Transit Deserts in San Antonio
Transit deserts are areas where there is high demand for public transportation, but a low supply of public transportation. Calculating transit deserts can be done by measuring the transit demand in a city, measuring the transit supply, and then subtracting the demand from the supply to find the "gap" in transit service. This gap is the transit desert.
In this example, transit deserts were calculated in San Antonio, Texas using the method described above. Both supply and demand were measured at the block group level. Transit demand is measured by calculating the "transit dependent population" within each block group. Data is collected from the Census (ACS 2012) for this. In order to measure the transit dependent population, the vehicles available must be subtracted from household drivers. The number of household drivers is the population age 16 and over minus people living in group quarters. Once the transit dependent population is calculated it is possible to find the percentage of the population that is transit dependent in each block group by dividing the transit dependent population by the total population. This is shown in the transit dependency map. Then, the transit dependent population within each block groups is divided by the acres in each block group to find the transit dependent population per acre. A z-score is calculated based on this that will eventually be subtracted from the supply z-score to identify the "transit deserts."
Measuring transit supply is also done at the block group level. Seven characteristics were taken into consideration for measuring the transit supply in San Antonio; intersection density, number of transit stops, number of transit trips (within a 24 hour period), number of bus routes, total length of sidewalks, total length of bike routes, and total length of low-speed roads. It should be noted that the total length of low-speed roads was used in the San Antonio Analysis instead of roads with speed limits under 45 miles per hour as were used in the Austin analysis. This was done because of the lack of speed limit data for the roads in San Antonio. Characteristics relating to the actual transit service, such as number of bus lines and frequency of service, as well as characteristics relating to transit access, such as intersection density and sidewalk lengths, were used so as to address both the service and access to the service. Each of these characteristics was measured in every block group and then divided by the number of acres in each block group to find a per acre measurement of each characteristic. Z scores were calculated for each of these seven characteristics and then aggregated to find an overall score for transit supply in each block group.
Once the z scores for supply and demand are calculated, the score for demand is subtracted from the score for supply to measure the gap in transit service, if there is one. Three maps were generated to visually display the data more clearly.
In San Antonio, transit supply is most heavily concentrated in the central part of the city, with supply becoming more and more stretched the further away from downtown. Transit demand is a bit less centralized than the supply and not necessarily in the same places. Block groups with higher demand appear to be a bit more prevalent in the northern half of the city. The gap map shows an excess of transit supply in downtown and in the neighborhoods that immediately surround downtown. Darker red areas that signify parts of the city where there are the largest gaps between transit supply and demand are not really concentrated in one particular area, but
spread out towards the outskirts of the city. The areas of San Antonio with the largest and smallest gaps in transit are highlighted in the carts below.
By identifying where there is a gap between transportation demand and transportation service in San Antonio, it might be possible to plan future transportation services more effectively.