Topic:
HIGHWAYS; ROADS; TRAFFIC ACCIDENTS;
Location:
TRAFFIC ACCIDENTS;
Scope:
Connecticut laws/regulations;

OLR Research Report


May 17, 2002

 

2002-R-0501

MERRITT PARKWAY TRAFFIC ACCIDENT ANALYSIS

By: James J. Fazzalaro, Principal Research Analyst

You asked for information concerning whether accidents on the Merritt Parkway (Route 15) are more frequent than on other limited access highways in Connecticut.

SUMMARY

The Merritt Parkway runs from Greenwich to Milford. Compared to most of the state's other limited access highways, the average number of accidents per mile of road is higher for the Merritt than on some of the state's other limited access highways, but is less than I-84, I-91, and I-95, the state's busiest expressways. According to a special accident database maintained by the Department of Transportation (DOT), 14 locations of a total of 44 on the Merritt Parkway could be considered to have accident rates above the norm for similar types of state roads. The accident frequency index for the two worst locations places them 88th and 98th on the list of approximately 1,500 locations on the entire state highway system with higher than normal accident frequency. By comparison, two non-limited access highways, Route1 and Route 6, account for 15 of the top 100 places on the list.

While the placement of the other Merritt locations on the overall list puts them slightly above the high-frequency locations on other state limited-access highways, the frequency rates do not appear inordinately high and are fairly modest when certain locations on non-limited access roads are also considered. The higher than normal accident rates are essentially clustered in three locations along the Merritt—from Exit 28 to the North Street interchange in Greenwich, at the Route 58 interchange in Fairfield, and at the Route 111 interchange in Trumbull.

The Merritt differs somewhat from the other limited access highways with respect to the proportion of its total road segment and ramp locations that have accident frequencies higher than the norm. This percentage (31.8%) is more than any other state route that is at least two-thirds limited access highway. I-95 has slightly more than 26% of its total road segment and interchange locations with higher than normal accident rates. However, if only the segment of I-95 that parallels the Merritt is considered (from Greenwich through Stratford), the Merritt is slightly less (32.8% compared to 31.8%). Similar factors, high traffic densities and closely spaced interchanges, which relate to both highways, could partially explain why these two road segments have a greater proportion of locations with higher than normal accident frequency.

The Merritt Parkway was designed and built in the 1930s, well before creation of the Interstate Highway System caused the development of modern expressway design standards. It is designated as a scenic highway and is listed on the National Historic Register. These and several other factors constrain the degree to which the parkway can be “modernized” despite the fact that it carries higher daily traffic levels than some of the state's other more modern limited access highways.

MERRITT PARKWAY TRAFFIC ACCIDENT FREQUENCY

The Merritt Parkway is the portion of Route 15 that runs from the Connecticut-New York state line in Greenwich to the eastern end of the Housatonic River Bridge in Milford. It is approximately 37.6 miles in length. From this point to Interstate 91 in Meriden, a distance of approximately 26.3 miles, Route 15 becomes the Wilbur Cross Parkway. It then continues for about 2.7 miles as a limited access highway until it joins Route 5 in Meriden (the Berlin Turnpike). At Route 314 in Wethersfield, it once again becomes a limited access highway for another 5.8 miles until it joins I-84 in East Hartford. This last section is known as the Wilbur Cross Highway. The total length of Route 15 is approximately 83.6 miles. Only one other highway in Connecticut is designated as a parkway. This is Route 796, also known as the Milford Parkway, which is a 2.9 mile road connecting Route 15 to U.S Route 1.

For the three-year period from 1997 through 1999, the latest period for which accident statistics are compiled in the Department of Transportation's Traffic Accident Survey Report (TASR), the Merritt Parkway portion of Route 15 experienced a total of 3,783 accidents. The TASR is one of the best sources of comparative accident data for Connecticut highways because it relates total accidents on a particular highway or section of a highway to factors such as road length, traffic volume, mileage traveled, and average system performance for similar road types. Considered by themselves, total accidents can be misleading because a particular road location may have a relatively small number of total accidents, but compared to roads with similar length, traffic levels and travel miles, its accident frequency might be considerably greater.

Average Accidents per Mile

The total of 3,783 accidents for the 37.6-mile length of the Merritt Parkway computes to an average of approximately 100.7 accidents per mile. This is higher than the average accidents per mile of road for some Connecticut roads with significant portions of limited access highway, such as Route 2, Route 9, I-395, and I-291, but lower than others such as I-84, I-91, and I-95. Table 1 shows the average accidents per mile of highway for Connecticut roads, all or significant portions of which include limited access highway segments.

Table 1: Average Accidents Per Mile for Major State Limited Access Highways

Route

Route Length (Portion Limited Access--LA)

Total Accidents

1997-1999

Average Accidents per Mile

Route 2

58.03 miles (38.2 mi. LA)

2,953

50.9

Route 3

14.98 miles (3.5 mi. LA)

771

53.2

Route 8

67.34 miles (58.3 mi. LA)

2,995

44.5

Route 9

40.89 miles (All LA)

1,845

45.1

Route 11

7.42 miles (All LA)

36

4.9

Route 15

(Total)

83.09 miles (72.4 LA)

7,702

92.7

Route 15

(Merritt Parkway Portion)

37.6 miles (All LA)

3,783

100.7

Route 25

28.59 miles (6.1 mi. LA)

900

31.5

I-84

97.9 miles (All LA)

11,557

118.0

I-91

58.0 miles (All LA)

7,765

133.9

I-95

111.57 miles (All LA)

18,464

165.5

I-291

6.4 miles (All LA)

275

43.0

I-384

8.53 miles (All LA)

302

35.4

I-395

54.69 miles (All LA)

1,525

27.9

I-691

8.92 miles (All LA)

535

60.0

High Frequency Accident Locations

The TASR database provides another way to assess whether the Merritt Parkway shows a higher accident frequency than other limited access highways. Accidents for a three-year period are compiled for every interchange or intersection area and for the segments of highway between intersections and interchanges for every state highway. DOT computes an average accident rate for various roadway types according to formulas that account for such factors as road section length, vehicle and mileage volumes, the overall time period, and the total number of similar type locations throughout the state highway system. By determining an average accident rate for each road type, comparisons can be made between dissimilar facilities.

DOT then develops a “critical accident rate” for each location. This calculation is a function of time, section length, traffic volume, and the system average accident rate for the category of highway being tested for abnormal accident experience. For each intersection or interchange location or road section, DOT calculates the ratio of the location's actual accident rate and its critical accident rate. If a location has more than 15 accidents and its actual accident rate exceeds the critical accident rate, it is eligible for inclusion on the Suggested List of Surveillance Study Sites (SLOSSS). Locations on SLOSSS are candidates for detailed review to determine if measures should be undertaken to mitigate geometric or roadway environmental problems that could be contributing to the abnormal accident experience.

SLOSSS locations are rank-ordered based on the size of the ratio. Locations where the ratio is less than 1.0 are excluded from SLOSSS because they are experiencing relatively normal or below normal accident frequency. The more a location exceeds the 1.0 index, the more abnormal its accident experience compared to the norm for all similar locations and the higher it ranks on the SLOSSS. For the 1997-1999 surveillance period, there were 1,503 locations listed on SLOSSS. DOT also maintains a companion TASR database for conducting a similar analysis for accidents producing injuries or fatalities.

There are 44 road segment and interchange locations on the Merritt Parkway included in the TASR database. Of these 44 locations, 14 had accident frequency ratios of more than 1.0. Seven of the sites are located in Greenwich, two in Norwalk, and one in Stamford, Westport, Fairfield, Trumbull, and Stratford. The highest frequency ratio (3.67) was in Greenwich for the road section between Old Mill Road and the North Street Interchange ranking it at #88 on the SLOSSS. Eight of the 14 locations had accident frequency ratios under 1.8. Table 2 shows the 14 locations, the number of accidents at each location for the 1997-1999 period, its accident frequency ratio, and its position on the SLOSSS.

Table 2: High Accident Frequency Locations on the Merritt Parkway 1997-99

Location

Accidents

Accident Frequency Ratio

SLOSSS Sequence Number

Greenwich

King Street Interchange

48

1.18

1,202

Between King Street Interchange and Mileage Point 1.80

129

1.79

634

From Mileage Point 1.80 to Exit 28 Interchange (Round Hill Road)

117

2.12

429

At Exit 28 Interchange

118

2.21

393

Between Old Mill Road and North Street Interchange

105

3.67

88

At North Street Interchange

219

3.57

98

Between North Street and Mileage Point 7.7

106

1.36

982

Stamford

At Route 104 Interchange

(Long Ridge Road)

123

1.33

1,019

Norwalk

Between Route 123 and Route 719 Interchange (Main Street)

80

1.08

1,352

At Main Street Interchange

131

1.77

644

Westport

Between Mileage Point 23.54 and Route 58 Interchange

144

1.03

1,451

Fairfield

At Route 58 Interchange

(Black Rock Turnpike)

163

2.59

251

Trumbull

At Route 111 Interchange

(Main Street)

342

2.89

186

Stratford

At Route 110 Interchange

(Main Street)

256

1.57

784

The difference in the relative frequency index for several of these locations compared to the total number of accidents experienced at them emphasizes the effect that overall traffic levels have on accident occurrence generally. For example, while 342 accidents occurred at the Route 111 interchange area in Trumbull and only 219 occurred at the North Street interchange area in Greenwich, the frequency index for the Greenwich location is considerably higher (3.57) than the Trumbull location (2.89). This difference is largely due to the fact that traffic levels at the Greenwich location were somewhat less than at the Route 111 interchange. Therefore the accidents at the Greenwich location were occurring at a more frequent rate even though there were fewer of them.

Two locations on the Merritt rank in the top 100 on the SLOSSS sequence list and a total of six are in the top third of the 1,500-location list. Five are relatively low in ranking compared to other SLOSSS locations (982 and above). The most significant problem areas on the Merritt appear to be clustered in three locations—the area from Exit 28 to the North Street Interchange in Greenwich, the area around the Black Rock Turnpike (Route 58) interchange in Fairfield, and the area around the Route 111 interchange in Trumbull.

Injury Producing Accident Frequency

Table 3 compares accidents and frequency ratios for the high-frequency locations on the Merritt Parkway with respect to all accidents occurring during the 1997-1999 period and those that in involved injuries or fatalities (injuries and fatalities cannot be separated in the database). As can be seen from the data, one location, the King Street interchange area in Greenwich shows only a 0.79 index level for injury accidents, but a 1.18 index for total accidents. This implies that while accidents are slightly more frequent than normal at this location, they are of a relatively minor nature because the injury/fatal accident frequency is well below the norm for similar locations on the state highway system.

On the other hand, four of the high-frequency locations on the Merritt Parkway have higher injury/fatal accident indices than for total accidents. The implication in these instances appears to be that the accidents occurring at these locations are relatively more serious than at the other high-frequency locations on the Merritt because they exceed the norm for injury-producing accidents for similar locations on the state system by a greater degree than for total accidents. Table 4 shows these five locations in green.

Table 3: Total Accident and Injury-Producing Accidents for High-Frequency Merritt Parkway Locations

Location

All Accidents

Injury/Fatal Accidents

Number

Frequency Ratio

Number

Frequency Ratio

Greenwich

King Street Interchange

48

1.18

11

.79

Between King Street Interchange and Mileage Point 1.80

129

1.79

35

1.50

From Mileage Point 1.80 to Exit 28 Interchange (Round Hill Road)

117

2.12

35

1.96

At Exit 28 Interchange

118

2.21

40

2.71

Between Old Mill Road and North Street Interchange

105

3.67

28

2.88

At North Street Interchange

219

3.57

51

3.04

Between North Street and Mileage Point 7.7

106

1.36

40

1.61

Stamford

At Route 104 Interchange

(Long Ridge Road)

123

1.33

38

1.27

Norwalk

Between Route 123 and Route 719 Interchange (Main Street)

80

1.08

27

1.12

At Main Street Interchange

131

1.77

29

1.19

Westport

Between Mileage Point 23.54 and Route 58 Interchange

144

1.03

55

1.28

Fairfield

At Route 58 Interchange

(Black Rock Turnpike)

163

2.59

40

2.33

Trumbull

At Route 111 Interchange

(Main Street)

342

2.89

109

2.89

Stratford

At Route 110 Interchange

(Main Street)

256

1.57

68

1.33

Proportion of Highway Locations with High-Frequency Accident Locations

The third way in which we have compared the Merritt Parkway to other limited access state highways is with respect to the proportion that the identified high-frequency accident locations represent of the total identified locations in the TASR database. Of the total of 44 road segments and interchange areas on the Merritt, 14 of them, or 31.8%, have relative frequency indices above 1.0, thus identifying them as high-frequency locations. Table 4 compares the Merritt to other state limited access highways in this respect. For relevance, the comparison has been limited only to those roads that, like the Merritt, are either entirely or substantially (at least two-thirds) limited access roads.

Table 4: Proportion of Road Segments and Interchange Locations with Higher Than Normal Accident Frequency for Selected State Limited Access Highways

Location

TASR Locations

Locations on SLOSSS

(Index of 1.0 or more)

Percentage

Route 2

248

23

9.3%

Route 8

118

11

9.3%

Route 9

64

3

4.7%

Route 11

6

0

0%

Route 15 (Merritt)

44

14

31.8%

Route 15 (All)

187

39

20.9%

I-84

111

19

17.1%

I-91

72

8

11.1%

I-95

161

42

26.1%

I-95 (Greenwich to Stratford)

61

20

32.8%

I-291

9

0

0%

I-384

12

0

0%

I-395

52

2

3.8%

I-691

11

1

9.1%

Based on this data, the Merritt Parkway portion of Route 15 has the largest percentage of its TASR locations with accident frequency ratios at or above the 1.0 index level. I-95 is the next closest with 26.1% of its TASR locations listed on SLOSSS. While this could be indicative of some special circumstance concerning the Merritt, it does not take into account the relative size of the accident frequency ratios only the total number of them. Also, when only the section of I-95 from Greenwich through Stratford is considered, it has a slightly higher percentage of SLOSSS-listed sites (32.8%) than the Merritt. This suggests the possibility that the characteristics of the travel corridor, i.e., high traffic levels and densities and relatively closely spaced interchanges, which are common to both roads may have some influence on their relatively high proportion of SLOSSS-listed sites compared to other limited access highways.

Merritt Parkway Accident Analysis

The data shown in the preceding tables allows for some mixed conclusions with respect to whether the Merritt Parkway experiences an unusual frequency of traffic accidents compared to the state's other limited access highways. On average, it experiences more accidents per mile than some of the state's other smaller limited access highways, such as Routes 2, 8, 9, and 11 and I-291, I-384, I-395, and I-691, but fewer than the three most heavily used interstate highways (I-84, I-91, and I-95).

Using the TASR relative accident frequency ratio as a measure, the 14 locations on the Merritt that showed a higher accident frequency than the norm for similar facilities on the state highway system represent a relatively high proportion of the road's total locations compared to other limited access highways, but for the most part are not particularly high in the SLOSSS rank order sequence. Only two locations are in the top 100 of the 1,500-location list and only four are in the top 350. In contrast, 15 of the top 100 SLOSSS locations are on Route 1 and Route 6 which, although not limited access highways, are heavily traveled roads. Frequency ratios appear to be slightly higher on balance for Merritt Parkway locations than for the other major interstate highways. However, this difference does not appear to set the Merritt significantly apart from those other roads.

The fact that about one-third of the TASR locations on the Merritt Parkway have higher accident frequencies than the norm for most of the roads of a similar nature may be of some significance. It is somewhat above the proportion for the other major interstates (I-84, I-91, and I-95). However, it is slightly less than the comparable portion of I-95 within the same corridor. This suggests that the corridor travel volumes and closely spaced interchanges, which are common to both roads and tend to increase opportunities for conflicts resulting in accidents while vehicles attempt to enter or leave the highway, have a similar impact on both highways.

The Merritt Parkway was designed and built in the 1930s. The last section of the Merritt was opened to traffic in September 1940. It was primarily intended to help relieve congestion on the Boston Post Road (U.S. Route 1). The Merritt significantly predates development of modern expressway design standards spurred on mainly by the creation of the Interstate Highway System in the mid-1950s. Despite these design differences, the Merritt carries average daily traffic of 40,000 to almost 80,000 vehicles depending on the location. This makes it significantly busier than several of the state's other limited access highways. (Route 9, for example, has average daily traffic levels of 25,000 to 65,000 vehicles.)

Many of the Merritt Parkway's design features, such as its landscaping and the unique designs of many of its overpasses, make it unlike the state's other limited access highways. Efforts have been made since the 1970s to retain its unique design despite the significant increase in traffic levels it has experienced. Among the most significant differences between the Merritt and most of the other state limited access highways are the relatively constrained ramp areas which tend to make entering and exiting the highway more difficult than on the more modern expressways.

After legislation failed in committee in 1976, the legislature in 1977 enacted a law creating a Merritt Parkway Commission as an autonomous body to advise the transportation commissioner on the care of the parkway. The commission was empowered to review plans of the DOT relating to the parkway's improvement, appearance, use, legal status, and envisioned future (PA 77-518). Although the legislation was vetoed by Governor Grasso, the transportation commissioner subsequently administratively created the 12-member “Merritt Parkway Advisory Committee” to oversee parkway affairs and advise DOT on its care.

In certain ways, it performs some of the same functions that were proposed in the vetoed law, but the commissioner is not bound by its recommendations.

The Merritt Parkway is also designated as a scenic highway and, since 1991, has been listed on the National Historic Register. These and other factors create a special situation regarding improvements that might be considered for the parkway. Other than routine maintenance activities, many of the improvement proposals for the Merritt undergo a unique process of review and comment by the advisory committee and others and are governed by special planning documents and design guidelines that have been adopted specifically for the parkway. While large-scale improvements such as the Route 7 and Route 8/25 interchange reconstruction projects have involved major changes to the parkway environment, many other improvements have been subject to the guidelines in place to maintain the parkway's historic character and aesthetics. Consequently, redesigning the parkway as a modern interstate-type expressway is not likely to occur.

JF:ts