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PV Panels, the US Interstate Highways, and some calculations on making use thereof


One can NOT induce accuracy with precision!
Mar 24, 2013
I've been considering suitable areas for emplacement of lot of solar panels. Many of you are familiar with the Land Art Generator Initiative and the global map they have used to demonstrate how much of various portions of the earth's surface would be needed to satisfy global total energy demand. My approach differs; one of such differences is that I am considering solely electricity production.

My methodology centers on land dual use. There are many valid arguments against using vacant land for large-scale PV farms. It might take away agricultural land; it might destroy - or at least ineluctably alter - a desert or "wasteland" ecosystem; it could be considered a visual blight; and so on.

But much area already is used. Residential and commercial rooftops; parking lots; aqueducts; highways. This exercise looks at one subset of highways: the USA Interstate Highway System. I chose this for a number of reasons, most importantly because it was the easiest to obtain size information.

The thesis is that a road's areal footprint could be dual-purposed - an array of PV panels could be constructed above the roads. Reasons for this are powerful: the land already has been sequestered under Rights-of-Way terms; it is immediately accessible by roads (that's a joke, son, but this also makes for an easier time in connecting such systems to a nationwide electrical grid); driving along a covered road is safer, in that sun glare and weather effects (rain & snow) would be mitigated by a properly-constructed panel array.

Below are my calculations - I ask for others' review of same and request any challenges or corrections.

Raw data: at present, the Interstate System is:

*length: 77 x 10^6 m long.

*width of lanes is mandated at 4m in rural ares; 3m in urban; 2 lanes per direction is the minimum; 3 is more common and many routes have far more. For calculations I have allotted 25m as a mean width of lanes by themselves

*width of shoulders also varies rural, urban and mountainous; I have agglomerated them to 10m

*width of median strip is a minimum of 11m rural; 3m urban. I have used 10m

*Right-of-Way strip varies enormously; to bring the sum to a round number I have allotted another 15m to the total.

25+10+10+15 gives us a usable mean width of 60 meters.

77 x 10^6 * 60 = 4.6 x 10^9 m^2. As an aside, this is equivalent (rounded) to 1,800 mi^2, or 1.1mm acres.

Aside: How many panels is that?
*In 2015, a run-of-the-mill PV panel is 2m^2 in area. Using round numbers, our highway system thus can be covered with approximately 2.3 X 10^9 panels. With a nameplate capacity of about 250 watts, that is a peak output potential of 580 X 10^9 watts.

Back to power generation - in other words, how many kWh might be generated?

Using a nationwide annual average of 5kWh/m^2/day**, and a round-number panel efficiency of 20%, we arrive at 1kWh/m^2/day (gotta love these rounded numbers), or 365kWh/m^2/yr.

For a total of 365 * 4.6 x 10^9 = 1700 x 10^9kWh/yr ---> 1700 x 10^6MWh/yr. At present, the US is generating approximately 4000 x 10^6MWh/yr***.


These calculations suggest that with today's technology, approximately 40 percent of the United States's entire electricity production could be accounted for by over-highway solar panels.

Critical data sources:
** nrel.gov/gis/il_solar_pv.html
***Electricity Data Browser


One can NOT induce accuracy with precision!
Mar 24, 2013
Fair enough. I was conservative in my width of the usable Interstate corridor so the fact that state highways rarely reach the width of Interstates should cancel out for purposes of these calculations.


One can NOT induce accuracy with precision!
Mar 24, 2013
That's part of my point, 911: although I know I am not the only one to have suggested paneling above roads, I have not been able to find any such data as what I just threw out. And I've been looking.


Model S VIN P01536
Oct 7, 2011
Portland, Maine, USA
While an interesting calculation, interstate highways and their margins are not the lowest-cost location for large-scale solar.
  • Installing panels over highways involves a lot of structures that wouldn't be needed on a open-field or roof-top installation.
  • Overhead infrastructure could be hit in a collision, possibly bringing the panels down onto the highway itself, exacerbating the disruption of an accident.
  • Panels in the margins of highways could likewise be damaged by vehicles leaving the highway at high rates of speed. There's a reason those margins exist.
  • Electrically, stringing the panels out over miles is inefficient. Solar panels operate at fairly low voltages and, therefore, have high power losses over distance. Concentrating 10,000 kW of panels in a compact area reduces losses relative to those same panels strung out along miles of highway.
Honestly, I don't think that the top issue with large-scale solar development is the lack of suitable locations. LA Basin could be completely powered by panels on the rooftops of warehouses and other big-box format commercial buildings. Infringing on the safety margins of the interstate highways doesn't seem necessary or prudent to me.


Well-Known Member
Aug 21, 2013
Los Altos, CA
Agree 100% with Robert.Boston regarding highway installations. Existing buildings and parking lots are sufficient for massive growth in solar.


Active Member
Mar 12, 2015
Kamloops BC Canada
While I'd love to see something like this implemented, I also have to agree with Robert. The height necessary to provide adequate clearance means a significantly more complex structure to support the weight, snow, wind load, even earthquake load. You'd want to be careful that snow didn't slide off between panels onto the road - similarly, rain can't be concentrated to fall like a waterfall at regular intervals as that would be really unsafe! As well, as was suggested above, besides the actual shoulder area, there is also a requirement for clear zone and recovery zone (if highway design in the USA is the same as up here). The greater the offset, the longer the unsupported span above the road. The cost and complexity gets somewhat exponential! Not to mention how it would work in steep areas where the highway is perched on the side of a mountain.

When it comes to messing with desert ecosystems, agricultural land etc, I take a slightly different position on whether it's a good thing or not. Given the way the climate will change, I think there is going to be some wiggle room in what is good and what is bad. There may be areas that would benefit from shade and provide much needed food - that would otherwise be parched wasteland. Who knows!


One can NOT induce accuracy with precision!
Mar 24, 2013
Some reasonable concerns bought up. Here are counterarguments for some, if not all, of them.

* Rain and snow cascades - correct, these have to be taken into account in the design, just as highway overpasses cannot be configured so as to create a similar torrent
* Safety on margins: highway over-signs also have poles that can be hit; those signs can - and on occasion do - collapse on freeways. Regardless, the avoidance technology is there. But the obviously greater number of such infrastructure need does increase the chances of a strike. Overall Cost/Benefit?
* Inefficiency of strung-out versus concentrated panels - that would have to be assessed, it seems to me, by looking at the comparative costs of (1) the larger number of transformers associated with strings vs (2) land, etc., acquisition costs for a solar farm. Phenomenologically, what is the difference in efficiencies of each of these two options versus the third: "thousand points of light" (i.e., on a large number of residential roofs) given the same number of panels?
* Ecological plusses and minuses of altering an ecosystem through large-scale panel siting: Go ahead, throw yourself on that grenade. I'll take the next one.

Disclaimer: I am absolutely a proponent of rooftop solar; of my four solar banks, two are rooftop (one tower-top; one field-mounted). I believe those who call rooftop solar unsightly are hypocrites of the first water: I defy them to demonstrate how that's any more of an eyesore than a plane of shingles, tiles or sheet metal. Regardless, it is an issue of the day. Also, most home roofs are not ideally pitched or sited for efficient solar capture. Lastly, dense urban areas - i.e., non-single-family housing - have too high a population per roof area for efficient capture.

One variant would be to consider the stretches of Interstate, etc., highways that have vast, appropriately sited ROW margins, and emplacing fields of panels there. This would not address Rob'tB's "string" concerns, but its appeal to me is includes its use of already-captured land, access to infrastructure and the consistent demonstration to the populace of one source of renewable energy.


Active Member
Jan 8, 2011
NE Tennessee
Parking lots are another option that would likely be cheaper than covering interstates. And most parking lots are naturally close to large energy users. Here is an estimate I found for a total of 3500 sq miles of paved parking lots in the USA. Log In - The New York Times

That is 9^9 sq meters of area so using the above factors that should cover 80% of our electric needs. Add in a few roof tops and we could power the country with no additional land. So this appears to be a possible option.
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One can NOT induce accuracy with precision!
Mar 24, 2013
I'm a very big fan of covering parking lots, and consider that ground-dragging fruit. It's a lot easier to calculate highway footprint than lot footprint, though.

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