. While planning public transport services, guidelines often use one-quarter mile or 400 meters (or at most upto one-half mile or 800 meters) as “walking distance to access public transport” as key measure (Daniels & Mulley, 2013). There can be many factors pushing the walking distance to much lower or higher limits. But, even a range of a few kilometers is rarely enough to get all our travel needs (commute, shopping, etc.) satisfied in a city. Thus, plans to encourage walking as a mode have to be integrated by encouragement of public transit use in an intermodal way, as well. However, public transit has its own challenges when it comes to mode choice, with its stop-to-stop structure, waiting/dwelling times, problems in accessing to stops and making transfers. As a result, it is very difficult to evaluate which of these two modes, walking or public transit, creates (or to what extent) challenges, such as:
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Limits of effective capture zone of public transit stations,
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Impact of infrastructure aspects, road network design or land use measures on the usage of public transit stations, or
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Whether people do not chose public transit because they do not want to/cannot walk, or vice versa.
It should be noted here that, in developing countries, walking limits and use of public transit may go up to levels much higher than developed countries, especially among certain traveler groups that cannot afford automobile; but, such mode choices made based out of necessity are easily reversed towards the favor of automobile, when a traveler’s socioeconomic demographics change over time, creating a setback in our efforts towards a more sustainable urban transportation management.