Topic:
TRAFFIC REGULATIONS; MOTOR VEHICLES; TRAFFIC ACCIDENTS;

OLR Research Report


January 21, 2004

 

2004-R-0035

CHANGES IN MOTOR VEHICLE ACCIDENTS AND ACCIDENT RATES FOLLOWING INCREASE OF SPEED LIMIT TO 65 MILES PER HOUR

By: James J. Fazzalaro, Principal Analyst

You asked for an analysis of any changes in the number of motor vehicle accidents and accident rates for the highways on which the speed limit was increased to 65 miles per hour (mph) in 1998.

SUMMARY

We are responding to your inquiry in two parts. In this report (Part I), we provide some preliminary findings regarding this question as it relates to the combined accident experience of all 18 road segments for which the speed limit was increased to 65 mph, as well as one segment on which the speed limit was also raised by as much as 10 mph over the then existing limit. Part I focuses on the actual changes on these segments only for the periods before and after the speed limit change, but does not compare them to either the entire limited access highway system or specific non-65 mph sections of the same highways. A subsequent, more-detailed report (Part II) will provide this comparison as well as an analysis of the changes specific to each individual segment that has the higher speed limit and a profile of each segment.

In 1998, the legislature required the State Traffic Commission to increase the maximum speed limit to 65 mph for any limited access highways it found suitable for the higher limit after taking into consideration their design, population factors, traffic flow, and any other relevant factors. On October 1, 1998, the speed limit was increased from 55 to 65 mph on 18 sections of limited access highway totaling approximately 334 route miles. At the same time, the State Traffic Commission also increased the speed limit from 40 and 45 mph to 50 mph on a 5.5-mile section of I-84 in Hartford and West Hartford.

The Department of Transportation (DOT) provided us with accident and related data for all 19 of these highway segments for an eight-year period from 1995 through 2002. This period covers the three years prior to the change in the speed limit in 1998 and the four years following the change.

Based on our analysis of this data, it appears that both the total number of accidents and the relative accident rate increased for the 65-mph roadways after the speed limit change, but the increases were not uniformly experienced on all 18 of the 65-mph roadways. The average number of accidents on these 18 sections for the three-year period following the speed limit change (1999-01) was 20.7% more than the average number of accidents on these same sections for the three-year period prior to the change (1995-97). While some of this increase can be accounted for by increases in traffic volumes, the average vehicle volume increase for the three post-increase years was only 14.6% more than the average vehicle volumes for the three pre-increase years.

While an increase in absolute numbers of accidents could be expected due to increased traffic volume and travel, the data also indicates that the accident rates as measured by millions of vehicles and vehicle miles traveled on the 65-mph segments also was, on average, more than 5% higher in the post-increase period when compared to the pre-increase period. However, the changes for each individual segment varied. All but one of the 18 sections experienced a real increase in the number of accidents as measured by the three-year pre- and post-change averages, but several showed no meaningful increase in relative accident rates and a few actually had lower three-year average rates for the post-change period.

Conversely, seven of the segments showed significant increases in both numbers of accidents and accident rates. These sections included all three on I-84 (including the West Hartford/Hartford section with the increase to 50 mph), I-91 from Windsor to Massachusetts, Route 20 in Windsor, Route 2, and Route 9 between Middletown and Old Saybrook. Four other 65-mph sections showed small increases in accidents and accident rates.

For reasons explained in the body of this report the 2002 data provided by DOT was not used in the analysis.

HIGHWAY SEGMENTS WITH 65-MPH SPEED LIMIT

The road segments for which speed limits were increased in October 1998 are as follows. There are slight variations in the segment lengths authorized for the higher limits in each direction (eastbound vs. westbound; northbound vs. southbound).

55 MPH to 65 MPH

● Route 2 from Route 94 in Glastonbury to Bozrah/Norwich town line (29.77 miles)

● Route 6 from Route 66 in Columbia to Route 66 in Windham (4.87 miles)

● Route 8 from Echo Lake Road in Watertown to Route 44 in Winchester (23.36 miles)

● Route 9 from I-95 in Old Saybrook to Saybrook Road underpass in Middletown (22.05 miles)

● Route 9 from Middletown/Cromwell town line to the I-84 connecting roadways in Farmington (15.68 miles)

● Route 11 from Route 82 in Salem to Route 2 in Colchester (7.37 miles)

● Route 20 from Special Service Road 401 in Windsor to I-91 in Windsor (3.22 miles)

● I-84 from Special Service Road 911 overpass in Danbury to the Middlebury/Waterbury town line (21.67 miles)

● I-84 from Roberts Street underpass in East Hartford to the Massachusetts border (33.02 miles)

● I-91 from the New Haven/North Haven town line to the Route 15 underpass in Meriden (14.99 miles)

● I-91 from the Route 66 underpass in Meriden to the Route 3 overpass in Wethersfield (13.26 miles)

● I-91 from Bina Avenue in Windsor to the Massachusetts border (16.48 miles)

● I-95 from the Hosley Avenue underpass in Branford to the Route 161 overpass in East Lyme (35.93 miles)

● I-95 from the Route 12 overpass in Groton to the Rhode Island border (16.24 miles)

● I-291 from Route 218 in Windsor to I-84 in Manchester (5.67 miles)

● I-384 from I-84 in East Hartford to the Route 6/44 underpass in Bolton (8.44 miles)

● I-395 from I-95 in East Lyme to the Massachusetts border (54.69 miles)

● I-691 from the North Wall Street underpass in Meriden to I-84 in Cheshire (7.36 miles)

40/45 MPH to 50 MPH

● I-84 from the Route 173 overpass in West Hartford to the east abutment of the Bulkeley Bridge in East Hartford (5.59 miles)

ACCIDENT AND ACCIDENT RATE EXPERIENCE OF 65-MPH SEGMENTS

Analytical Method

To assess the changes in accidents and accident rates for the highway segments with increased speed limits, we took the DOT data for the eight years from 1995 through 2001 and separated it into the three-year pre-change period (1995-1997) and the three-year post-change period (1999-2001). The data for 1998 was not used in the analysis because it included both pre-change and post-change periods. The combined accident and accident rate data for the 18 segments on which the speed limit was increased to 65 mph was averaged for the two three-year periods on either side of the 1998 change year. The three-year averages were compared to show how the experience of these 18 roads changed following the change in speed limit.

2002 Data

Although DOT provided data for 2002 as well, we decided not to include them in the analysis. For several of the road segments, the number of accidents reported for 2002 was so much lower than in 2001, in some cases by 40-50%, that the decreases appeared illogical. While it might be expected that there could be a significant change from one year to the next, these decreases seemed extraordinary and not at all close to the fluctuations evident in the prior seven years of data. For example, for the I-91 segment from North Haven to Meriden, the total accidents in the prior seven years ranged from a low of 431 in 1997 to a high of 540 in 1999. Yet in 2002, there were only 284 accidents reported, a decrease of 41% from 2001. The accident data analysts at DOT agreed that the decreases seemed anomalous and could be due to an underreporting problem even though the state police barracks had provided all the data they had. We decided that including the questionable data for 2002 might bias the trend analysis and sufficient data existed for the three years before and after the speed limit change to provide a meaningful comparison without the additional year of data. Once accident data is compiled for 2003, it should be possible to determine if the 2002 data actually reflects what really happened or was affected by significant underreporting.

Table 1 shows the comparative data for the 18 segments for which the speed limit was increased from 55 to 65 mph in 1998.

Table 1: Accident Experience for 65-MPH Roads Before and After Speed Limit Increase of October 1, 1998

Year

Total

Accidents

Rate Per One Million Vehicle Miles Traveled

Rate Per One Million Vehicles

1995

4,905

0.92

17.33

1996

5,439

1.0

18.99

1997

5,212

0.92

17.38

1998

5,078

0.86

16.36

1999

6,151

1.01

19.18

2000

6,440

1.02

19.20

2001

6,185

0.96

18.20

Average Accidents and Rates for 3 years BEFORE Speed Limit Change

(1995-1997)

5,185

0.95

17.9

Average Accidents and Rates for 3 years AFTER Speed Limit Change

(1999-2001)

6,259

1.00

18.86

Percentage Increase Following Speed Limit Change

+20.7%

+5.3%

+5.4%

The data for the segment of I-84 in West Hartford and Hartford for which the speed limit was increased from 40 and 45 mph to 50 mph at the same time as the other roads were increased to 65 mph was not included in the above table. This segment showed the greatest increases in accidents and accident rates after the speed limit changed of any of the 19 segments for which limits were raised. Had it been included in the overall averages, it would have raised the post-change averages for accidents from 20.7% to 24.6%, for accidents per 1 million vehicle miles traveled from 5.3% to 8.59%, and for accidents per 1 million vehicles from 5.4% to 9.0%.

While it could be expected that the number of accidents would be likely to increase after the speed limit increase as vehicle volume and miles of travel were greater, the 20.7% increase in the three-year accident average after the speed limit increase is more than the growth in average vehicle volume and average miles of travel that occurred after the speed limit increase (14.6% and 14.8% respectively). However, the more than 5% increase in accident rates in the period following the speed limit increase is probably of more significance since it directly accounts for these increases in traffic volume and travel. A change in the number of accidents would not generally be significant if the accident rate remained relatively the same because the accident experience would be essentially the same given the increased exposure resulting from the additional traffic and travel. However, this is not what appears to have happened.

The increase in accidents and accident rates was most evident on 65-mph segments of I-84 (Danbury to Waterbury and East Hartford to Massachusetts), I-91 (Windsor to Massachusetts), Route 20, Route 2, and Route 9 (Middletown to Old Saybrook). The I-84 segment from Danbury to Waterbury showed a 33% increase in total accidents and approximately a 10% increase in accident rates. The I-84 segment from East Hartford to Massachusetts showed a 28.5% increase in total accidents and approximately a 15.5% increase in accident rates. The I-91 segment from Windsor to Massachusetts showed a 49% increase in accidents and a 36% increase in accident rates. Route 20 (the Bradley Airport connector) showed a 38% increase in accidents and approximately a 24% increase in accident rates. Route 2 showed a 30% increase in accidents and a 13% increase in accident rates. The Route 9 segment from Middletown to Old Saybrook showed a 33% increase in accidents and a 13% increase in accident rates.

Four other 65-mph segments (I-95 from Branford to East Lyme, I-384, I-395, and I-291) showed smaller increases in accidents (13-24%) and accident rates (3-6%). The remaining eight segments showed increases in accidents but accident rates that changed little or slightly decreased from pre-change levels.

The 50-mph segment of I-84 in West Hartford and Hartford showed the greatest increase in accidents and accident rates following its speed limit increase. The average annual accidents increased by 48.6% in the three years following the increase and the accident rate increased by 31%.

In Part II of this study, we will provide a profile and analysis of each of the individual 65-mph road segments and a comparison of the 65-mph segments with a comparable road segment on which the speed limit remained at 55-mph and with the overall expressway network.

JF:ro