Time Value vs. Speed

When evaluating the value of time vs speed we must first recognize that as concerns energy consumption, the optimum speed for cars is about 35-45 MPH. After this peak, efficiency falls off rapidly and a law of physics called "drag" states the power (energy) required is quadrupled when speed is doubled. This means the faster you go, the more you waste. A 55 MPH speed limit is a compromise between speed and efficiency.

The argument that the time saved justifies additional cost because of lost earnings fails for the following reasons:

First off, who gets paid for driving to work? But let's play along anyway to see where this goes.

Let's do the math! The average per capita income in America is about $36k for working about 1,800 hours. That works out to about $20 an hour. A commuter driving 30 miles per day will spend 10 extra minutes on the road when slowing from 80 to 55 MPH, but will save 28% for fuel, thus offsetting the potential loss in productivity. **

Assuming she could get paid for her time behind the wheel, it would work out to $3.33 she could have earned if she spent the extra $3.21 for fuel to save 10 minutes. This works out to a .12 cents per day loss in potential earnings vs. guaranteed savings of up to $1,200 year for those 10 minutes a day, again that is if she could get paid for driving to work.

Another way to look at it would be to use the IRS allowance for mileage at .50 cents per mile. If we reduce that cost by 20% the driver saves about .10 cents per mile, or $3.00 per day, $15 per week, $780 per year. Lost potential earnings IF the driver could get paid for the time behind the wheel would be .33 cents per day, $1.65 per week, $85 year vs guaranteed savings of up to $1,200.

What about people who drive all day from job to job? Alas, these drivers must share the roads with everyone else, so there must be a compromise for the safety of all. A mile-a-minute is reasonable. Every business has to include the costs for travel in their pricing, and in some cases perhaps the time value offsets the energy costs, but it is nearly a wash at todays fuel prices and as energy costs go up the equation continues to tilt in favor of moderating consumption.

What about congestion from slow drivers? Congestion will actually decrease due to reduced collisions providing an increase in productivity rather than a decrease.

Consider this metaphor from the Missouri DOT website; "If traffic slows down as they approach a congested area and all the drivers stay at a constant speed, traffic will get through the congested area faster. Imagine the highway as a funnel. Now, imagine the traffic which has to travel along the highway during a certain time as a container of rice. If you pour all the rice into the funnel at the same time, it gets congested at the bottom of the funnel and takes some time to work through the funnel. Now, if you slowly pour the rice into the funnel – keeping it at a steady pace – the rice moves through the funnel evenly and doesn’t cause congestion. In fact, even though the rice is entering the funnel slower, all the rice gets through the funnel (to its destination) faster.

The Missouri Department of Transportation is starting variable speed limits along I-270. When congestion starts building along stretches of I-270, MoDOT will use changeable speed limit signs to vary the speed limit on the road. Speed limits along I-270 could range from 60 mph during extremely light traffic, to as low as 40 mph during extreme congestion. This would also include congestion due to crashes along I-270.

Note: this metaphor is based on an idea from Paul Haase of Sammamish, Washington, in response to a challenge by Washington State Department of Transportation Secretary Doug MacDonald."

Also important to note, it is the top speed allowed that determines how much energy is used to accelerate, and how much is wasted in braking.

It will be interesting to see the energy Secretary's response to Senator Warner's July 2008 letter asking "at what speeds vehicles would be most fuel efficient, how much fuel savings would be achieved, and whether it would be reasonable to assume there would be a reduction in prices at the pump if the speed limit were lowered."

If scientists are allowed to tell the story rather than political appointees this could be the foundation for significant change.

Fuel @ $5 gal, 28% MPG improvement from 80-55MPH
Speed Time MPG Cost/day Cost/year
55 MPH 33 min. 18.0 $8.33 $3,040.45
60 MPH 30 min. 17.5 $8.57 $3,128.05
65 MPH 28 min. 16.6 $9.04 $3,299.60
70 MPH 26 min. 14.9 $10.07 $3,675.55
75 MPH 24 min. 13.9 $10.79 $3,938.35
80 MPH 23 min. 13.0 $11.54 $4,212.10

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