As the discussion over where the next wave of energy generating technologies will come from increases, many individuals and companies have expressed interest in replacing the roadways of the United States with durable photovoltaic cells. Replacing traditional (and petroleum-based) asphalt with solar cells provides an enormous opportunity for energy generation, as anyone who has walked barefoot on asphalt on a hot day can attest, but requires and enormous infrastructure overhaul to be feasible.
According to the Federal Highway Administration, the United states had 4,058,347 miles of rural and urban roads in 2008.  Subtracting unpaved and gravel roads from this total leaves 2,655,528 paved miles of roadway - 1,016,387 urban and 1,639,141 rural. Assuming an average width of 12 feet per lane, an average rural road width of 2 lanes, and an average urban road width of 4 lanes, this gives a total paved road area of 16,690 square miles, or 43,227 square kilometers.
High efficiency solar cells available on the market today (see, for example, reference ), can convert around 18% of absorbed power from the sun into electricity. A surface perpendicular to the sun's rays at the top of Earth's atmosphere will receive an average of 1.368 kW/m2, while the average solar radiation striking a point on the surface of the earth (I) can be calculated with the following formula: 
|I = S cos(Z)||(1)|
Here, S is the solar constant (1.368 kW/m2) and Z is the zenith angle, which measures the angle between the sun's position in the sky and a point directly above the surface in question. Z is a combination of latitude (Φ), solar declination angle (δ), and hour angle (H):
|Z = cos-1(sinΦsinδ + cosΦcosδcosH)||(2)|
The solar declination angle for the northern hemisphere is based on the date - it is 0° at vernal and autumnal equinoxes, 23.5° at the summer solstice, and -23.5° at the winter solstice. The hour angle is based on the time of day:
|H = 15° x (t - 12)||(3)|
The time, t, is measured in hours with midnight as zero, such that 8AM would be 8 and 8PM would be 20.
Ruling out nighttime, the absolute worst case for power generation would be a northeastern location on a winter morning or afternoon, while the best case would be a southwestern location at midday in summer. Using Augusta, Maine (Φ 44°18'38"N) and San Antonio, Texas (Φ 29°25'N) as examples:
With an efficiency of 18%, this means the best case for power generation is 244.93 W/m2, and the worst case is 88.09 W/m2. To obtain a ballpark estimate of how much power a photovoltaic highway system would generate, 100 W/m2 will be used as a nation-wide average - this should give an appropriate picture of the order of magnitude that could be expected from such an infrastructure overhaul.
43,227 square kilometers of road mean a total power generation of 4.32 x 1012 W for the United States if all days were clear and sunny - but data from the National Climatic Data Center implies that the national average number of sunny days is approximately 60% of the year.  Assuming massive drops in power generation during cloudy days and a useful window of power generation of around 5 hours (10AM - 3PM), this gives a one year total of 4.73 x 109 MWh of generated electricity, or about 1,781 MWh per mile of roadway. For comparison, the U.S. generated a total 3.95 x 109 MWh of electricity in 2009 across all energy sources.  The world generated a total of 1.91 x 1010 MWh of electricity in 2008. 
The main cost and challenge facing a photovoltaic highway are one and the same: how to go about replacing the highway infrastructure of the United States - a country which has stubbornly attempted to resist infrastructure overhaul in everything from the use of metric units to healthcare. Convincing the populace and politicians that such an overhaul is necessary, though, becomes a miniscule problem in the face of the fact that replacing a one-lane road with photovoltaic cells could cost as much as $4.4 million per mile.  Replacing the 2,655,528 miles of paved roadway in the U.S. is prohibitive to say the least, but the technology may be feasible for smaller-scale generation - the tradeoff then is $4.4 million for 1,781 MWh of electricity, or a cost of $2,470 per MWh. The Energy Information Administration's projections for electricity costs in 2016 list energy generated from conventional photovoltaics at $396 per MWh, and energy generated from coal at $100 per MWh.  It is conceivable, then, that breakthroughs in fabrication of the photovoltaic highway could lower its cost by a factor of ten and make it a cost-competitive option.
The second challenge is making the solar cells used in the highway durable enough to survive the constant beatings inherent with serving as one of our nation's roadways. The $4.4 million cost cited in takes this into account - the cells examined are weather-proof and encased behind a thick layer of translucent hardened glass or polycarbonate, allowing a large percentage of incident light through but simultaneously providing a layer of strong protection against the vehicles using the roadway. 
The final challenge is maintaining the highway - an asphalt highway requires routine maintenance to fix potholes, repaint lines, etc., and a photovoltaic highway, too, will require regular cell replacement as older cells die. If the cells have a lifespan of 20 years, this can be modeled as 1/20th of the cells needing replacement every year - and for two and a half million miles of cells at $4.4 million per mile, this works out to over $500 billion in maintenance per year.
The idea of a photovoltaic highway - roadways made of solar cells that replace asphalt and power the surrounding area - will not be realized until the cost of installation can be brought down to make the technology comparable with other methods of power generation. However, the core idea behind the highway is simple: replace everyday objects that don't generate power with ones that do. If the technology can be made cost-competitive, the photovoltaic highway may be just the first in a line of new paradigms for energy generation.
© Seth Winger. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 "Highway Statistics 2008," Federal Highway Administration, October 2009.
 M. Green et al., "High Efficiency Silicon Solar Cells," IEEE Transactions on Electron Devices, 31, 679 (1984).
 G. Stickler, " Solar Radiation and the Earth System," National Aeronautics and Space Administration, 20 Aug 08.
 "Sunshine—Average Percent Possible," National Climatic Data Center (2008).
 " International Energy Statistics," U.S. Energy Information Administration (2010).
 A. Schwartz, "Recharging Roadway Startup Wins GE Prize," Fast Company, 8 Oct 10.
 "Annual Energy Outlook 2010," U.S. Energy Information Administration (2009).