Traffic Safety Facts

Traffic Tech – Technology Transfer Series

Number 357 January 2009

U.S. Department of Transportation 1200 New Jersey Avenue SE., Washington, DC 20590

National Highway Traffic Safety Administration

The	number	of	older	licensed	drivers	in	the	United	States	is	

growing	at	a	rate	faster	than	the	overall	population.	As	people	age,	they	are	more	likely	to	take	one	or	more	potentially	

driver-impairing	 (PDI)	 medications.	 TransAnalytics,	 LLC,	

recently	completed	a	pilot	study	to	gain	a	better	understanding	of	the	safety	impact	on	older	drivers	of	taking	multiple	

PDI	 medications.	A	 final	 report	 titled	 A Pilot Study to Test 

Multiple Medication Usage and Driver Functioning provides	an	

update	on	the	prevalence	of	prescription	medications	in	the	

older	population,	and	the	effects	on	driving	of	specific	drugs	

and	 drug	 classes.	 Research	 activities	 included	 a	 literature	

review,	data	mining	exercise,	identifying	other	databases	for	

future	data	mining,	and	a	field	study	of	occupational	therapist	on-road	evaluations	of	older	drivers	who	take	multiple	

PDI	medications.	

Literature Review 

This	project	examined	recently	published	research	to	update	

a	 prior	 NHTSA	 report	 (Literature Review on Polypharmacy 

and Older Drivers, DOT	HS	810	558)	on	the	effects	of	different	types	of	PDI	drugs	on	driving.	New	information	about	

specific	 drugs	 and	 drug	 classes	 on	 driving	 is	 provided	 for	

an	 anti-seizure	 medication	 (topiramate)	 used	 for	 migraine	

prevention	and	other	therapies;	acute	and	stable	dosing	of	

opioids;	 sedating	 and	 non-sedating	 antihistamines;	 antidepressants;	 short-	 and	 long-half-life	 sedative-hypnotics;	

an	 immediate-release	 versus	 extended-release	 anti-anxiety	

medication	 (benzodiazepine);	 a	 skeletal	 muscle	 relaxant	

(carisoprodol);	and	anti-diabetic	medications.	

Studies	that	assessed	the	behavioral	consequences	of	medication	usage	among	older	people,	i.e.,	gauging	the	risk	associated	with	prescription	and	over-the-counter	drugs	versus	

the	 medical	 conditions	 treated	 by	 the	 medications,	 were	

reviewed.	Vehicle	crash	involvement	among	older	drivers	

has	 been	 significantly	 associated	 with	 a	 history	 of	 falling	

down	in	the	previous	two	years.	The	same	medications	that	

mediate	the	risk	of	falling	may	also	mediate	motor	vehicle	

crash	risk.	Although	certain	drugs	increase	the	risk	of	falls	

in	 older	 persons,	 chronic	 medical	 conditions	 were	 often	

more	 important	 than	 medications	 in	 causing	 falls.	 Since	

cardiovascular	 events	 often	 contribute	 to	 falls,	 this	 report	

reviewed	studies	that	examined	adverse	drug-induced	cardiovascular	events	that	are	reversible	after	the	withdrawal	

of	specific	drugs.

Identifying Preferred Patient-Level Administrative Databases for Data Mining

This	 project	 defined	 an	 ideal	 database	 to	 study	 the	 crash	

involvement	of	older	drivers	taking	PDI	medications—	which	

would	contain	linked	medical,	hospital,	and	pharmaceutical	

data	for	each	eligible	person	and	would	be	the	only	provider	

of	services—then	evaluated	a	number	of	candidates	for	such	

work.	The	“ideal”	database,	which	would	capture	the	complete	record	of	drug	utilization	for	a	patient	(driver),	unfortunately	does	not	presently	exist.	However,	several	promising	

candidates	for	future	NHTSA	investigations	were	identified,	

and	information	on	each	is	contained	in	the	final	report.	

Proportion of Crash-Involved Drivers Taking Two or More 

PDI Medications, by Age Cohort

0

5

10

15

20

25

30

35

40

45

16-49 50-54 55-59 60-64 65-69 70-74 75+

Age Cohort

Percent of Age Cohort

Testing Multiple Medication Usage and 

 Driving Functioning

Data Mining Exercise 

This	project	examined	a	patient-level	administrative	claims	

database	 containing	 prescription	 information	 and	 E-codes	

identifying	the	incidence	of	motor	vehicle	injuries	to	identify	

drivers	who	were	taking	PDI	medications	and	were	involved	

in	crashes.	The	number	of	PDI	drugs	taken	by	crash-involved	

individuals	within	the	age	cohorts	from	16	through	49	and	

then	in	5-year	cohorts	up	through	age	75+,	ranged	from	zero	

to	16.	The	use	of	multiple	PDI	medications	by	crash-involved	

drivers	50	and	older	climbs	steadily	with	age,	until	leveling	

off	at	the	65-	to	69-year-old	cohort.	At	the	same	time,	onethird	to	one-half	of	crash-involved	drivers	in	each	of	these	

cohorts	were	taking	no	PDI	medications.	From	this	exercise,	
TRAFFIC TECH	is	a	publication	to	disseminate	information	about	

traffic	 safety	 programs,	 including	 evaluations,	 innovative	 programs,	and	new	publications.	Feel	free	to	copy	it	as	you	wish.	If	

you	would	like	to	receive	a	copy,	contact	Dr.	Angela	Eichelberger,	

Editor,	fax	202-366-7394,	e-mail:	angela.eichelberger@dot.gov.

1200	New	Jersey	Avenue	SE.,	NTI-130

Washington,	DC	20590

a	set	of	two	PDI	drug	combinations—hypotensives	in	combination	with	one	or	more	other	classes	of	PDI	medications	

such	as	lipotropics,	beta	blockers,	calcium	channel	blockers,	

NSAIDS,	SSRIs,	and	gastric	acid	secretion	reducers—emerged	

as	inclusion	criteria	in	the	subsequent	field	study.

Field Study 

Forty-four	healthy	older	drivers	between	the	ages	of	57	and	

89,	 who	 lived	 in	 residential	 communities	 in	 Delaware	 and	

Maryland	 and	 drove	 an	 average	 of	 50	 miles	 and/or	 three	

days	each	week,	participated	in	a	pilot	study	to	examine	their	

use	of	PDI	medications	and	driving	abilities.	A	pharmacist	

collected	data	on	each	participant’s	medication	usage	via	a	

one-on-one	 “brown	 bag”	 medication	 review.	 Each	 driver’s	

functional	 status	 was	 measured	 using	 a	 computer-based	

test	 battery.	 Then,	 an	 occupational	 therapist	 (OT)	 or	 certified	 driver	 rehabilitation	 specialist	 measured	 drivers’	 onroad	performance.	This	included	onboard	measures	of	brake	

response	time	under	alerted	and	unalerted	conditions.	

Due	to	the	small	sample	size	relative	to	the	number	of	drugs	

and	 drug	 classes,	 logistic	 regression	 was	 unable	 to	 significantly	associate	medication	usage	with	observed	differences	

in	functional	(cognitive)	status,	driving	evaluation	outcomes,	

or	brake	response	time.	However,	the	drivers	who	“failed”	

the	OT	evaluation	were	also	among	the	oldest.	

PDI	 medications	 may	 be	 more	 impairing	 to	 driving	 performance	with	increasing	age,	due	to	a	wide	range	of	agerelated	 physiological	 changes	 and	 changes	 in	 how	 these	

drugs	are	metabolized.	ACE	inhibitors,	generally,	and	ACE	

inhibitor/thiazide	diuretic	combinations,	in	particular,	may	

be	deserving	of	special	attention	in	future	research;	but	this	

must	be	regarded	as	a	tentative	conclusion	given	the	research	

limitations	noted	above.

Instrumented Vehicle Analyses 

The	 private	 cars	 of	 a	 sub-sample	 of	 five	 individuals	 were	

equipped	with	instruments	to	collect	video,	GPS,	and	speed	

recordings	 to	 examine	 the	 variability	 in	 selected	 behaviors—surrogates	of	driver	attention/distraction,	plus	speed	

choice—during	 independent	 driving.	 These	 same	 instruments	were	present	during	their	drives	with	the	OT,	which	

took	place	in	the	same	area,	under	comparable	conditions.

Comparing	 behavior	 during	 independent	 driving	 versus	 a	

formal	 driving	 evaluation	 showed	 that,	 on	 road	 segments	

common	to	both	sets	of	drives,	an	82-year-old	woman	was	

more	likely	to	drive	slower	on	her	own	than	during	the	OT	

evaluation	when	other	traffic	was	present,	but	faster	on	her	

own	 than	 when	 observed	 by	 the	 OT	 under	 “empty	 road”	

conditions	where	other	traffic	could	not	affect	speed	choice.

In	 the	 aggregate,	 the	 sub-sample	 of	 5	 drivers	 with	 instrumented	vehicles	spent	more	time	looking	down	and	inside	

the	vehicle	and	less	looking	toward	the	inside	rearview	mirror	when	driving	independently	than	when	the	OT	was	present,	 including	 during	 intersection	 negotiation	 when	 active	

scanning	of	the	road	environment	is	most	critical.	This	highlights	a	difference	between	independent	driving	and	older	

drivers’	behavior	during	a	driving	evaluation	that	may	have	

significant	safety	implications.	

Conclusion

Small-sample	 empirical	 investigations	 are	 not	 a	 practical	

route	to	understanding	of	multiple	medications	and	driving	

impairment.	The	prevalence	of	PDI	drugs	in	any	populationbased	sample	work	against	successfully	modeling	the	predictor-criterion	relationships	of	greatest	interest,	but	sample	

recruitment	is	daunting.	

This	report	recommends	two	promising	methods	for	future	

research.	First,	databases	highlighted	in	this	report	can	be	

used	 to	 mine	 patient-level	 information	 that	 can	 pinpoint	

drugs	and	combinations	of	drugs	to	target	in	future	information	and	education	interventions.	Second,	unobtrusive,	

miniature,	 in-car	 instrumentation	 packages	 can	 monitor	

drivers’	 behaviors	 to	 measure	 behavioral	 variability	 as	 a	

function	 of	 driving	 context	 and	 to	 determine	 normative	

exposure	levels.

How to Order

To	order	A Pilot Study to Test Multiple Medication Usage and 

Driving Functioning (92	pages	plus	appendices),	prepared	by	

TransAnalytics,	LLC,	write	to	the	Office	of	Behavioral	Safety	

Research,	 NHTSA,	 NTI-130,	 1200	 New	 Jersey	Avenue	 SE.,	

Washington,	DC	20590,	fax	202-366-7394,	or	download	from	

www.nhtsa.dot.gov.	John	Siegler,	Ph.D.,	was	the	Contracting	

Officer’s	Technical	Representative	for	this	project.