Traffic Safety Facts

Traffic Tech – Technology Transfer Series

Number 338 December 2007

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

National Highway Traffic Safety Administration

In	 an	 effort	 to	 address	 restoring	 credibility	 to	 speed	

limits,	 the	 National	 Highway	 Traffic	 Safety	

Administration	 and	 the	 Federal	 Highway	

Administration	 (FHWA)	 have	 jointly	 undertaken	

tests	 of	 speed	 limits.	 These	 speed	 limits	 have	 been	

established	by	a	formal	engineering	review	that	starts	

with	 the	 85th	 percentile	 speed	 of	 free	 flowing	 traffic.	

However,	 these	 speed	 limits	 could	 be	 set	 as	 low	 as	

the	50th	percentile	speed	based	on	access,	pedestrian	

activity,	crash	history,	and	other	factors.	Speed	limits	so	

set	are	referred	to	as	“rational	speed	limits.”	The	overall	

objective	of	these	tests	is	to	determine	whether	speed	

limits	so	set,	when	combined	with	well	publicized	and	

targeted	 enforcement,	 result	 in	 greater	 compliance,	

more-uniform	speeds,	and	improved	safety.

This	approach	of	setting	speed	limits	is	based	on	work	

showing	 the	 85th	 percentile	 to	 be	 an	 acceptable	 limit	

from	 a	 safety	 perspective,	 as	 well	 as	 the	 assumption	

that	most	motorists	will	select	a	safe	speed	on	their	own	

when	given	the	opportunity.

This	report	presents	the	evaluation	results	from	the	first	

of	six	projects	being	conducted	jointly	by	NHTSA	and	

FHWA	on	the	demonstration	and	evaluation	of	rational	

speed	 limits.	 The	 evaluation	 was	 conducted	 by	 an	

independent	 contractor	 (Westat,	 Inc.).	 Following	 data	

collection	and	engineering	analyses,	the	speed	limits	on	

various	portions	of	a	demonstration	road	in	Gulfport,	

Mississippi	(a	7.5-mile	segment	of	U.S.	Route	49),	were	

raised	 5,	 10	 or	 15	 mph	 at	 selected	 sites,	 creating	 five	

speed	zones	(35,	40,	45,	50,	60	mph).	The	increases	in	

limits	 were	 accompanied	 by	 public	 information	 and	

education,	and	stricter	enforcement	of	the	raised	limits.

Data	 on	 speeds,	 crashes,	 citations	 and	 enforcement	

hours	were	collected	in	both	Gulfport	and	a	comparison	

community	prior	to	and	quarterly	during	the	one-year	

demonstration	period.

Findings

Figure	 1	 shows	 the	 improvement	 in	 speed	 limit	

compliance	following	introduction	of	rational	limits	in	

the	 demonstration	 community.	 The	 black	 bars	 depict	

the	proportion	of	drivers	exceeding	the	limit	by	10	mph	

or	more	at	each	of	6	representative	speed	measurement	

locations	along	U.S.	Route	49	during	the	baseline	period	

prior	to	the	introduction	of	rational	limits.	The	grey	bars	

depict	 the	 proportion	 exceeding	 the	 limit	 during	 the	

period	 rational	 limits	 were	 implemented.	 This	 figure	

shows	 that	 compliance	 with	 the	 newly	 introduced	

rational	speed	limits	was	better	than	compliance	with	

the	prior	limits	(except	for	site	1,	where	the	speed	limit	

was	unchanged).

Figure 1. DEMONSTRATION COMMUNITY: Percent of 

Vehicles Traveling at Least 10 mph Over the Speed Limit

A Field Test of Rational Speed Limits in 

Gulfport, Mississippi

0

5

10

15

20

25

30

35

40

45

1 2 3 6 10 13

Site

% of Vehicles traveling at least

10 mph over the speed limit

(35 mph, 35 mph) (35 mph, 40 mph) (35 mph, 45 mph) (35 mph, 50 mph) (45 mph, 50 mph) (50 mph, 50 mph)

3.7

35.7

38.3

8.2 8.5 6.6 8.1

4.3 5.7 6.9 5.4 3.4

Predemonstration

Demonstration

Figure	2	shows	the	comparable	data	for	10	mph+	over	

the	speed	limit	for	the	comparison	community.	Note	

that	compliance	with	the	limit	was	mostly	unchanged	

or	somewhat	worse	during	the	equivalent	periods.
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.

Figure 2. COMPARISON COMMUNITY: Percent of Vehicles 

Traveling at Least 10 mph Over the Speed Limit

It	is	also	noteworthy	that	total	crashes	increased	in	a	

comparison	community	where	no	rational	limits	were	

imposed	(see	Figure	4).	This	provides	some	support	to	

the	view	that	the	rational	limits	did	not	have	a	negative	

impact	on	safety	since	no	corresponding	increase	was	

found	in	the	demonstration	community.	However,	it	is	

not	 possible	 to	 determine	 the	 effect	 of	 rational	 speed	

limits	on	crashes	from	this	limited	amount	of	data.

Figure 4. COMPARISON COMMUNITY: Speed-Related and 

Total Crashes

1200	New	Jersey	Avenue	SE.,	NTI-130

Washington,	DC	20590

0

10

20

30

40

50

60

70

80

90

100

Number of Speed-Related Crashes

2000 2001 2002 2003

Years

Number of Total Crashes

Baseline Years

Total Crashes

Speed-Related Crashes

Speed Crashes

Total Crashes

400

500

600

700

800

900

1000

1100

1200

1300

Demonstration

Year

0

10

20

30

40

50

60

70

80

90

100

2001 2002 2003

Years

Number of Speed-Related Crashes

200

220

240

260

280

300

320

340

360

380

400

Number of Total Crashes

Speed Crashes

Total Crashes

Baseline Years Demonstration

Year

Total Crashes

Speed-Related Crashes

0.6

5.4

23.1

2.7

4.6

1.1 1.7 1.9

6.4

27.8

5.3 4.2

0

5

10

15

20

25

30

1 2 3 4 5 6

Site

% of Vehicles traveling at least

10 mph over the speed limit

Demonstration

Period

(45 mph) (45 mph) (45 mph) (45 mph) (55 mph) (55 mph)

Predemonstration

Period

Figure	 3	 shows	 that	 total	 crashes	 and	 speed-related	

crashes	decreased	in	the	demonstration	year,	reversing	

a	 3-year	 upward	 trend.	 Nevertheless,	 this	 should	 be	

interpreted	 with	 caution	 because	 compared	 with	 the	

prior	3	years,	both	speed-related	and	total	crashes	were	

somewhat	higher	during	the	demonstration.

Figure 3. DEMONSTRATION COMMUNITY: Speed-Related 

and Total Crashes

Further	 evaluation	 studies	 (currently	 ongoing	 in	 6	

other	 communities)	 are	 required	 to	 determine	 the	

overall	safety	effect.	However,	these	findings	suggest	

that	 rational	 limits	 can	 be	 implemented	 without	

compromising	safety.

How to Order

For	 a	 copy	 of	 Field Test of the Impact of Setting and 

Enforcing Rational Speed Limits (38	 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	 http://www.nhtsa.dot.gov.	 Paul	 J.	

Tremont,	Ph.D.,	was	the	project	officer.