The goal is to identify a group of people whose travel choices are not likely to be limited by the inability to use a car for a given outing. Logical criteria are that they drive and own a car.7 In addition, it seems important to account for the number of cars in households with multiple people, to distinguish between those who have to negotiate and compromise over use of shared vehicles versus those who can use one basically whenever they want. Common measures used by other researchers are the ratio of vehicles to household members (of all ages), or to adults (e.g. Delbosc & Currie, 2012), workers (e.g. Chu, 2012), drivers (e.g. Anggraini, Arentze, &
Timmermans, 2008; Scheiner & Holz-Rau, 2012), or driving-age household members (e.g.
Blumenberg & Smart, 2010).
As shown in Table 21, there are large differences in the portion of households with one or more vehicles per person of all ages including children, versus per adults, drivers, or teens plus adults: only 55.6% of households have 1 per person (all ages), and 92.1% of these are households with no children (among households with any children, only 14.0% have one vehicle per person of any age) and 41% are people living alone. Many households have one vehicle per driver (80.2%), and for most of these (93.3%) this also means one per adult. However, 11.7% of car-owning households have some non-driving adult household members. Limiting the criteria
7 Throughout, I use “car” and “vehicle” interchangeably to mean any type of privately owned vehicle, including cars, motorcycles, vans, RVs, pickup trucks, other trucks, but excluding light electric vehicles, such as golf carts or electric-assist bicycles.
to households with one vehicle for every adult, whether or not they are drivers, reduces the qualifying share to 74.8% of all households.
The handling of teen drivers makes only a small difference for the overall sample because they are such a small minority of the population: about 5.5% of households have any driving-age teens. Among these households, however, it makes a big difference in how vehicles are counted: 79.4% of them have one or more car per adult, but only 42.9% per person over age 16 (including driving and non-driving teens; see Table 21). This seems too restrictive a criterion, since only 60.3% of households with driving-age teens have teens who drive. While potentially endogenous, I opted for the compromise of only counting teens who drive as potential vehicle users. Among households with any driving-age teens, 61.8% meet this criterion – one vehicle per adult plus teen driver (or equivalently, one per person but excluding minors if they do not drive).
Table 21. Percent of households owning one or more vehicles per person, defined in different ways 1 or more vehicles
Weighted N (households) 150,147 8,288 4,998
% of total households 100.0% 5.5% 3.3%
% of households with any 16- or 17-year-olds 100.0% 60.3%
Source: 2009 NHTS version 2.1, weighted sample of households (using a version of WTHHFIN that is a redistribution weight rather than an expansion factor; the total sample size remains the same but portions change).
Because counting children (having one car for every person, even children) seems overly restrictive – the households fulfilling this criteria might be aberrant and different from the mainstream — and because children have less autonomous activity (typically at least somewhat dependent on their parents), I exclude them from the ratio I use to identify “unlimited”
ownership levels. Rather, I opt to count all of the autonomous household members who might lay claim to the car, including teens if they drive, as well as all adults, whether or not they drive.
The rationale for including non-driving adults is that they have autonomous transportation needs regardless of their driver status. Having extra non-driving adults in the household might put demands on the household vehicles(s) and so access to vehicle transportation for the remaining (driving) household members should not be considered unlimited in their presence.
(By contrast, I assume the travel of non-driving minors would be accommodated as part of their parents’.)
The final criterion, that individuals themselves drive, affects only a small minority of cases, given the ownership criteria already established. Almost all (98.1%) of the adults in households that qualify with respect vehicle ownership do drive (Table 22). However, 1.9% of adults in these households do not, and are excluded from the “unlimited” category. By contrast, only 76.0% of those in households owning fewer cars and 45.0% of those with no cars drive, with 99.6% and 57.7% living with someone else who drives, in the fewer- and no-car
households, respectively.
Among those that do drive in the higher ownership group, there is an additional minority (6.1%) who answer affirmatively to having “a condition or handicap that makes it difficult to travel outside of the home,” including 0.4% who indicate that they have “given up driving” because of the condition (see Table 23). However because these respondents are coded as drivers, either because they actually drove on the survey day and/or self-identify as current drivers, I retained them in the group considered to have “unlimited” vehicle access (comprising n=1,385 out of a total N= 204,265, unweighted, in this subsample). By being included, they serve to represent the mobility levels that are likely among people burdened by such conditions but with vehicle access, which seems important for then contrasting this to the mobility levels of those with such conditions but without vehicles.
Table 22. Percent of individual adults who drive, by household vehicle ownership level
Weighted N 244,928.2 15,642.3 59,758.5 169,527.4
Percent of total weighted N 100.0% 6.4% 24.4% 69.2%
Source: 2009 NHTS version 2.1. Among individuals, weighted, and only including adults (age 18+).
“More” includes households with one or more vehicles per person excluding non-driving minors;
“fewer” includes households with at least one vehicle but fewer than the total number of adults (driving and non-driving) plus teen drivers in the household.
Table 23. Percent of individual adults with limiting medical conditions, by household vehicle ownership level
Overall
Weighted N 250,789.0 15,892.8 64,114.0 170,782.2
Percent of total weighted N 100.0% 6.3% 25.6% 68.1%
Source: 2009 NHTS version 2.1. Among individuals, weighted, and only including adults (age 18+). “More”
includes households with one or more vehicles per person excluding non-driving minors; “fewer” includes households with at least one vehicle but fewer than the total number of adults (driving and non-driving) plus teen drivers in the household.
In summary, I defined three levels of access as shown in Table 24. The “unlimited” (high-access) segment is used as the basis for the benchmark model. About 67.9% of adults meet the criteria for this high-access segment, and about 72.5% of households have at least one person who meets the criteria.8
8 These portions are for the weighted sample, to give a sense of what the shares are likely to be in the general population. I used the weights provided by the NHTS for this purpose, applied separately per person (using WTPERFIN) and per household (using WTHHFIN).
Table 24. Definition of three vehicle-access levels and their portion of the sample People Households People Households High Individual drives, and lives in a household with at
least one vehicle per person (excluding non-driving minors, but including minors who drive as well as all adults, whether or not they drive)
67.9% 72.5% 198,448 119,659
Medium Individual lives in a household with plenty of vehicles, but does not drive; or lives in a household with at least one vehicle, but with more adults or drivers than vehicles (and may or may not drive)
25.7% 18.8% 49,365 23,283
Low Individual lives in a household with zero vehicles;
may or may not drive
6.4% 8.7% 8,765 7,205
Total 100.0% 100.0% 256,578 150,147
Data are from the 2009 NHTS version 2.1. All counts exclude respondents out of town on the survey day. The people counts are for all individual adults (excluding children), potentially including multiple members from the same household. Household counts reflect the level of the highest-access member among everyone in the household (that is, categorized as “high” if at least one individual has high access, but “medium” only if the highest level of access among anyone in the household is “medium”). The unweighted counts show the number of cases available that meet the criteria for each access category. Weighted portions are for data weighted to make the sample representative of the U.S. population as a whole, using weights provided by the NHTS (WTPERFIN for individuals and WTHHFIN for households.)