Some Gratuitous Highway Design Info

[AzNMpower32]

I've spent a fair amount of time on the road alone, racking up the miles (or is that kilometres?) within a 3-state area. Along the way, I've noticed a lot of details in the engineering and design of the roads in various states, and it all affects the ease of driving. After all, it's hard to zip from A to B at speed if various factors get in the way.

One thing that irritates me about many highways in NC is that there's no natural barrier on rural stretches. At night, this means very limited use of the full/high beams because on-coming traffic is in direct view. Often, I'm limited in terms of speed to about 110-120km/h in rural areas because of the risk of overdriving the dipped (low) beam xenons.

Since I tend to be on good terms with both VDOT and NCDOT, I sent an inquiry to NCDOT's Highway Design division, and someone got back to me in a couple days.

Why doesn't NC design its rural highways with a tree median (utilizing the existing trees) or a tall dirt/grass mound? This is more the common practice in Virginia, so drivers in each direction are more often able to use their high beams for better illumination at highway speeds due to a tall visual separation between the two directions of traffic.

Matt,

The short answer to your question is - money. To have a median with trees requires the median to be much wider. To have a wider median, we have to buy more right of way. Providing a planted/naturalized median wide enough to to be a visual barrier and to allow acceptable clear distance to the trees from the roadway would require us to purchase upwards of 100' to 150' more right of way in most instances. In the early years of interstate development, "bifurcated" medians (wide natural areas between the directions of travel) were utilized in many undeveloped, rural areas. Remnants of these median types can be seen along I-95 and I-85 in NC and VA. In modern times, with prices for all aspects of road construction being considerably more expensive, medians in the range of 46' to 70' are the standard practice. These narrower medians reduce right of way impacts, as well as, environmental impacts because our roadway "footprint" can be much less. Even a tall dirt/grass mound as you suggest would require wider medians, would cost more to construct (fill dirt), would require special drainage considerations and would necessitate additional maintenance operations.

In urban areas we do utilize concrete median barriers to allow us to shrink our median even more or to build additional lanes where our grassed median used to be. These barriers can be fitted with "glare screens" (concrete, special fencing or other materials) to reduce headlight glare, but again concrete is very expensive and less forgiving in a crash than a grassed median with a guardrail or cable guiderail barrier.