Seeing as how I am a Highway Design Engineering Technician by trade, as well as a certified arborist, I have a very good perspective as to how and why the trees are cleared and maintained along the NH Highways.
Our DOT is very interested in keeping the trees as far away from the pavement edge as practical for the following reasons:
Public Safety:
1) The most obvious reason is for safety of the traveling public; so that if the driver does leave the road they have some space available to maneuver without hitting a solid object (i.e.. trees). Ditch lines and back slopes, with no trees, provides the space for the vehicle to be redirected on a parallel path with the road and have some time to regain control. If trees are allowed to grow close in, there will be a sudden stop and possibly a very serious accident will occur. The more time the vehicle has to slow down or get redirected the less likely the seriousness of the accident. There are national standards set that reflects distances for the limits of vegetation to pavement edge along a typical interstate. Two way/Secondary roads are a little different in the distance to be provided to the clearing line, as the speeds are slower but the potential for off the road accidents is still high if trees are allowed to grow so close to the traveled way. Safety implications and benefits here are well documented. In some cases the Department does not own enough Right-of-Way to clear so that all the shaded areas are eliminated but certainly the amount of time the sun gets to the pavement is extended. Public safety is affected by both deciduous and conifer tree types.
Reduction of the Environmental Impacts due to excessive Salt use:
2) The next best reason is to help prevent shading the highway pavement areas in the winter time. The shading causes the Department to use more salt to help provide bare pavements after a storm event, to prevent black ice from re-occurring, to create lenses of brine when it is snowing, or remove snow pack after the storm. The Department uses salt in much larger quantities to melt ice in a shaded area then in an unshaded area. Environmentally, heavy salt use in specific areas is not desirable. It affects wildlife, drinking water in wells and plant life in the local watersheds. As you know, considerable amount funding is use to purchase salt and dispense it to the highway every year. The cost per ton of salt is also on the rise and in the future the budgets will continued to be constrained. In areas where the pine trees or evergreens are adjacent to the pavement, the Departments strategy is keep them a greater distance away from the traveled way as they retain their leaves and shade the highway year round. Due to the type of tree they are, they also hold the snow on the branches causing more shade along with the potential to "snap" under wind and snow loadings. We have just experienced a fatality with a pine tree falling on a traveling car on the I-93 here in NH.
Highway Signing Visibility:
3) Another aspect is providing the sight distance for the numerous signing needed on the all roads. These signs vary in distance from the edge of pavement based on size and scope of the message. It is imperative that the signs be legible from a distance and be clearly exposed so the public can acknowledge the message and decide their reaction to message on the sign. By recognizing the sign message well in advance, the Department feels that the radical movements by vehicles are reduced making the roads safer. The impact of tree growth has accelerated due to the absence of mowing as well in the past several years. In areas where mowing maintained the visibility, we now have to hand cut brush to re-expose the sign or have to trim upper branches of trees that block viewing the sign. Where practical mowing is preferred but steep slope areas have to be hand cut, so it is likely that they will grow to a certain tolerable height and then we will have to cut them away.
Guardrail Function:
4) We also need to provide for the deflection of guardrail when hit by vehicles. In some areas we have trees growing in the guardrail on the secondary roads and immediately behind the guardrail on the interstates. The guardrail is designed to deflect and redirect the vehicle back into the road. If there is a tree there this will not happen. Some cable guardrail is design to deflect up to 8' from where it is driven into the ground. Steel W-beam deflects 4' 6" when deflecting as designed when hit. So it is important that tree cutting or trimming occur to provide for the function of the guardrail to be maintained over the life of the rail.
Drainage of Ditches:
5) Another reason is to keep the ditch lines free of trees so they can convey the water from either rain or snow melt. Ditch lines with trees in them can plug, cause erosion, and cause other damage that is costly to repair. We have had a hard lesson in the last 5 years on the repair costs of over taxed ditch lines not operating at capacity. Many of these ditches are immediately adjacent to the pavement and thus causes to the Department to clear or mow yearly.
Trees falling into Paved Areas:
6) Another winter-time or even summer time storm issue would be that trees could be up-rooted or just sheared off in an ice storm, hurricanes or micro-bursts of wind. They, then fall into the road as many lean out into the pavement area chasing exposure to the sun. As the weight of the snow or ice builds up on them, they bend over and in some cases will block passage of the highway. In a real ice storm, like we had in the 90's, we could not open the roads fast enough to get emergency vehicles or the electrical companies access to areas they needed to travel to restore power. By keeping the trees back from the edge of pavement, it will alleviate the possible closure of the road and possible injury in hitting a tree blocking the road. We have just experienced a fatality with a pine tree falling onto a traveling car on the I-93 due to high wind.
Easier Animal Recognition by Drivers:
7) By the clearing trees back from the edge of pavement, the Department provides for space for the driver to recognize animals, particularly moose and deer, so as to allow the driver to react to the animal's movements. Hitting a moose at 65 mph has the potential to be a very serious accident including the potential for human loss of life. When the trees are close to the edge of pavement the driver does not have the time to react to avoid striking the animal. This is true both at night as well as day time travel.
Tree Browning:
8) A lesser issue is the browning of pine tree needles and evergreen branches immediately adjacent to the highways (mostly occurs, in guardrail sections, where the trees have grown up immediately adjacent to the pavement). The cause of this issue is an environmental one and the degree of browning varies site to site. It is thought to be partly caused by salt affecting the soils and also the fuel exhaust of all the vehicles continually passing by. This issue has made the front pages of local papers over the years and thus, if we can avoid it occurring, the public will not be exposed to this. A good impression will be seen by the visitors of NH's mountains and Tourism will not be affected. Simply keeping trees cut back from the pavements will solve this visual problem.
When the Department decides to target a larger area for clearing back the trees, we contact the Environmental folks and have them review the locations for wildlife impacts and endangers species. Only after we have gotten all the approvals do we proceed. We are currently not stumping as the risk for erosion and expense of reseeding is beyond our available funding. For smaller areas we do a self-review for wetlands and impacts to wetlands internally before moving forward.
I hope that this gives you guys some reasoning as to why we need to cut trees along our highways. It has been many years, in some cases, since we had manpower to deal with overgrowth issues along our highways.
