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INTERSTATE RURAL SPEED LIMITS
THE EFFECT OF SPEED LIMIT CHANGES ON FATAL CRASHES
OREGON TRANSPORTATION INSTITUTE
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Portland, OR 97229-
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RURAL INTERSTATE SPEED LIMITS
After Congress repealed the National Maximum Speed Limit in December of 1995, all
but five states (Oregon, Kentucky, Maine, Vermont and Virginia) increased rural interstate
speed limits. Twenty-
Louisiana, Minnesota and West Virginia increased speed limits in 1997 and New Jersey changed limits in 1998. Florida raised speed limits from 65 to 75 MPH on April 8, 1996 but was not included in our analysis. Early studies led reviewers to doubt the reliability of the Florida data. [1] Even after the Florida data was revised, we were not sufficiently assured of its reliability. For example, of the 2,151 fatal crashes reported by the Fatality Analysis Reporting System of the National Highway Traffic Safety Administration (FARS) for 2001, 1,595 were classified as an “Unknown” roadway function class. In other years, “unknown” was among the highest numbers of reported fatal crashes for rural road classifications in Florida.
Sixteen other states were not used because the speed limits were not raised uniformly, pertaining to only to specified segments. In our view, the lack of uniformity could be a greater contributing factor to fatal crashes than the speed by causing less uniform understandings of the speed limits and, therefor, greater variations in driving speeds.
In this paper, we set forth the experience for fatalities and fatal crashes comparing the years before and after the change for these states as well as principles promulgated over the years by many researchers which provide information to aid the reader to better understand the findings.
We show the effects on rural interstates, all rural roads and all rural roads except for interstates because a speed limit change on an interstate can affect the route choices of drivers.
During the 1995-
Herein, we use “fatal crash” data (the number of fatal crashes) as opposed to “fatalities.” Fatalities have the potential of skewing results because the number of fatalities is affected by the number of people in a vehicle, a random factor which may or may not have an effect on the crash. In before and after change comparisons, we compare the “fatal crash rate” derived by dividing the number of fatal crashes for a roadway function class (i.e. rural interstate) by the vehicle miles traveled (VMT) for the same roadway function class in the same state. All rates are per 100 million VMT.
We depart from the methods used in many other studies in two major respects: the groups compared and the periods measured. We measure the states that made changes against their own performance before and after the change, as a group which consists of all changes and as separate groups of those states that made the same miles per hour change. We believe this is vital as the effects on safety due to changes depend on the size of the change as well as the speeds themselves.
Unlike studies that measure the effects of changes immediately after a speed limit change is enacted, we measure the effects over a longer period of time. It takes a long time to implement a speed change.
class=Section3>
Oregon Department of Transportation estimates six months before the signage is installed: Two to four weeks for the specifications to be prepared, one month to advertise and receive bids and three to four months for the contractor to complete the work.[2]
Even after the signage is in-
RURAL INTERSTATE SPEEDS
In 1941, President Franklin D. Roosevelt appointed a committee to evaluate the need
for a national expressway system. The Committee supported such a system in 1944 and
the Congress passed the Federal Aid-
Uniform design standards called for design speeds of 70 MPH, lanes 12 feet wide, gradients not to exceed 6 percent and bridge clearances of fourteen feet. [5] As people who drove the system when it opened know, the speed limit, with very few clearly marked exceptions, was 70 MPH.
On October 17, 1973, the Organization of Petroleum Exporting Countries (OPEC) raised oil prices from $3 per barrel to $5.11. In 1974, OPEC raised prices again to $11.65 per barrel. To promote oil conservation from driving at a speed which was the most fuel efficient, Congress legislated a National Maximum Speed Limit (NMSL) of 55 MPH which, also, provided funding for states to hire more state police to enforce the new fuel conservation limit.
In 1987, with oil prices ranging from $16.45 to $19.32 per barrel, Congress enacted legislation which allowed states to raise their speed limits to 65 MPH on rural interstates. 40 states did so immediately.
In December 1995, Congress repealed the NMSL and states, once again, could set speed limits at whatever they thought was safest. As indicated above, all but five states made changes.
BACKGROUND
Factors affecting fatal crashes and other accidents are:
1. Stopping distances increase with higher speeds.
2. Higher speed results in higher severity of accidents with the sharpest severity increase above 70 MPH. [6] This study was prior to completion of the interstate system designed for faster speeds than preceding main rural roads and pertained to cars without safety devices and with equipment such as non steel belted radial tires designed for lower speeds.
3. Variations in speed increase accident probability. As Solomon reported in 1964, accident rates increased as reported speed deviated from the mean speed. [7] Cirillo [8] found similar results on the interstate system in 1968. In a similar study, West and Dunn found slower drivers had higher accident rates than faster than average drivers.[9] Their data showed that low speed and high speed drivers 15.5 MPH above or below the mean speed had six times the number of accidents as drivers closer to the mean speed. Hauer summarized “The greater the deviation from the median speed, the greater the number of times a vehicle is overtaken or passed which increases accident risk.” [10]
What is the “comfortable speed” for drivers on rural interstates or other roads, the speed that they would drive regardless regardless of speed limit posting or with no speed limit? In 1975, Morris and Berry did a study of drivers on rural interstates with their speedometers covered. [11] They found that 69.9 was the preferred speed on rural interstates. This followed a study which found that the safest speed was the speed traveled by 85% of drivers.[12] This is what is referred to as the “85th percentile.
The Office of Safety and Traffic Operations R & D of the Federal Highway Administration sponsored a study of speed limit changes which was performed between 1985 and 1992, eleven to 18 years after imposition of the artificial fuel conservation 55 MPH limit and after the 1987 change to a 65 MPH maximum, [13] The report showed that lowering speed limits by as much as 20 MPH or raising speed limits as much as 15 MPH had little effect on driving speeds. They found that lowering speed limits below the 50th percentile did not reduce accidents but did increase driver violations. They found that raising posted speeds did not increase speeds materially or increase accidents.
After the 1987 congressional action which allowed states to raise their speed limits from 55 to 65 on rural interstates and 40 states did so, Lave and Elias studied the effects of those changes on fatalities in the entire system. [14] Lave and Elias theorized that traffic shifted back to the rural interstates with the higher limit and state patrol resources were allowed to be allocated from rural interstates to other roads once the requirement to enforce the 55 MPH conservation limit was changed and both affected fatalities. This study found that the statewide fatality rate was reduced by the speed limit change.
STATES CHANGING SPEED LIMITS IN 1995 -
State Date Enacted Old Limit New Limit
RHODE ISLAND MAY 12, 1996 55 65
TEXAS DEC 8, 1995 65 70
CALIFORNIA JAN 7, 1996 65 70
KANSAS MAR 7, 1996 65 70
MISSISSIPPI FEB 29, 1996 65 70
MISSOURI MAR 13, 1996 65 70
WASHINGTON MAR 15, 1996 65 70
ALABAMA MAY 9, 1996 65 70
NORTH DAKOTA JUN 10, 1996 65 70
GEORGIA JUL 1, 1996 65 70
ARKANSAS AUG 19, 1996 65 70
MICHIGAN AUG 1, 1996 65 70
NORTH CAROLINA AUG 5, 1996 65 70
OKLAHOMA DEC 15, 1995** 65 75
ARIZONA DEC 8, 1995 65 75
WYOMING DEC 8, 1995 65 75
NEVADA DEC 8, 1995 65 75
SOUTH DAKOTA APR 1, 1996 65 75
IDAHO MAY 1, 1996 65 75
NEW MEXICO MAY 15, 1996 65 75
UTAH MAY 1, 1996 65 75
COLORADO JUN 24, 1996 65 75
NEBRASKA JUN 1, 1996 65 75
MONTANA * DEC 8, 1995 65 *
* In 1996-
** On December 15, 1995, a change was enacted from 65 to 70 MPH and raised, on August 29, 1996, to 75 MPH.
RATE CHANGES FOR SPEED CHANGES FROM 65 TO 70 MPH
All of the fatalities and fatal crashes have been provided by the Fatality Analysis Reporting System (FARS) of the National Highway Traffic Safety Administration. The Vehicle Miles Traveled (VMT) data was supplied by the Federal Highway Administration (FHWA). The rates were obtained by dividing the number of fatalities or fatal crashes for specific roadway segments by the VMT for the same roadway segment (in millions).
It should be noted that the reporting of fatalities and fatal crashes is hardly an exact science. Not all rural interstate accidents are observed and many have no survivors to talk about the details. FARS supplies data on fatalities and fatal crashes to FHWA but a review of the web site for both agencies shows conflicting data indicating that a process of refinement is ongoing.
It is also true that the other ingredient used to establish rates, VMT, is a rough estimate. Some of the reported VMT data looked suspicious when compared to reported VMT by other states and when compared to the gas tax collections by the same state.
Table 1 shows the rates for the twelve states that changed speed limits from 65 MPH to 70 MPH on rural interstates that did not have inclusion problems referred to above.
Rates Prior To The Speed Limit Change
1990 to 1994 -
1990 to 1995 -
The enactment dates started in December of 1995 so the new speed limits were not, in fact, implemented until 1996 and later.
Rates After The Speed Limit Change
1996 to 2001 -
1997 to 2001 -
1998 to 2001 -
In 1996, a transition year, the rate jumped to 1.212 . In every year thereafter, the fatality rate fell. The fatal crash rate was lower in 2001, the last year for which there was data, than any of the years before the speed limit was changed.
The steady decline indicates, for the changes from 65 MPH to 70 MPH, the speed limit increase had no adverse effect on fatal crashes and seems to have had a beneficial effect.
Observations by the Oregon Transportation Institute on some rural interstates with a speed limit of 65 MPH showed the 85th percentile speed at 71 MPH. The speed limit change merely legalized the speed at which 85 % of drivers were already driving. The reduction of fatal crashes may well be due to the fact that fewer drivers hit the brakes when they saw or thought they saw a law enforcement vehicle.
OTHER RURAL ROADS -
Rates Prior To The Speed Limit Change
1990 to 1995 -
Rates After The Speed Limit Change
1996 to 2001 -
1997 to 2001 -
1998 to 2001 -
Since implementing the new limits, the fatal crash rate has declined slowly but steadily indicating a beneficial effect.
ALL RURAL ROADS -
Rates Prior To The Speed Limit Change
1990 to 1995 -
Rates After The Speed Limit Change
1996 to 2001 -
1997 to 2001 -
1998 to 2001 -
The pattern, shown in more detail on Table 3 is one of improvement, increasing as time passes.
RATE CHANGES FOR SPEED CHANGES FROM 65 TO 75 MPH
Table 4 shows the before and after fatal crash rates for rural interstates changing speeds from 65 to 75 MPH. Table 5 shows the same for rural roads, not including the rural interstates. Table 6 shows data for all rural roads, including interstates.
RURAL INTERSTATES
Rate Prior To The Speed Limit Change
1990 to 1995 -
Rates After The Speed Limit Change
1996 to 2001 -
1997 to 2001 -
1998 to 2001 -
While the speed changes from 65 to 70 showed a steady decrease in the rates, the speed changes from 65 to 75 showed a slight increase which turned into a slight decrease in the average for the last four years. One explanation, which may be valuable in enacting speed limit changes, is that large changes, particularly when the change is above the 85th percentile, might be avoided in favor of small increment changes depending on the relationship of the existing speed limit to the 85th percentile at the time of the change.
Studies show that the 85th percentile is the safest speed limit. However, the 85th percentile is not a fixed speed. It may be that the comfort speed level increases with time and the 85th percentile increases accordingly. The average fatal crash rate for the last four years for the 65 to 75 MPH change would appear to point in this direction. Of course, this is increasingly less applicable as limits continue to increase above the road design standards.
OTHER RURAL ROADS LESS THE RURAL INTERSTATES
Rates Prior To The Speed Limit Change
1990 to 1995 -
Rates After The Speed Limit Change
1996 to 2001 -
1997 to 2001 -
1998 to 2001 -
While the fatal crash rate rose for the first two years after the change for the rural interstates, the beneficial effect on the other roads showed increasingly beneficial effects immediately.
ALL RURAL ROADS -
Rates Prior To The Speed Limit Change
1990 to 1995 -
Rates After The Speed Limit Change
1996 to 2001 -
1997 to 2001 -
1998 to 2001 -
The overall effect on all rural roads was a lowering of the fatal crash rate, showing a steady downward trend.
OTHER SPEED CHANGES
Two other states made changes in the period studied. Rhode Island enacted a speed change from 55 to 65 on rural interstates in May of 1996. Since the speeds were so dissimilar and Rhode Island has only 21 miles of rural interstates, it was deemed inappropriate for inclusion.
Montana enacted a speed limit change from 65 to basic rule (prudent speed) during the day and 65 MPH at night in December of 1995. In 1999, another change was made setting speed limits at 75 MPH for day and night. Montana was not included in the 65 to 75 MPH category because of the short time span at the 75 MPH speed and that the change, in 1999, could be regarded as a reduction in speeds. For whatever it’s worth, the fatal crash rate on Montana rural interstates dropped from 1.755 before the change to 1.680 (average for 1996 to 2001). On rural roads less interstates, the rate rose from 2.732 to 2.812.
TABLES
ENDNOTES
[1]. Charles Farmer, Richard Retting and Adrian Lund, ”Changes in Motor Vehicle Occupant Fatalities After Repeal of the National Maximum Speed Limit, ” October 23, 1997, Revised 1998.
[2]. E mail from Douglas J. Tindall, Director of Statewide Maintenance, Oregon Department of Transportation, November 29, 2001.
[3]. Charles Lave, “Higher Speeds May Save Lives,” Access, Number 7, Fall
1995, pp. 23-
[4]. Federal highway Administration, “The Dwight D. Eisenhower Interstate System,” 2003, http://www.fhwa.dot.gov/programadmin/interstate.html .
[5]. “The States and the Interstates-
[6]. D. Solomon, “Accidents on Main Rural Highways related to Speed, Driver and Vehicle,” Bureau of Public Roads, July 1964.
[7]. Ibid.
[8]. J. A. Cirillo, “Interstate System Accident research Study II, Interim Report II,” Public Roads, Vol 35, No 3, August 1968.
[9]. L.B. West, Jr. and J.W. Dunn, “Accidents, Speed Deviation and speed Limits,” Traffic Engineering, Vol 41, No 10, July 1971.
[10]. E. A. Hauer, “Accidents, Overtaking and Speed Control,” Accident Analysis and Prevention, Vol 3, July 1971.
[11]. R. S. Morris and C. Berry, “An Analysis of Comfortable Driving Speed,” Safe Journal, Vol 7, No 1, 1975.
[12]. K. B. Joscelyn, R. K. Jones and P. A. Elston, “Maximum speed Limits:
Vol I -
[13]. Martin R. Parker & Associates, Inc., “Effects of Raising and Lowering
Speed Limits,” Report No. FHWA-
[14]. Charles Lave and Patrick Elias, “Did the 65 MPH speed Limit Save Lives?,”
Accident, Analysis and Prevention, Vol 26, No 1, 1994, pp. 49-