Federal Speed Limit

It is a topic which has been fought ever since implementation, and successfully, too, as there are currently no national speed limit guidelines. A federally-mandated speed limit was initially implemented in 1974 with the passage of the National Maximum Speed Law. As a response to the 1973 oil embargo, it was an attempt to curb fuel consumption through the reduction of speed on all roads to 55 MPH or less. Fast-forward 13 years, and oil is no longer a scarce resource (at least at the pump). With fuel prices (comparatively) low, and drivers clamoring for higher speeds, the federal government passed the Surface Transportation and Uniform Relocation Assistance Act, allowing states to raise rural speed limits to 65 MPH. Forty-one of them did exactly that. Another eight years pass, and President Clinton signs the National Highway Designation Act, proposing, among other things, the elimination of all national speed limits. States were now free to raise speed limits on national roads as they saw fit.

This is where America’s highway policies currently reside. Most highways have posted speed limits of 65 or 70 MPH, while actual vehicle travel rates may be 5-10 MPH higher. Speeds of 80 MPH are not uncommon to see in both rural and urban road conditions. Besides showing how much of a rush Americans are often in, what problems does this create? Keeping in line with the company policies of 3P (People, Planet, Profit), let’s begin with the human aspect.

Speed kills. This isn’t a new and surprising phrase, however, the area in which it is describing may be. According to a study published in the American Journal of Public Health in September, 2009, the progressive elimination of national speed limits has resulted in an estimated total of over 12,500 additional traffic accident deaths since 1987. How could this be? Vehicles have undoubtedly become safer, yet people continue to die in high-speed accidents. One could explain it away with the increase of automobiles on the road, however, that does not explain these facts: In the year following the passage of the National Maximum Speed Law, car crash mortality fell 16.4%. Additionally, following the elimination of set speed limits nationally, in states which did not change their speed rules, highway deaths fell again. On the other side, crash fatalities following the Surface Transportation and Uniform Relocation Assistance Act increased 9.1% in states which changed their speed limits from 55 to 65 MPH. With the elimination of all national set speed limits in 1995, urban highway deaths increased 4%. In addition to the over 12,000 deaths considered to be a direct result of higher speeds, the study found over 36,500 injuries as a result of those fatal crashes. A federal speed limit is likely to have similar results today as it did in 1974, reducing highway deaths by a comparable percentage.

The second “P” highlights the Planet. In this case, the environmental impacts of high-speed highway driving. According to the Bureau of Transportation Statistics, in 2007, there were 254,403,082 vehicles registered in the United States. On average, each vehicle was driven 11,900 miles annually, consuming approximately 1 gallon of fuel for every 17.2 miles traveled. This results in an annual consumption of 692 gallons per vehicle. According to the EPA, a gallon of gasoline burned in an engine produces 8.8 kilograms (19.4 pounds) of CO2. Using the previous statistics, each vehicle should produce 6,089.6 kilograms (13,424.8 pounds or ~6.7 tons) of CO2 each year. Multiplied by the total number of registered vehicles in the country, and the results show that American driving habits culminate in the release of 1,549,060,366,298 kilograms (1,707,655,248 tons) of CO2 annually.

So what does this have to do with a Federal speed limit? Higher speeds require more energy to maintain, and the energy source in most vehicles is gasoline (or a petroleum product, including diesel). A basic rule of thumb is that, once at highway speeds, the energy required to fight aerodynamic drag increases exponentially. Put simply, if speed doubles, the energy required to do so quadruples. For a vehicle doing 50 MPH in one run and 100 MPH in another, it will have burned twice the fuel in the latter test (4 times higher consumption in half the time). According to research by Consumer Reports on a stock Toyota Camry, the vehicle was consuming at a rate of 40 MPG at 55 MPH, however, it fell to 35 MPG once accelerated to 65 MPH. At 75, it was down to 30 MPG. Assuming that car was a perfect national average example it would be driving 11,900 miles, with 45% on the highway, and 55% in the city. Gleaning the appropriate amount of miles for highway driving only, the vehicle was driven 5,355 miles at high speeds. If the driver kept the vehicle at its optimum speed of 55 MPH, they would have consumed 133.875 gallons, releasing 1,178.1 kilograms (2,597.175 pounds or ~1.3 tons) of CO2. However, if the vehicle was consistently doing 75 MPH, the consumption would be 178.5 gallons, releasing an additional 392.7 kilograms (865.725 pounds or ~0.4 tons) of CO2.

This may not seem like a big deal, but remember those national totals? Let’s assume that every registered vehicle drove at 75 MPH on the highways, and that the percentage decrease in consumption was similar (75% of optimum) on all vehicles (this is an under-exaggeration, as many vehicles with poor aerodynamics decrease much faster than the example Camry). Now, we will assume the national average of highway MPG is 25 (deliberately overestimated, considering combined is only slightly over 17 MPG). Achieving only 75% of this value in real-world driving provides an actual highway average of 18.75 MPG. Now, this isn’t taking into account the minimal decrease in engine operation time (due to the faster speed). Such a value would likely pose as only a fraction of the additional usage, since the consumption curve increases exponentially, not in a linear fashion. Regardless, such a mileage reduction equates to annual CO2 emissions in the United States, simply as a result of driving 75 instead of 55 MPH, of 159,846,544,482.24 kilograms (352,388,973,063.12 pounds or ~176,194,486 tons). Astounding in its own right, more so when it is seen that it is 10% of total consumer automobile emissions in the U.S.!

Getting outside of the complicated math, that value bears repeating. It is entirely possible (as shown above), that up to 10% of nationwide passenger vehicle fuel-based CO2 emissions are avoidable simply by slowing down! Of all climate change policies in deliberation, this seems to be a “low-hanging fruit”. Please understand, these values are simply for presentation and educational purposes. While determined to the highest accuracy, actual results may vary due to a variety of factors. Nonetheless, the writer maintains its significance.

So the first P, People, showed how lower highway speeds reduce the number of fatal accidents. The second P, Planet, provided an environmental rationale to driving slower. The final P, Profit, aims to provide a financial incentive to accomplish the same goal.

At the time of writing, the current national average for regular gasoline is $2.585. Using the values determined above, the national expenditure of driving all those vehicles on the highways for 5,355 miles at 25 MPG is $140,864,767,324.97. However, at the adjusted value of 18.75 MPG, the cost increases to $187,819,689,766.63. The cost savings potential nationally as a result of driving slower is an astronomical $46,954,922,441.66. Per vehicle, this results in an annual savings of $184.57. That type of return resembles the recent tax rebates, and yet this needed no public or private expenditure!

We hope this has been an informative look into the true costs of high-speed highway driving.

Summary of “3P”-based analysis on lower highway speeds:


Over 12,500 deaths could have been avoided since 1987

Over 36,500 injuries within fatal car accidents could have been avoided in same time period

Fatality rates increase following speed limit increases

Fatality rates decrease in states with no speed limit changes


MPG falls as speed exceeds 55 MPH

Potentially 10% of nationwide passenger vehicle fuel-based CO2 emissions a result of high-speed driving


National cost savings through speed reductions on highways: $46,954,922,441.66 annually

Per vehicle cost savings: $184.57 annually

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