TECHBRIEF The Safety Impacts of 

Differential Speed Limits on 

Rural Interstate Highways

BACKGROUND

The Surface Transportation and Uniform Relocation Assistance Act

(STURAA), enacted on April 2, 1987, permitted individual States to raise

speed limits from the previously mandated national speed limit of 88

kilometers per hour (km/h) to 105 km/h (55 miles per hour (mi/h) to 65

mi/h) on rural interstate highways. After this date, some States uniformly raised their speed limits for passenger cars and trucks. Other

States raised the speed limit for passenger cars only while leaving the

truck limit at 88 km/h (55 mi/h), creating a Differential Speed Limit

(DSL)—different speed limits for cars and heavy trucks traveling on the

same roadway. Speed limits that are the same for both passenger cars

and trucks are known as Uniform Speed Limits (USL). On November 28,

1995, the national maximum speed limit was repealed, giving States

further flexibility in setting their limits. By 2002 several States had

experimented with both DSLs and USLs.

OBJECTIVE

Previous studies conducted during the early 1990s that compared

USLs and DSLs were constrained because of the limited data available.

Over the past decade several States have either eliminated or implemented a lower limit for trucks providing the opportunity for a before

and after study that might provide more reliable results. In 1994,

Virginia switched from a differential speed limit of 105 km/h (65 mi/h)

for cars and 88 km/h (55 mi/h) for trucks to a uniform speed limit of

105 km/h (65 mi/h) for all vehicles. In 1996, Arkansas adopted a differential speed limit by raising the speed limit for cars to 113 km/h (70 mi/h)

but maintaining 105 km/h (65 mi/h) for trucks. In 1998, Idaho changed

from a uniform speed limit of 121 km/h (75 mi/h) for all vehicles to 

a 105 km/h (65 mi/h) limit for trucks. With more than a decade having

elapsed since the passage of the STURAA, the Federal Highway

Administration (FHWA) sponsored a long-term study to investigate

the effect of USLs and DSLs on vehicle speeds and crashes on rural

interstates nationwide.

Research, Development, and

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.tfhrc.gov
LITERATURE REVIEW

The safety effects of differential

speed limits for cars and trucks

have been inconclusive in previous studies. Some studies found

no difference between USL and

DSL (references 1, 2, 3, and 4).

Other studies found one or the

other to be a better policy

choice.(5,6) The studies were mainly cross-sectional comparisons of

speed or crashes in States with

different speed limits for cars and

trucks to those in nearby States

with uniform speed limits. The

differences (or lack thereof) observed between States could be

due to variations in traffic density,

weather, and many other factors.

Impact of DSL on Mean Speed

In 1990, Freedman and Williams

analyzed speed data collected at

54 sites in 11 Northeastern States

to determine the effect of DSL

on mean and 85th percentile

speeds.(5) Six States had retained

a USL of 88 km/h (55 mi/h), three

had raised USLs to 105 km/h (65

mi/h), and two States employed

a DSL of 105 km/h (65 mi/h) for

cars and 88 km/h (55 mi/h) for

trucks. For passenger cars in the

DSL States, the mean speed and

85th percentile speeds were not

significantly different from the

105 km/h (65 mi/h) USL States.

For trucks in the DSL States, the

mean and 85th percentile speeds

were not significantly different

from those of the 105 km/h (65

mi/h) USL States. Similar results

were obtained when comparing

the percentage of vehicles complying with the speed limit.

In 1994 Harkey and Mera found

no significant difference between

passenger car and truck mean

speeds when comparing USLs

and DSLs.(1)

Impact of DSL on Speed Variance

The implication of increased

speed variance is an increase in

interactions between vehicles,

leading to a potential increase in

crashes. Council et al. in 1998

found that for rear-end collisions

between cars and trucks, a highspeed differential increases the

severity of the crash.(6) However,

Harkey and Mera found no significant differences between car

speed variances at the USL and

DSL sites.(1) Furthermore, they

found no difference between

the speed distributions for both

cars and trucks for the 105/97

km/h (65/60 mi/h) and 105/105

km/h (65/65 mi/h) speed limits.

A 1974 study by Hall and Dickinson showed that speed differences contributed to crashes,

primarily rear end and lane

change collisions.(2) 

Impact of DSL on Crashes

Harkey and Mera investigated

crash results from 26 sites in 11

States.(1) The study investigated

the percentage of three different

crash types (rear-end, sideswipe,

and all other) for USLs and DSLs.

Table 1 shows that a higher

proportion of car-into-truck and

truck-into-car crashes occurred in

USL States, except for rear end

crashes where more car-intotruck collisions happened in the

DSL group.

A study by Garber and Gadiraju

conducted in 1991 compared

TABLE 1. Accident Proportions by Speed Limit, Collision Type, and Vehicle Involvement.

Speed Limit

USL: 105 km/h and 

88 km/h (65 mi/h and

55 mi/h)

10.91 10.78

13.70 6.86

22.12 21.07

21.52 14.96

2.57 2.01

2.07 0.99

DSL: 105/88 km/h and

105/97 km/h (65/55

mi/h and 65/60 mi/h)

Rear End

Car-Into- TruckTruck Into-Car

Sideswipe

Car-Into- TruckTruck Into-Car

Other

Car-Into- TruckTruck Into-Car

(Adapted from table 32, reference 1)
crash rates in the adjacent States

of Virginia (DSL) and West Virginia (USL).(3) The increase in the

posted speed limit for trucks to

105 km/h (65 mi/h) did not result

in a significant increase in fatal,

injury, and overall accident rates.

There was, however, some evidence that the DSL may increase

some types of crash rates while

reducing others.

According to Hall and Dickinson,

the existence of a posted DSL,

however, was not related to the occurrence of truck crashes.(2) Finally,

an evaluation conducted by the

Idaho Department of Transportation

found that a change from USL to

DSL did not increase crashes.(4)

Caveats to the Following Study

There are six limitations that may

apply to the speed and crash

rates results of this study:

● Selected sites may be a biased

sample.

● It was not possible to obtain

speeds by vehicle type (passenger cars and truck).

● Durations used in this study are

relatively short.

● Rural interstates were analyzed

at an annual level of detail.

● Sample size used in the statistical tests associated with the

speed analysis was defined as

the number of speed monitoring sites and varied by state.

● Law enforcement patterns during these time periods are unknown.

METHODOLOGY

Three general steps comprised

the methodology used in this

research:

● Speed and crash data were collected from States that had

been identified as having

changed their speed limits at

least once during the 1990s

from USL to DSL or vice versa.

TABLE 2. Types of Speed Limits Throughout the 1990s on Rural Interstate Highways.

Maintained USL Maintained DSL Changed from USL to DSL Changed from DSL to USL

Arizona

Iowa

North Carolina

Illinois

Indiana

Washington

Arkansas

Idaho

Virginia

Figure 1. Mean Speed for All Vehicles.

Mean Speed Trend

93

97

100

103

106

109

113

116

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001

Year

Mean Speed in km/h

Iowa

Illinois

Indiana

Idaho

Virginia

1 mi/h = 1.6 km/h
● Conventional statistical approaches (analysis of variance –

Tukey’s and Dunnett’s tests)

were used to analyze speed and

crash data from these States.

● Empirical Bayes procedure was

applied to these crash data.

Nine States were selected so they

could be divided into four policy

groups based on the type of

speed limit employed during the

period, as shown in table 2.

RESULTS

Vehicle Speeds

Five speed measures (mean

speeds, speed variance, 85th percentile speeds, median speeds,

and noncompliance rates) were

analyzed for the five States where

such speed monitoring data were

readily available. Speed data

were generated from speed monitoring stations throughout the

States; individual speeds on specific interstates were not always

available. It was not possible to

obtain speeds by vehicle type

(passenger cars and trucks).

Figure 1 illustrates the trends in

mean speeds, for all vehicle types,

among the five States with speed

data. Data could not be obtained

for all years during the time periods. Except for Virginia, the main

observation is that all speeds appear to be increasing over time,

regardless of speed limit type.

Crashes

Figure 2 presents an overall representation of crash data from the

various States. While the data in

figure 2 are based on crash rates

and validates the results generated by the Empirical Bayes

method, it should be noted that

the Empirical Bayes method did

not use crash rate in the modeling

process but included annual average daily traffic (AADT) and section length as independent variables. Only North Carolina showed

a significant increase in the total

crash rate; the other States

showed no significant change in

the total crash rate.

Caveats to the Use of 

Empirical Bayes Method 

Several data limitations might

have influenced the results of the

Empirical Bayes analysis.

● Comparison groups were imperfect. Ideally, the comparison

group would have been selected from the same State at the

same time as the studied group.

● Although speed monitoring data

were available to understand

statewide speed trends, specific

speeds for every interstate section used in the crash analysis

were not available.

● The crash estimation model

used only two variables—AADT

and section length. There may

have been other relevant variables that were not included in

the model.

Total Crash Rates

0

20

40

60

80

100

120

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Year

Total Crash Rates 

(Crashes Per 100 Million VMT)

Arizona

North Carolina

Idaho

Arkansas

Virginia

Figure 2. Total Crash Rates per million vehicle-miles traveled (VMT).

Note that speed limits changed in Idaho (1996, 1998), Arkansas (1996), and Virginia (1994)
FINDINGS

To evaluate how a treatment affects safety, the Empirical Bayes

method predicts what the expected crash frequency would

have been during the after period had there been no such treatment and then compares it to

the actual number of crashes

that occurred during the after

period. Using the Empirical

Bayes technique, the ratio θ of

the “actual” after crashes to the

“would have been” after crashes

is calculated. If the ratio θ is

greater than 1.0, then the treatment (e.g., a change from one

type of speed limit to another)

resulted in an increase in the

number of crashes.

In most cases, θ was greater

than 1.0, as shown in table 3, indicating an increase in crashes.

However, the data in table 3 are

not consistent. The ratio θ for

total crashes in Virginia, which

changed from DSL to USL, is

higher than one of the States

that changed from USL to DSL

(Arkansas) but lower than the

other state that changed from

USL to DSL (Idaho). The table

also shows that for total crashes,

θ was approximately 1.0 for the

State that maintained DSL (Washington) while it was greater than

1.0 for States that maintained

USL (Arizona and North Carolina).

Additional crash types, such as

rear-end type crashes, are discussed in the final report. 

TABLE 3. Impact of Speed Limit Changes, Confidence Intervals and Crash Increases According to the

Empirical Bayes formulation.

Crash Type

Confidence

Interval

Lower Bound

Increase

Confidence

Interval

Upper Bound

Increase

Crash

Effect

Ratio θ

Total crashes 1.26 24.2% 28.6% Increase

Total crashes with truck involved 1.16 12.1% 20.7% Increase

Total crashes 1.26 19.9% 31.9% Increase

Total crashes with truck involved 0.91 -19.7% 1.5% No change

Total crashes 0.99 -6.6% 5.0% No change

Total crashes 1.07 0.4% 13.4% Increase

Total crashes with truck involved 1.31 18.9% 42.8% Increase

Total crashes 1.29 13.2% 46.7% Increase

Total crashes with truck involved 2.46 68.6% 224.9% Increase

*North Carolina maintained their uniform limit but also raised this limit for both passenger car and trucks

Total crashes 1.15 12.9% 17.2% Increase

Total crashes with truck involved 1.25 20.0% 29.8% Increase

Maintained a uniform limit (Arizona)

Maintained a uniform limit (North Carolina)*

Maintained a differential limit (Washington)

Changed from uniform to differential (Arkansas)

Changed from uniform to differential (Idaho)

Changed from differential to uniform (Virginia)
Researcher—This study was performed by the Virginia Transportation Research Council.

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution is being

made to the Divisions and Resource Centers.

Availability—The report will be available in Fall 2004 and may be obtained from the FHWA Report Center 

by e-mail to report.center@fhwa.dot.gov, by fax to 301–577–1421, by phone to 301–577–0818, or online at

www.tfhrc.gov/safety/intersect.htm.

Key Words—Differential speed limit, universal speed limit, truck speed limit, speed limit

Notice—This TechBrief is disseminated under sponsorship of the U.S. Department of Transportation in the interest of information exchange. The TechBrief provides a synopsis of the study’s final publication. The TechBrief

does not establish policies or regulations, nor does it imply FHWA endorsement of the conclusions or recommendations. The U.S. Government assumes no liability for the contents or their use.

SEPTEMBER 2004 FHWA-HRT-04-156

HRDS-05/09-04(500)E

REFERENCES

1. Harkey, D. L. and Ruben Mera: Safety Impacts of Different Speed Limits on Cars and Trucks, U.S. Department of

Transportation, Federal Highway Administration, Publication No. FHWA-RD-93-161, Washington, D.C., May 1994.

2. Hall, J. W. and L. V. Dickinson. An Operational Evaluation of Truck Speeds on Interstate Highways, Department

of Civil Engineering, University of Maryland, February, 1974.

3. Garber, N. J. and R. Gadiraju., Impact of Differential Speed Limits on Highway Speeds and Accidents, 

Department of Civil Engineering, University of Virginia, Charlottesville, VA, 1991, pp13.

4. Idaho Transportation Department Planning Division, Evaluation of the Impacts of Reducing Truck Speeds

on Interstate Highways in Idaho, -Phase III, Final Report, December, 2000, National Institute for Advanced

Transportation Technology, University of Idaho.

5. Freedman, M. and A. F. Williams. “Speeds Associated with 55 mi/h and 65 mi/h Speed Limits in Northeastern

States,” ITE Journal, pp. 17-21, Vol. 2, No. 2, 1992, Institute of Transportation Engineers, Washington, D.C.

6. Council, F. M. et al. “Applying the Ordered Probit Model to Injury Severity in Truck-Passenger Car Rear-End

Collisions,” Transportation Research Record 1635, Transportation Research Board, National Research Council,

Washington, D.C., 1998.

CONCLUSIONS

The results presented in table 3

are on a State-by-State basis.

Overall, the study was not able to

isolate or measure the effect of

USL/DSL changes. The effect of

the DSL, if any, is not enough to

be detected in the aggregate

speed data that were analyzed.

Speed characteristics were generally unaffected by a USL or

DSL policy. Except for Virginia,

mean speeds tended to increase

over the 1990s regardless of

whether the State maintained a

USL, maintained a DSL, or

changed from one to the other.

In some cases the increase in

speed was significant, in other

cases it was not.

No consistent safety effects of

DSL as opposed to USL were

observed within the scope of

the study. The mean speed and

crash rates tended to increase

over the 10-year period, regardless of whether a USL or DSL

limit was employed. The Empirical Bayes methodology suggested that crash risk during the

study period increased for all

four policy groups.