Traffic Safety Facts

Traffic Tech – Technology Transfer Series

Number 339 January 2008

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

National Highway Traffic Safety Administration

Law	enforcement	officers	are	trained	in	accordance	with	

NHTSA/IACP	guidelines	to	administer	Standardized	

Field	 Sobriety	 Tests	 (SFSTs)	 to	 drivers	 suspected	 of	

alcohol	 impairment.	 The	 officer	 may	 later	 testify	 at	 a	

trial	about	the	evidence	that	led	to	the	arrest,	including	

the	driver’s	performance	on	the	SFST.	SFSTs	consists	of	

three	tests:	Horizontal	Gaze	Nystagmus	(HGN),	Walkand-Turn	(WAT),	and	One-Leg	Stand	(OLS).	

Courts	 generally	 accept	 testimony	 about	 WAT	 and	

OLS,	but	may	not	admit	testimony	about	HGN.	During	

an	 HGN	 examination,	 the	 suspect	 stands	 with	 feet	

together	 and	 arms	 at	 the	 side.	 A	 suspect	 must	 then	

follow	the	movement	of	a	stimulus	with	the	eyes	and	

the	 officer	 examines	 and	 scores	 each	 eye	 separately.	

Sometimes,	minor	procedural	differences	occur	in	the	

administration	 of	 an	 HGN	 test	 due	 to	 environment,	

weather,	and	the	suspect’s	level	of	cooperation.

Courts	have	accepted	arguments	that	variations	from	

standard	 procedures	 in	 HGN	 administration	 may	

affect	its	validity	and	as	a	result	render	HGN	testimony	

inadmissible.	 The	 effect	 of	 deviations	 from	 standard	

procedure	on	HGN	scores	has	never	been	systematically	

studied.	In	addition,	questions	have	been	raised	about	

the	validity	of	the	test	when	a	suspect	has	functional	

vision	in	only	one	eye.	

To	 ascertain	 whether	 minor	 variations	 in	 procedure	

affect	 the	 validity	 of	 HGN	 tests,	 NHTSA	 examined	

variations	in	HGN	administration	through	laboratory	

experiments	and	field	data	collection.	Under	contract,	

the	 Southern	 California	 Research	 Institute	 conducted	

three	experiments	to	examine	the	effects	of	procedural	

variations	in	the	administration	of	an	HGN	test	on	the	

accuracy	of	the	test.

Method

In	 each	 of	 the	 three	 experiments	 (summarized	 in	

	Table	1),	SFST-trained	and	experienced	officers	examined	alcohol	dosed	participants	under	standard	and	altered	administration	conditions.	The	experiments	were	

conducted	in	a	double-blind	procedure	where	neither	

the	participants	nor	officers	were	aware	of	participants’	

BAC	levels.	Alcohol	dose	amounts	were	based	on	gender,	age,	and	body	composition	and	were	calculated	to	

produce	expected	peak	BAC	≤	.12	at	the	end	of	a	30-

minute	absorption	period.

Table 1. HGN Procedural Variations Examined

Experiment Variable Definition

Stimulus Rate of speed of the stimulus as it 

1

Speed passes in front of a participant’s eyes

Stimulus Vertical position of the stimulus relative 

Elevation to the participant’s eye-level gaze

Stimulus Distance of the stimulus from the 

Distance participant’s face

2 Participant’s Participant standing, sitting, or lying 

Posture down during examination

3 Participant’s Participant having monocular vision

vision

The	 goal	 of	 the	 first	 experiment	 was	 to	 examine	 the	

effect	 of	 varying	 stimulus	 presentation	 during	 HGN	

examinations	on	officers’	correct	observations.	Stimulus	

speed	was	evaluated	at	the	standard	2-second	pass	and	

compared	with	a	1-second	pass.	Stimulus	elevation	was	

evaluated	at	the	standard	2	inches	above	eye	level	and	

compared	with	presentations	at	eye	level	and	4	inches	

above	eye	level.	Finally,	stimulus	distance	was	evaluated	

at	the	standard	12-	to	15-inch	range	and	compared	with	

a	10-	and	20-inch	distance	from	the	participant’s	face.	

The	 second	 experiment	 examined	 the	 effect	 of	 a	

suspect’s	position	during	an	HGN	test.	A	standard	HGN	

examination	is	conducted	with	a	suspect	standing,	feet	

together,	and	arms	at	the	sides.	However,	sometimes	a	

suspect	is	unable	to	stand,	and	the	test	is	administered	

with	 the	 suspect	 sitting	 or	 lying	 down.	 In	 this	 study,	

dosed	 participants	 received	 an	 HGN	 examination	

standing,	sitting,	and	lying	down.	

The Robustness of the Horizontal Gaze 

Nystagmus Test in Standardized Field Sobriety 

Tests
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	Patricia	Ellison-Potter,	

Ph.D.,	Editor,	fax	202-366-7096,	e-mail:	traffic.tech@dot.gov.

The	 goal	 of	 the	 third	 experiment	 was	 to	 assess	 the	

	validity	of	an	HGN	test	when	the	suspect	has	monocular	 vision.	 When	 officers	 conduct	 the	 HGN	 test,	 they	

examine	a	suspect’s	eyes	and	score	each	eye	separately,	

which	 may	 lead	 to	 discrepancies	 when	 a	 suspect	 has	

monocular	vision.	Therefore,	an	HGN	test	was	conducted	on	participants	with	functional	sight	in	one	eye.	

Experiments	 1	 and	 3	 were	 conducted	 in	 a	 laboratory	

setting.	Experiment	2	was	conducted	in	a	field	setting.

Findings

Overall,	the	laboratory	experiments	revealed	that	the	

	officer-examiners	did	not	err	when	participant	BACs	

were	.10	grams	per	deciliter	or	greater	and	rarely	erred	

when	 participants’	 BACs	 were	 .08	 g/dL	 or	 greater	

regardless	 of	 variations	 in	 stimulus	 presentation,	

participant	 position,	 or	 when	 participants	 had	

monocular	vision.

With	 respect	 to	 specific	 variations	 in	 stimulus	

administration,	a	1-second	stimulus	speed	significantly	

increased	the	number	of	false-negative	errors.	In	other	

words,	 a	 participant	 receives	 a	 score	 of	 no	 alcohol	

impairment	when	the	participant	was	in	fact	impaired.	

The	 standard	 2-second	 speed	 neither	 increased	

observational	 errors	 nor	 improved	 observations.	

Varying	the	level	of	the	stimulus	so	that	it	was	4	inches	

above	the	eye	or	at	eye	level	was	no	different	than	the	

results	 from	 the	 standard	 administration	 of	 2	 inches	

above	 eye-level.	 Finally,	 varying	 the	 distance	 of	 the	

stimulus	 from	 the	 face	 did	 not	 alter	 the	 HGN	 signs	

observed	at	20	inches	when	compared	to	the	standard	

distance	 of	 12	 to	 15	 inches	 from	 the	 participant’s	

face.	However,	holding	the	stimulus	10	inches	from	a	

participant’s	face	increased	the	number	of	HGN	signs	

an	examiner	correctly	observed.	

In	 Experiment	 2,	 participants’	 positions	 were	 varied	

for	 each	 HGN	 examination.	 Similar	 to	 the	 results	 of	

Experiment	1,	standing,	sitting,	or	lying	down	did	not	

make	 a	 difference	 in	 number	 of	 correct	 observations	

(see	Figure	1).	

Figure 1. Officer’s Scores by Participant Position

91.0 87.7 91.9

5.2 8.7 6.1 3.6 3.8 2.0

0

10

20

30

40

50

60

70

80

90

100

Percent

Standing Sitting Lying Down

Subject’s Position

Correct False Positives False Negatives

1200	New	Jersey	Avenue	SE.,	NTI-130

Washington,	DC	20590

The	 goal	 of	 the	 third	 experiment	 was	 to	 determine	

if	 the	 HGN	 test	 did	 not	 correctly	 identify	 impaired	

individuals	 if	 the	 person	 in	 question	 has	 monocular	

vision.	The	results	of	the	study	revealed	that	the	mean	

number	 of	 HGN	 signs	 was	 smaller	 in	 non-functional	

than	 functional	 eyes.	 The	 difference	 was	 statistically	

significant	but	small	in	magnitude.	While	the	results	are	

preliminary	due	to	the	small	sample	size,	there	was	no	

evidence	that	participants’	non-functional	eyes	affected	

HGN	 in	 their	 functional	 eyes.	 Moreover,	 there	 is	 no	

evidence	that	HGN	signs	in	monocular	individuals	will	

lead	to	false	arrests.	The	findings	cannot	be	generalized	

to	 individuals	 whose	 visual	 deficits	 have	 differing	

etiologies.	

Implications

The	results	of	this	study	reveal	that	HGN	is	a	robust	

phenomenon.	 Minor	 procedural	 variations	 do	 not	

compromise	the	validity	of	the	HGN	examination.	

How to Order

For	 a	 copy	 of	 The Robustness of the Horizontal Gaze 

Nystagmus Test (36	pages	plus	appendices),	write	to	the	

Office	of	Behavioral	Safety	Research,	NHTSA,	NTI-130,	

1200	 New	 Jersey	 Avenue	 SE.,	 Washington,	 DC	 20590,	

send	 a	 fax	 to	 202-366-7096,	 or	 download	 from	 www.

nhtsa.dot.gov.	 James	 Frank,	 Ph.D.,	 and	 Jenny	 Percer,	

Ph.D.,	were	the	project	officers.