Fuel Consumption

by Michael N. Plei, PE

Introduction

Last year U.S. vehicles traveled nearly 3 trillion miles^[1] – and in the process consumed roughly 135 billion gallons of gasoline, cost nearly $470 billion^[2] at the gas pumps and released over 1.8 billion metric tons of green houses gases^[3].

And even though the travel distance has been roughly the same over the last 5–6 years, (just a mere 62 billion off the peak in 2007), this number has nearly doubled since 1980^[4].

Fig 1. U.S. Vehicle Miles Traveled

Lower Fuel Consumption

Finding ways to lower our fuel consumption and emissions is crucial to the health of our country. Climate change and oil dependency puts us on our future economic growth at risk. The adoption of hybrid and battery powered vehicles will help reduce emissions as the power industry shifts to non-fossil fuels. And other trends, such as car sharing may help reduce the impact of local, but not highway driving. Continuing with a focus on our roads, an interesting question to ask is “what is the impact of road pavement on our vehicle’s fuel consumption”?

Today’s roads are typically built using either asphalt (flexible pavement) or concrete (rigid pavement). Both types of pavement have advantages and disadvantages (which is a post for another time), but for now we would like to consider how pavement types and their condition affect fuel consumption.

Usually one would turn to experts to get answers to these questions. Like many things, this question is not as simple as it appears. Many experts in the pavement fields have sponsors. This is nothing new, but it makes it more difficult to see what is the real answer (unless there is clear cut consensus). This is not to say that experts would misrepresent the facts, but like a good legal team, experts will present issue in the best light given their sponsor’s position.

Roughness, Texture and Deflection

It appears that most experts agree there are three major contributing parameters to fuel consumption as it relates to pavement vehicle interface (PVI). These parameters are: pavement roughness (macro-texture); pavement texture (micro-texture); and pavement deflection. There are additional parameters that affect fuel consumption like tire pressure, momentum, air drag, etc. However, for the purpose of this article, we want to hold the vehicle and the environment constant and look solely at the contribution of pavement on fuel consumption.

Fig 2. Roughness (macro-texture) and Texture (micro-texture)

When it comes to pavement roughness, everyone seems to agree that smoother is better for lower fuel consumption. Texture is like roughness, but on a much smaller scale. With regards to texture, the contributing factor to fuel consumption is still unknown. Texture is, however, an important factor of stopping efficiency — where more texture is better.

Fig 3. Deflection

With regards to deflection, historically there has been a debate. But recent research has shown that deflection does plays a significant impact on a vehicle’s gas mileage.

Peer reviewed work at MIT^[5] indicates that when a pavement deflects it acts like a little valley that the wheel is constantly trying to climb out of throughout the duration of the trip.

Conclusion

So in conclusion, Fuel Consumption relative to Pavement Type/ Condition:

• The impact of Pavement Roughness is well understood. A smoother road will lower fuel consumption.
• The impact of Pavement Texture on fuel consumption is not well understood. Smoother texture probably lowers fuel use, but also negatively affects the ability to stop quickly.
• The impact of Pavement Deflection on fuel consumption is a recent discovery. Stiffer payment (less deflection) yields better gas mileage. A rigid pavement is stiffer for the same thickness and less affected by temperature and moisture compared to a flexible pavement. A flexible pavement can have a similar deflection but it will take a thicker structure.

Interestingly, making stiffer pavements could save the US 3% of its transportation fuel bill — a savings that could add up to 273 million barrels of crude oil per year, or $15.6 billion at today’s oil prices. In addition, this would reduce CO2 emissions by 46.5 million metric tons.^[6] on an annual basis.

Something to consider the next time a road is being built.

Notes

1. ^ US Vehicle Miles Traveled, FRED Economic Data, St. Louis Fed, Retrieved on Sept 3, 2014.
2. ^ Based on an assumed $3.50 fuel price.
3. ^ Sources of Greenhouse Gas Emissions,, US EPA, Retrieved on Sept 3, 2014.
4. ^ Ibid 1,
5. ^ MIT News Release, May 23, 2012
6. ^ Ibid 5