Transit Deserts in Dallas
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 Dallas, 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 Dallas; intersection density, number of transit stops, number of transit trips (within a 24 hour period), number of bus routes, total length of bike routes, and total length of roads with speed limits below 45 mph, and total length of sidewalks. It should be noted that total sidewalk length in Dallas was measured differently than in the other cities. The City of Dallas provides a polygon shapefile instead of a line shapefile for sidewalks. The perimeter of each sidewalk polygon was measured and then divided by 2 to get a rough estimation of the sidewalk length. While this is not precise, it does reveal which block groups have more sidewalks than other block groups. 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 length of bike routes, 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.
Transit supply in Dallas is heavily concentrated in downtown and the neighborhoods that immediately surround it. There is also a corridor of higher supply that appears to follow Interstate 75, heading northwards. Block groups with higher percentages of transit dependent populations are more prevalent in the northern part of the city, but are spread throughout. Downtown is characterized by exceptionally low percentages of transit dependent populations. There is also a cluster of block groups with high percentages of transit dependent populations near the southern edge of the city. Once the demand z scores were subtracted from the supply z scores it was found that the transit deserts in Dallas are primarily located in the northern part of the city, but with a couple near the central core. Block groups with large differences between supply and demand were scattered sporadically for the most part without much grouping. Block groups with the smallest gaps, if any, between supply and demand were all located in the central part of the city, mostly in downtown. The charts below highlight the block groups with the largest and smallest gaps between transit supply and demand in Dallas.
By identifying the transit deserts in Dallas, future expansions of transportation services can be planned more effectively and current services can be assessed more easily.