Research Notes- 2003
Medical Conditions and Other Factors in Driver Risk
By Mary Janke
In November 1998, a teenaged girl crossing the street was hit and killed by an elderly driver. The resulting public concern led to a law named for the victim, the Brandi Mitock Safe Drivers Act, Senate Bill 335 (Hayden, Ch. 985, Stats. 2000), that became effective January 1, 2001. Among other things, SB 335 amended CVC 12818 significantly by requiring that all drivers reported to DMV by a local health officer under the "lapse reporting law," Health & Safety Code Section 103900, demonstrate their driving ability in a road test as part of their reexamination. (Previously it had not been departmental policy to give road tests to all "lapse of consciousness" drivers--many are reported on account of a seizure disorder that would certainly impair their driving during a seizure if they were driving at the time, but apparently does not affect their functioning between seizures.) Consistent with its focus on physical and mental fitness for driving, SB 335 required DMV to evaluate the effects of "physical conditions, ailments, and other factors" on the ability to drive safely. In its evaluation DMV was to include indicators and predictors of impaired driving ability, not excluding driver records. The assignment was given to the Research and Development Branch (R&D).
In addition to providing the mandated information, we thought it might be useful to give the Legislature additional unrequested, but relevant, information--predictors of unsafe driving within the general driving population, and a description of a more comprehensive system of licensing tests (the "3-tier" system) being studied by the department.
Much of the legislative report was based on this author's previously published review of the scientific literature on conditions required to be reported by physicians in California (examples are epilepsy, certain sleep disorders, and dementia). The research on other driving-relevant conditions that are not required to be reported (like vision disorders, Parkinson's disease, and cardiovascular disease) was discussed as well. Also a great deal of the report, analyzing DMV driving record data specifically, was based on an unpublished 2001 study by R&D's Emilie Mitchell and Mike Gebers. (Both the legislative report, Report No. 190, and the Mitchell and Gebers paper are available from R&D.) These authors compared the crash rates of DMV's six P&M groups (Alcohol, Mental, Physical, Lapses, Drugs, and Lack of Knowledge or Skill) with the driving population rate. They also replicated a study, performed by R&D's Dell Dreyer in 1973, which had found that groups of orthopedically disabled drivers--restricted to hand controls, a steering knob, or a leg prosthesis--had crash rates less than, or at worst equal to, those of the driving population as a whole. Dreyer's results were confirmed by Mitchell and Gebers; the hand-controls group had a lower 2-year crash rate than the general driving population, and the other two restriction groups were statistically indistinguishable from the general population.
Below is a figure from the legislative report based on Mitchell's and Gebers' data. It shows ratios of the odds of having a crash (crash odds) for each orthopedic restriction group to crash odds for the general driving population, adjusting for age and sex differences among the groups. Because the general driving population group was compared with itself, its crash odds ratio equals 1.00
The legislative report drew two general conclusions about medical conditions as predictors of crash risk:
- The effect of a medical condition on driving can't be determined just from the diagnosis; more importantly it depends on the severity of functional impairments caused by the condition. In turn, those depend to a great extent on the stage of disease a driver has reached.
- Some kinds of serious vision problems, and cognitive
impairment great enough to prevent a driver from making
good, timely judgments of the best action to take
in a traffic situation, are probably the most driving-relevant
functional impairments caused by disease.
Evaluation of crash risk for the general driving population was not required by SB 335; often the risk factors here do not so much impair the ability to drive safely as the inclination to drive safely.
However, because the information was judged to be of possible interest to the Legislature, general risk factors were discussed briefly in the report. Some main points were:
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A brief description of the department's emerging 3-tier assessment system was also included in the SB 335 report, since 3-tier is aimed at identifying and assessing drivers with functional driving disabilities like those SB 335 was concerned with.
| It was argued in the legislative report that the findings of medically related crash risk from the literature, and the crash rate data presented, provide support for supplementing the department's present licensing program with a system that will improve identification and evaluation of driverswith physical or mental disorders that might impair their driving. This possibility has been studied by R&D since 1993, funded in part by the National Highway Traffic Safety Administration. As an operational 3-tier system is envisioned, most or all renewal applicants appearing in field offices would take very brief screening tests (for instance, visual contrast sensitivity) in addition to the two standard licensing tests used now; only if they showed impairment on this "first tier" of tests would they go on to the automated tests of perceptual and cognitive speed and accuracy on the second tier. (Referrals, already identified by someone as being impaired, would take all tests necessary to reliably evaluate their driving ability.) Renewal applicants doing poorly on the second tier, but not so poorly that it would be too hazardous to test them on the road, would enter the third tier, a road test called the Supplemental Driving Performance Evaluation (SDPE). The SDPE was designed by a departmental task force for use especially with experienced but impaired drivers; it is commonly used in the field now. It includes exercises in resisting distraction and wayfinding, as well as ability to scan for hazards and ability to maneuver the vehicle. Performance on the SDPE would be the most important factor in deciding whether an impaired applicant could be safely relicensed. |  |
A potential advantage of this sort
of assessment system is that drivers would not be selected
for testing on the basis of inherent factors that are
not causally related to safe driving ability, like chronological
age and sex. On the contrary, they would be selected
on the basis of their performance on a battery of tests.
That battery will be finalized only after considering
test validity, feasibility, and acceptability to the
public. Funded by the Office of Traffic Safety, R&D
is currently studying the performance of the 3-tier
test battery in several field offices.