National Highway Traffic Safety Administration

Evaluating Drivers Licensed with
Medical Conditions in Utah, 1992 – 1996

Executive Summary

Introduction

OBJECTIVES

METHODOLOGY

RESULTS

DISCUSSION

RECOMMENDATIONS

REFERENCES

The Utah Driver License Division operates a specialized licensing program for drivers who have medical conditions. The program was developed by the division under the guidance of the Utah Medical Advisory Board. The program's guideline describes the physical, mental and emotional capabilities appropriate for various types of driving and determines license eligibility by medical condition or functional ability category, and functional ability level (1-12). The intent of the board was to create the least restrictive program possible that was consistent with public safety. Drivers who are licensed with medical conditions may receive a full unrestricted or restricted license depending on their functional ability level. Restricted licenses may include speed, area and/or time of day limitations. The functional ability or medical condition categories include:

1. diabetes mellitus and other metabolic conditions,
2. cardiovascular,
3. pulmonary,
4. neurologic,
5. epilepsy and other episodic conditions,
6. learning/memory/communications,
7. psychiatric or emotional conditions,
8. alcohol and other drugs,
9. visual acuity, musculoskelatal abnormalities/chronic medical debilities,
10. functional motor ability, and
11. hearing

Utah CODES was funded to evaluate the effect of the existing medical condition licensing program on public safety. The project was funded in part by NHTSA, with the support of the Utah Driver License Division in the Utah Department of Public Safety, and the Utah Department of Transportation.

In order to determine the effects of this public safety program, we compared the citation, crash and at-fault crash rates of drivers licensed with medical conditions to those of similar drivers matched on age group, gender and county of residence. Comparison drivers were obtained randomly from the general driving population and rates of adverse driving events were examined over a five year period, 1992-1996. A two-to-one matching strategy was used. Sampling was performed with replacement.

Analyses were conducted for each functional ability or medical condition category by restriction status. Analyses for drivers licensed with multiple medical conditions were conducted separately, by restriction status. The previous analyses were also conducted separately for drivers who maintained one restriction status during the study period, and drivers whose restriction status fluctuated during the study period. We used probabilistic linkage to link data elements relating to the same driver from several different databases in order to combine the elements needed for the study (i.e., crash, violation and driver license databases).

The rates of citation, crash and at-fault crash varied between the populations and events of interest. Overall, unrestricted drivers licensed with single medical conditions had higher rates of citation, crash and at-fault crashes than the chosen comparison drivers. The differences were statistically significant, but of small magnitude. Restricted drivers licensed with single medical conditions during the study period had higher rates of crash and at-fault crash than unrestricted program drivers, but similar rates of citation. Analysis by individual functional ability categories (medical conditions), showed great variation. Of interest, the citation risk for unrestricted drivers licensed in the categories "cardiovascular" and "pulmonary" had significantly lower rates of citation than their chosen comparison drivers, but similar rates of crash and at-fault crash. The greatest citation risks were found in the restricted categories "learning, memory and communication disorders" and "alcohol and other drugs" where the rates were 11.63 and 5.83 times higher respectively than of the selected comparison drivers. However, these populations were extremely small (N=6 and N=24 respectively) so their impact on public safety was negligible. Similarly, the greatest risks for all crash and at-fault crash occurred in small, restricted driving populations licensed in the "musculoskelatal abnormalities" and "alcohol and other drugs" categories (N=32 and N=24 respectively).

Drivers who were licensed with more than one medical condition during the study period were analyzed separately. The risks for crash and at-fault crash were higher than those of the chosen comparison drivers for both restricted and unrestricted drivers. The magnitude of risk was highest for at-fault crash for drivers who had restrictions imposed on their driver licenses; the rate was 1.76 times higher than those of the chosen comparison drivers (95% CI 1.40, 2.28). The rates of citation for unrestricted drivers were similar and citation rates for restricted drivers were significantly lower than those of their respective comparison groups.

The results of this study provide contextual information on the effects of the medical conditions licensing program on public safety. Specifically, we found that the overall rates of adverse driving events varied between medical condition or category type, and restriction status. Of interest, in the largest functional ability category, cardiovascular (N=18,990), the rates for all adverse events were similar to those of their comparison groups for both restricted and unrestricted drivers; however, unrestricted drivers in this category had a slight but significantly lower rate of citation. For unrestricted drivers, the highest risk of at-fault crash was found in the learning, memory and communications category where the risk of at-fault crash was 3.63 times higher than their respective comparison group (95% CI 2.00, 6.60). The greatest differences in at-fault crash rates were found in restricted license categories. Restricted drivers in the musculoskelatal abnormality or chronic medical disability group had a rate 11.29 times higher than their comparison drivers (95% CI 2.39, 53.25).

As with any injury control intervention, evaluation is an essential component of the program in order to identify areas of increased risk and to provide feedback to the administering agency. Further research should be performed to evaluate the rates of adverse driving events by individual functional ability levels in order to determine if there are distinct levels for which risk increases or decreases, and to describe the effects of co-existing medical conditions for large categories. This information will help to identify areas where the program could be improved, as well as, help to identify functional ability categories where unnecessary restrictions could be eliminated.

The Utah Driver License Division implemented a program in 1979 to license drivers with medical conditions. In 1981, the Utah Driver License Division Medical Advisory Board redesigned the written standards of the program that describe the physical, mental and emotional capabilities appropriate for various types of driving. The intent of the board was to create the least restrictive program possible that was consistent with public safety.

The program uses a general questionnaire to screen all license applicants within the state to identify medical conditions related to the applicant’s physical, mental and emotional health. Applicants who report a medical condition when completing the questionnaire are placed into at least one of twelve broad functional ability categories by medical history. The categories include:

1. diabetes mellitus and other metabolic conditions,
2. cardiovascular,
3. pulmonary,
4. neurologic,
5. epilepsy and other episodic conditions,
6. learning/memory/communications,
7. psychiatric or emotional conditions,
8. alcohol and other drugs,
9. visual acuity,
10. musculoskelatal abnormalities/chronic medical debilities,
11. functional motor ability, and
12. hearing.

Applicants who identify themselves as having a medical condition are further then classified by functional ability level. The ability level characterizes the driver's physical, mental or emotional condition according to the Utah program's guidelines. Scaling is done by a medical professional according to detailed guidelines provided by the program (Appendix B). The functional ability level corresponds to driver license privileges and license limitations as shown in Table 1. Drivers who place in functional ability levels 3 – 5 are unrestricted and have full license privileges while drivers at functional ability levels 6 – 11 are restricted with limitations on their licenses (e.g., speed, area and/or time of day limitations). Drivers at level 1 and 2 were not included. Level 1 is used for commercial drivers, and level 2 indicates a history of medical condition with recovery and these drivers are not required to participate in the program.

Functional Ability Profile Level	Driving Risk/Responsibility, License Class or Limitations
1 through 5	Driving of commercial vehicles, depending on individual profile category. Driving of private vehicles.
6	Driving with speed limitations
7	Driving with speed and area limitations
8	Driving with speed, area and time of day limitations
9	Driving accompanied by licensed driver with limitations of speed and/or area and/or time of day limitations as recommended by health care professional
10	Special driving limitations recommended by health care professional not covered above
11	Under evaluation – may or may not drive, according to circumstances as Determined by director, with medical advice as appropriate
12	No driving

Based upon the results of the questionnaire, an applicant may have a driver license immediately issued, or the applicant may be required to complete a more extensive health history form An applicant identified as having a medical condition may be required to provide documentation by a health care provider to verify his or her functional ability level before a license will be issued. Depending upon the functional ability category (medical condition) and functional ability level (1 – 12), an applicant who has a medical condition may receive full-unrestricted or restricted driving privileges, or the license application may be denied. Applicants who disagree with the level assigned by their health care provider may contest the level and have it reviewed by the Utah Driver License Medical Advisory Board. A copy of the State of Utah Functional Ability in Driving: Guidelines and Standards for Health Care Professionals is located in Appendix A. Copies of the general screening questionnaire, and the corresponding forms that are completed by health care professionals are located in Appendix B.

The Utah Crash Outcome Data Evaluation System (CODES) was created in 1992 at the University of Utah School of Medicine through a successful competitive funding application from the National Highway Traffic Safety Administration (NHTSA). Utah was one of the initial seven CODES states that used probabilistic linking techniques to link computerized data from motor vehicle crashes with those from several health care related data sets including emergency medical services and hospital inpatient and emergency department databases. The initial objective of CODES was to measure the effectiveness of safety belts and motorcycle helmets . Since its creation, Utah CODES has linked and analyzed state crash, ambulance, hospital inpatient and outpatient data in part or entirely for the years 1991 - 1997.

Utah CODES has become an integral partner with the Utah Department of Public Safety, the Utah Department of Health and the Utah Department of Transportation in injury control efforts related to traffic safety in the state. Analysis of the linked data sets has identified the medical and resulting financial outcomes for injuries caused by crashes. Utah specific data have been used to analyze the effects of pending legislative issues and to support changes that would be of benefit to public health and safety (e.g., implementation of a primary seatbelt law and graduated licensing program for teens). Because program staff have the capability and experience in probabilistic linkage and analytical techniques, Utah CODES was in a unique position to be able to perform the necessary linkages in order to evaluate the existing program of licensing drivers with medical conditions.

Several agencies had approached Utah CODES about evaluating the medical conditions licensing program in the state. According to the Department of Public Safety, the licensing program is controversial; many drivers feel the program is unwarranted while others feel the standards set forth compromise public safety. Both the Utah Medical Advisory Board and the Utah Driver License Division were interested in evaluating the effects of the program. Additionally, NHTSA is in the process of developing a Functional Ability Driving Guide for state driver licensing agencies use to help design programs for drivers with functional disabilities. Research in this area is sparse; thus, the successes or failures of existing state programs related to functional ability are of particular interest. According to NHTSA, Utah’s program and the corresponding guidelines have been considered by many states to be one of the most comprehensive functional ability and driving programs in the nation [2]. Because of these agencies and the applicability of the research to public policy, Utah CODES was funded to evaluate the effect of the existing system. The project was funded in part by NHTSA with the support of the Utah Driver License Division in the Utah Department of Public Safety and the Utah Department of Transportation.

The purpose of the study was to compare the crash and citation rates of drivers with medical conditions to drivers without medical conditions, matched on age group, gender and county of residence, obtained from the general driving population. Analyses were performed for each functional ability (medical condition) category by restriction status for the study period 1992 – 1996. Analyses for drivers licensed with multiple medical conditions were performed separately, by restriction status. Additionally, the same analyses were performed separately for drivers who maintained one restriction status during the study period, and drivers whose restriction status fluctuated during the study period.

Probabilistic linkage was used to link data elements from several different databases in order to combine the elements needed for such a study . Probabilistic linkage is an iterative tool which can overcome inaccuracies or differences in the separate databases, (e.g., incorrect, missing or duplicate data, typographical errors, changes in surnames, etc.) which exact matching cannot. Data linkages were performed using Automatch SoftwareÒ and are described below:

Variables from the Utah Department of Transportation Crash Files were linked to variables from the Utah Master Driver License File for the years 1992-1996. Fields used to link these two files included the license state of the crash driver, name (last, first, middle initial), sex, date of birth, and driver license number. The medical condition database was provided in a relational file to the Utah Master Driver License File.

The crash file identified 397,849 Utah licensed drivers as having a crash during the study period. The Utah Master Driver License File contained 1,750,918 drivers license records. Of the Utah licensed drivers in crashes, 384,311 (97%) drivers were successfully matched to the corresponding driver license records. A copy of the match file is located in Appendix C.

Probabilistic linkage was used to identify persons who held valid driver licenses and died either during the study period, or in the five years previous to the study period. This linkage was performed because drivers licensed with medical conditions were thought to have a higher mortality rate than the general population of drivers and deaths would effect the eligible number of driving days. Variables used to link these two files included name (last, first, middle), city, state, residential zip code, sex, date of birth, and social security number.

The death certificate database was subset to include persons ages sixteen years and over (i.e., persons eligible for a driver license). Thus, the resulting data set contained 100,248 death certificates for the years 1986-96 that were linked to 1,750,918 drivers license records from the 1997 Utah Driver License Master File. Of these, 59,709 (59.6%) were successfully matched.

This matching procedure allowed a date of death variable to be created for drivers who died during the study period. By creating this variable, drivers who held valid driver licenses when they died could be excluded at the date of death (i.e., deceased persons cannot drive even though their license is still valid) and allowed the replacement of comparison drivers who died prior to the study start date. This procedure was performed in order to minimize misclassification bias of the number of days a driver was eligible to drive in the study. A copy of the match file is located in Appendix C.

For an ideal comparison, crash and citation rates should be related to exposure, expressed as events per mile driven and controlled for risk factors that affect the likelihood of the event occurring. For example, if two drivers have the same number of crashes per year but one drives only half as much as the other, the rates are the same per unit time but two-fold higher when comparing driving distances. Additionally, factors such as weather, road surface, traffic conditions and speed limit may affect crash risk. Likewise, local law enforcement patterns in areas where drivers frequently drive affect the risk of citation.

This concept of measure of exposure is important when comparing the crash and citation rates of different populations, particularly in the older persons or persons who have medical conditions that may affect driving. During the study period, drivers who reported medical conditions in Utah were much different than the general population. Figure 1 illustrates the differences in ages between the medical condition drivers by restriction status and the rest of the driving population. Note that the general driving population is much younger overall than the medical conditions driving populations. Additionally, restricted drivers tend to be older than unrestricted drivers licensed with medical conditions. For this study, only drivers without medical conditions were eligible to be chosen as a comparison driver. This category excludes all drivers with medical conditions and drivers with incomplete information in the master driver license file.

Many studies have shown that drivers who do not feel that they can safely drive limit the amount they drive, or limit their driving to times or conditions when they feel comfortable to drive . For example, persons who do not see well at night may schedule trips during daylight hours.

While ideally a study to evaluate the medical conditions program would consider these factors, such data would have to be collected by a survey tailored to each functional ability category by restriction status. Because collecting these data would have been cost prohibitive, we determined that the best method to approximate these factors would be to match drivers with medical conditions to a comparison group. For each driver with a medical condition, two driving records of drivers without medical conditions from the same age group, gender and county of residence were selected for comparison.

Drivers in the medical conditions program were subdivided by functional ability category. The category "hearing" was excluded from analysis since this category was only used for commercial drivers. Drivers in each functional ability category were further subset by restriction status. If a driver with a medical condition fluctuated between restricted and unrestricted categories, he or she was counted in each category for the appropriate time period. The same comparison drivers were used for each medical condition driver who fluctuated for both restriction statuses. Comparison drivers were followed for the duration if they held a valid driver license during the study period. Drivers listed in multiple functional ability categories were analyzed by restriction status separately. These groupings were further separated into categories by age group, county of residence and gender. Age groups included years 10-14, 15-19, 20-24, 25-29, 30-39, 40-49, 50-59, 60-69, 70-79, 80 and older. Driver's age was calculated at the midpoint of the study period using the date of birth available in the Master Driver License File. We included the age group 10-14 in order to capture new drivers entering the study near the endpoint (1995-1996).

Comparison drivers were selected randomly from all licensed drivers not in a functional ability category from the 1997 master file. Commercial drivers who were licensed at functional ability level 1 (no history of disease/condition) were not included in the population from which a comparison driver could be selected. Similarly, drivers licensed at functional ability 2 (past history of disease/condition but licenses are issued the same as the general driving population) were excluded. For each driver with a medical condition, two comparison drivers fitting the grouping criteria (age group, county of residence and gender) were chosen at random from the Utah Master Driver License File. Sampling for comparison drivers was performed with replacement, meaning that each possible comparison driver was eligible to be selected even if that driver was chosen previously to be a comparison. This method was used because there were not enough drivers in some groupings to select two unique comparison drivers for each medical condition driver from the same age group, sex and county of residence.

The results of probabilistic linkage were used to determine the number of eligible licensed driving days by restriction status; and the number of crashes, at-fault crashes and citations occurring at that restriction status for each driver. Drivers with medical conditions who fluctuated between restriction status had the corresponding number of days at each status level assigned. As mentioned previously, drivers who died during the study period had their corresponding number of eligible driving days adjusted so that the date of death was included but the following days excluded. Similarly, if a chosen driver had his or her driving privileges suspended because of citations or crashes, they were not excluded from the study; the number of eligible license days was adjusted to reflect the suspension.

Comparison drivers were followed for the duration of the study (1992 – 1996) by their eligible number of driving days (the number of days they held a valid driver license) during the study period. The number of days used for these groups was higher than the number of days for drivers with medical conditions because the licensing periods are much shorter for drivers who have medical conditions. For example, if a driver with a medical condition was in the database for 1 year of the study period, he or she would be counted for 365 days. However, his or her corresponding driver would have been followed from 1992 – 1996, or 1,825 days. This was done in order to simplify the matching process and minimize the computer time used to generate the comparison drivers. The eligible number of driving days for both drivers with medical conditions and their comparisons reflects the data of the Utah Driver License Division. The same two comparison drivers were used for drivers whose medical condition fluctuated their restriction statuses. Events and eligible license days for these comparison drivers were counted at each restriction status.

Crashes were considered to be "at fault" if a driver received a citation for the crash or was marked as having contributed to the crash. Only crashes and citations that occurred during the period of time the driver was licensed were considered. Events (citation or crash) were corresponded to the driver's record, and restriction status. Citation, crash and at fault crash rates per eligible licensed driving day were calculated separately for restricted and unrestricted drivers with medical conditions and their corresponding drivers for each functional ability category. These data were then used to estimate the relative risk for each medical condition category, allowing a comparison of the crash or citation risk of drivers licensed with medical conditions to similar drivers licensed without medical conditions from the general driving population. The relative risk approximates a Chi Square distribution with one degree of freedom. Using this distribution we calculated a 95% confidence interval for the estimate of relative risk . Relative risk describes the influence of a particular variable on the likelihood of an outcome. For instance, unrestricted drivers in the visual acuity group have a relative risk for crashes of 1.35; this means that they were 1.35 times as likely to be in a crash as were members of the control group.

During 1992–1996, there were a total of 68,770 drivers in the medical conditions program excluding all commercial drivers and drivers in functional ability level two. The majority of drivers (54,825, 79.7%) were licensed in only one functional ability or medical condition category. A small number of these drivers had relatively unstable medical conditions (2,099, 3.8%) causing them to fluctuate between unrestricted, restricted or ineligible licensing functional ability levels during the study period. Table 2 shows the actual number of drivers reporting a single medical condition by functional ability category, and the corresponding numbers and percentages of drivers who fluctuated between restricted, unrestricted or ineligible license privileges during the study period.

The age group and sex distributions for each functional ability category, and each restriction status (unrestricted and restricted) are very different. Generally, restricted drivers tend to be older than unrestricted drivers. Population demographics shown by age group and sex histograms are presented in Appendix D.

The remaining drivers (13,832, 20.1%) were licensed in one or more functional ability categories during the study period. Most of the drivers reporting multiple conditions were licensed in two functional ability categories; however, the number per licensed driver ranged from one to seven categories during the study period . This is shown in Table 3.

Table 4 shows the two way categorical combinations for drivers who were licensed in multiple functional ability categories. Note that drivers may appear more than one time, depending upon the number of functional ability categories reported. The most common two way combinations of functional ability categories were "cardiovascular and diabetes," "cardiovascular and vision," and "cardiovascular and pulmonary."

Figure 2 shows the shows the percentages of drivers who were licensed in multiple functional ability categories, by each functional ability category. Over half of these drivers with multiple conditions were in the respective "cardiovascular" and "diabetes and other metabolic conditions" functional ability categories.

Analyses were performed for drivers who reported single medical conditions in the following ways: 1) combining all drivers licensed in one functional ability category who had either a constant restriction status (restricted or unrestricted) or whose status fluctuated during the study period, 2) for drivers who had a constant restriction status during the study period, and 3) for drivers who had a fluctuating restriction status during the study period. Drivers licensed in multiple functional ability categories during the study period were analyzed separately in the same manner.

The overall results for single medical condition drivers by combining restriction statuses for citations, crashes and at-fault crashes are presented in Tables 5 - 7. These tables show the estimates of risk for the event as compared to the risk for the event in the comparison drivers. Table 8 shows the estimates of risk for drivers licensed with multiple medical conditions, restriction statuses combined as compared to comparison drivers for the events of citation, all crash and at-fault crash. Similarly, the table shows the same comparisons for all single functional ability categories combined.

Tables 9 - 14 show in detail the rates and the estimate of risk for single medical condition drivers with combined restriction statuses for citation, crash and at-fault crash. These tables show the details of the information presented in the summary (Tables 5 - 7). Stratified analyses for the single condition drivers who had a constant restriction status for the study period and then those drivers who changed statuses during the study period for citation, crash, and at fault crash are shown in Tables 15 - 20. This analysis was performed to determine whether or not drivers with unstable conditions affected the combined analysis disproportionately. In these tables, drivers whose status changed are listed only once in each restriction status, regardless of the number of times their status fluctuated during the study period.