Journal of Environmental and Public Health 5 24 22 20 18 16 00 14 1 / y cen 12 u reqF 10 8 6 4 2 0 0 20 40 60 80 100 120 140 160 180 200 Miles traveled to opioid treatment program Figure 1: Distribution of travel distance (n = 23,141). In the third block, both age and race/ethnicity were related to travel distance. Patients between 44 and 81 years of age traveled 0.899 times as many miles as patients 18 to 29 years of age. Hispanics, African Americans, and patients in other racial/ethnic groups traveled 0.709, 0.635, and 0.896 times as many miles as non-Hispanic white patients. In the fourth and ﬁnal block, travel distance was positively related with prescription opioid use and negatively related with heroin use. Patients using prescription opioids in the month before enrollment traveled 1.085 times as many miles as patients not using prescription opioids. We checked for multicollinearity in the model with all blocks entered, and all variance inﬂation factors were below 2.0, suggesting no problems. 3.3. Supplementary Analysis. Since frequency of racial/ethnic groups was strongly associated with urbanicity (e.g., percent non-Hispanic white in low, moderate, and high density areas was respectively, 93%, 89%, and 70%) a stratiﬁed analysis involved estimating the contrasts between patient racial/ethnic groups only for patients (n = 6, 902) enrolling in programs located in moderately or less populated counties (n = 39). Consistent with our hypothesis, African American patients traveled 0.679 and Hispanic patients traveled 0.852 times as many miles as non-Hispanic white patients (both P < .01). 3.4. Cross-State Commuting. Eight percent of all patients commuted to another state to attend an OTP, traveling an average of 50 miles (median = 36). More than 20% of the patients in 10 OTPs traveled across a state border to attend their program. Average (mean) distance traveled among these interstate patients from these 10 OTPs was 49 miles (median = 34). The average (mean) distance traveled among those same 10 OTPs who did not cross a state border was 25 miles (median = 14). An additional 4 OTPs had 5% to 19% of their patients commuting across state lines to attend the program. Cross-state commuting was more prevalent in the southeast (14% of patients) and Midwest (24% of patients) compared to the West (<1% of patients) and the Northeast (2% of patients). Among patients attending programs located in counties of diﬀerent population densities, cross-state commuting was 8% in low, 15% in moderate, and 5% in high population density areas. 3.4.1. Correlates of Cross-State Commuting. Table 3 shows the results of the multilevel logistic regression analysis on cross- state commuting. Patients were more likely to cross a state border if their OTP was located in the Midwest (compared with the Northeast) and less likely if their OTP was in the West compared with the Northeast. Patients attending an OTP in relatively low densely populated areas (250 K − 1M) compared with a metropolitan area (>1 M) as well as those whose OTP was located in a large ZIP code area were more likely to commute across a state border. The one signiﬁcant patient covariate was race/ethnicity: African-Americans and other race/ethnicities (excluding Hispanics) were less likely to cross a state border than whites. The adjusted odds ratios for these same variables were also signiﬁcant except for Midwest OTP location. 3.5. Location of Study OTPs. Figure 2 displays a map repre- senting the distribution of the participating OTPs (n = 84) to the distribution of nonparticipating OTPs (1,142) within the 4 US regions. Regionally, the respective distribution of study OTPs and all OTPs is Southeast (38% and 35%); Northeast (25% and 34%); Midwest (17% and 7%); West (20% and 24%). The respective Beale distribution of study OTPs and all OTPs is densely populated counties (>1M: 54% and 62%); moderately populated counties (250 K − 1M; 29% and 31%), and least densely populated counties (<250 K: 18% and 7%). Among the 12% (n = 2,878) of respondents who attended an OTP in a county with a diﬀerent Beale code, 91% (n = 2,631) went to an OTP located in a more densely populated county. We also found that the more OTPs in a respondent’s ZIP code the more likely they would travel within their ZIP code (OR = 14.9; CI = 13.3 to 16.8), within their state (OR = 7.5; 5.3 to 10.8) and a shorter distance (β = −.17, P < .001). 3.6. Comment on Missing Data. Approximately three quar- ters (76%) of respondents had complete data. Missing data on a self-administered survey is not surprising. With the exception of Employment and First Methadone Treatment, each item was answered by >95% of the respondents. But since our multivariate approach was listwise deletion only those cases with complete data were retained in the multivariate analysis. Mean distance traveled for respon- dents with complete data is greater than distance traveled among respondents with missing data (15.4 versus 13.8 miles, P < .001). This does suggest some bias, although