Hypothesis
To highlight how ageing places urban planning practices and conventions under stress, let alone when they are taken as normative guides, a simple hypothesis is examined, which states that ageing corresponds to a physically limited active mobility capacity for older people and, more specifically, slower gait speed. If true, the temporal-referenced and population-wide conventions used in spatial analyses and planning practice models, such as the 15MC, are expected to overestimate the ability of older people to cover equal distances in comparison to younger adults within a given timeframe. Furthermore, as the urban population continues to age, the same fallacy can lead to ever-shrinking perceived accessible areas, and (possibly) age discrimination. To test this assumption, the focus is specifically on the radius implied by the 15 min city theory through walking, which is by far the most common form of an active and non-car-dependent mean of daily mobility.
Methods and Analysis
To explore how the 15MC walking isochrone corresponds to two age groups, defined here as 'adults' and 'older adults', an online database search has been conducted using Google Scholar in research published in English (in early 2022), extending from 1995 to 2020 and concerning reports of objectively measured walking speeds per age group and especially for older people (usually defined as over 60 or 65 years old). It is well established that free-living walking speeds fluctuate with terrain features, natural conditions such as temperature, visual stimuli, socioeconomic status, culture, or movement purposes \cite{Fitzpatrick_2006,Finnis_2008,Levine_1999}. To identify less subjective measures that better represent the physiological limitations of the human body and to exclude as many environmental factors as possible, the online search narrowed to studies measuring pedestrian speed while crossing crosswalks. The assumption here is that moving on a crosswalk is performed at normal to vigorous speed, without distractions, as quickly as possible, but not fast enough to become too inconvenient.
A significant number of papers were retrieved, of which five were selected on the basis that they provided not only average values but also estimations in the 15th percentile. The 85th and 15th percentiles of a normal distribution are two parameters that are commonly used in traffic safety, as the 15th percentile speed represents a threshold that can be exceeded by at least 85% (significant majority) of the population involved to be used in recommendations.
To test this hypothesis, two new average speeds were calculated, one referring to the average speeds measured and one referring to the 15th percentile estimations. Accordingly, the distance covered in 15 minutes was recalculated for each age group, as well as the time needed to cover 1000 meters (1km), which is another well-known walking distance convention for defining a neighborhood. Finally, to visualize the differences between groups in spatial terms, a network analysis was conducted using QGIS in the city center of Athens (municipality of Athens) and service areas were analyzed around the Athens-Metro stations, as an example.
Results
Based on the values presented in Figure 1, the reported walking speed for younger adults was found to be significantly (21%) faster than for older adults in average measure terms, and (32%) in the 15th percentile. The difference between average measures and 15th percentile estimations was significant, indicating that using mean values overestimates speed conventions more for the elderly than for younger adults. Reverting the speed-time-distance calculations for the 15th percentile estimations, the equal 15-minute walking distance ranged from 805 m for the elderly to 1060 m for younger adults. Taking the 1000-meter (1 km) convention as a reference, the travel time was close to 15 min for the younger adults and 20 min for the elderly.