The Hippo


May 31, 2020








: The Diverging Diamond Interchange design, which would be the first of its kind in New England.

Accident trap
Four alternatives considered for Exit 6

By Ryan Lessard

 If you regularly drive through Manchester on Interstate 293, you probably dread the traffic, particularly during rush hour. The stretch of highway that snakes along the Merrimack River is notoriously dangerous, especially near Exit 6, and officials say it’s due for an update. Recently, the first public information meeting was held to discuss some of the early findings of a study into the area’s traffic, accidents and sensitive environmental and cultural resources.

The problems
Highway planners with consulting firm VHB were contracted by the Department of Transportation to oversee the redesign of I-293. They studied accident statistics on the highway from the curve north of Exit 5 to just past Exit 7 between 2012 and 2015. There were more than 300 car accidents, 51 of which resulted in injuries. When plotted on a map, they found that most of the accidents and most of the injurious accidents occurred in and around Exit 6.
“There’s a whole pocket of … injury accidents as well as vehicle damage crashes in the area of the northbound off ramp and southbound on ramp area of Exit 6,” said Keith Cota, the DOT project manager for the 293 project. “It’s quite evident that the geometry of 293 in conjunction with the old geometry of the roundabout design of the interchange at Amoskeag inhibits traffic and exhibits quite a bit of crash history there.”
Cota said the interchange was designed in 1954 and worked fine for a while but as traffic increased, it became increasingly risky to navigate. 
VHB studied how many cars were traveling I-293 at different times and found that cars numbered near 3,000 at the peak morning hour of 7 a.m. and about 2,500 during the peak evening hour of 5 p.m. Fridays were the busiest days for northbound traffic, and overall traffic in July and August saw the most vehicles — around 54,000 — in 2015. 
Based on these statistics, planners determined that the highway needs to be a lane wider on both sides, which would increase it to three lanes, at least up to Exit 7.
Survey findings
As part of the first phase of planning, VHB surveyed the project area for any environmental resources, like rare plants, and contracted archaeologists and historians to locate any cultural resources.
An architectural historian catalogued every structure in the area more than 50 years old and a team from Independent Archaeological Consulting in Portsmouth dug about 400 test pits to see if they could find any artifacts. They found artifacts in about 60 of those pits, Kathy Wheeler of IAC said in an email.
Those artifacts were found in five separate sites after digging within most of the 16 zones identified as being likely sensitive.  
Wheeler said the Merrimack River was long used as a major travel artery for Native Americans and had many fishing stations along its shores. Some of the pre-contact native artifacts discovered by archaeologists include some fragments of tools such as a drill and the stone chips produced by producing stone tools, known as “lithic debitage.”
Surveyors also found the remnants of an old mill foundation, believed to be built in the 19th century. 
Depending on what routes planners wish to use, Wheeler and her team will do more intensive research on any potentially affected sites to determine whether they might contain new knowledge of the past, which would necessitate an alternative route to avoid damaging the site.
Environmental researchers, meanwhile, mapped out the area wetlands, floodplains, farmland, groundwater resources, wildlife and hazardous waste sites. 
One key finding was the presence of licorice goldenrod, a plant on the state’s endangered species list, at a narrow passage of Black Brook, a nearby tributary to the Merrimack River. Peter Walker with VHB said the goldenrod happens to be in a location ideal for a bridge that’s part of a proposed change to the Exit 7 area. Its presence doesn’t exclude the possibility of building the bridge, but there might need to be some environmental mitigation to offset the damage. Ironically, those plants were previously placed there by humans for a similar mitigation.
Alternative designs
After surveying the area and compiling all the needed information, VHB representatives presented their findings and some ways to improve the exits. 
“We’ve collected all the data. … Now we look at the potential corridors, the potential alignments of roads. We’ve got to see what those impacts are and make good decisions to minimize those impacts,” VHB principal Marty Kennedy said.
For Exit 6, there are four alternative buildouts. One would make a section of the highway into a bridge, under which an X-shaped interchange of north- and southbound exits would converge with traffic lights. A second design, known as a “diamond interchange,” would spread the ramps farther apart and add traffic lights, and a similar “offset diamond interchange” would eliminate the second bridge south of Amoskeag and force all southbound on-ramp traffic west of the highway and would require a ramp that would go over a section of Eddy Road.
The design that had the most people at the meeting scratching their heads was known as a diverging diamond interchange, or DDI. It looks like a Celtic knot of roads with some sections directing motorists to drive on the left side before winding back to the right. Kennedy explained the movements with a video animation and said the interchange would be the most efficient design. 
The challenge would be getting motorists used to it. Kennedy said that while this is a fairly common design in the Midwest, there are no interchanges like this in all of New England.
Goffstown development
Perhaps the most popular proposal presented at the public information meeting, which was attended by about 50 people (including some Manchester aldermen, the head of Manchester Police Department’s traffic division and 23 members of a condo association), was moving Exit 7 farther north just past Manchester Community College.
This would involve adding a roadway to connect Front Street westward to Dunbarton Road, which planners predict would not only increase access to the Hacket Hill area, but also reduce traffic on Goffstown Road, a major connector between Manchester and Goffstown.
Planners also showed that if they went a step further and connected Dunbarton Road with a new road that crosses Black Brook to Goffstown Road, it would reduce traffic on Goffstown Road even more — as much as 34 percent.
But perhaps even more significant than the traffic reduction is increased access to Goffstown’s industrial zoned land, which is ripe for development. 
Either way, planners said improving Exit 7 at its current location and adding a northbound on-ramp there would be problematic given its proximity to Exit 6.
Next steps
There was some concern expressed at the meeting by members of the public about the new roads connecting Manchester and Goffstown potentially impacting deer, vernal pools and wetlands; others were worried that the DDI design for Exit 6 would cause confusion and accidents.
Mike Gardener, who spoke on behalf of the Point at Riverfront condo association, said he and his members were concerned about noise and whether the trees currently between their condo and the highway would be cut down. 
“In general, people are often most concerned about noise effects,” Walker said.
But Keith Cota with the DOT said that while trees would likely be cut, they will look into putting up sound barriers if needed.
The project now enters Part B, which involves refining the options with a greater level of environmental and cost detail, some preliminary engineering and a public hearing next May. From that, planners hope to narrow down their choice into a single preferred action plan. 

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