Socioeconomic features of home county
| Helmet N=230 (33.8%) Mean (SD) | No helmet N=450 (66.2%) Mean (SD) | P value | |
| Median income (US$) | 57 571 (18 374) | 52 111 (12 438) | <0.001 |
| Per capita income (US$) | 29 584 (7959) | 27 252 (5411) | 0.003 |
| Per cent living below poverty line | 12.8% (4.0) | 13.9% (3.6) | 0.001 |
| High school graduation rate | 87.3% (4.65) | 85.8% (4.32) | 0.002 |
| College graduation rate | 25.7% (13.4) | 21.6% (9.71) | <0.001 |