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shallowdeep -> RE: Would you drive 55 MPH if ? (5/17/2008 12:55:49 AM)
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Interesting topic, but discussing the supposed benefits without critically examining the assumptions makes little sense. My take:
1. Fuel savings
Aerodynamic drag does begin to reduce fuel economy beyond around 55-60 mph, and the reduction can be around 20% for an increase from 55 mph to 70 mph (see Oak Ridge National Laboratories Transportation Energy Data Book, p. 4-26 - or here for a non-technical graph and explanation). If all driving occurred at steady highway speeds, 20% savings might be possible, but that's not the case. The International Energy Agency's 2005 report Saving Oil in a Hurry examined the potential savings of a 55 MPH speed limit, among many other ideas, and concluded the United States and Canada together would save 363,000 barrels of fuel per day, equivalent to 3.2% of the transport fuel they consume, or 2.4% of all oil fuels consumed by them (see pp. 96-106). A separate 1996 German government study cited in the report estimated a 100 km/h speed limit could reduce Germany's consumption by 4.8%.
I ran my own sanity check on the numbers by looking at the drag forces. I used Wikipedia's claim for an average full-size car's drag area to calculate the difference in drag force between 50 and 70 mph. I then multiplied that by the vehicle miles traveled on interstates in 2006, adjusting for 31.6% congestion, to estimate the decreased energy requirements of reduced speed. Assuming a 20% engine efficiency, I calculated daily savings of 385 thousand barrels of gasoline, which matches reasonably well.
Assessments of the effect of the National Maximum Speed Limit indicate savings were around 1-2% of oil consumption, but that was limited by poor compliance and was higher initially when compliance was greater. Still, even with perfect enforcement, the savings are likely to be less than 5%.
2. National security issues
Using recent refinery figures, a 42 gallon barrel of crude results in around 26.8 gallons of gasoline and diesel. So the reduction in imported crude oil might be greater than the transport fuel savings alone imply. 363,000 barrels of vehicle fuel translates into 569,000 barrels of crude. While quite a bit of fuel, it only amounts to 4.25% of the 13.4 million barrels of daily US imports, so the country would still be heavily dependent on foreign supplies. As has been pointed out, there are plenty of others eager to buy what we don't, so a relatively small cutback in demand isn't going to keep exporting nations from selling their oil, although there might be a modest reduction in price.
3. Reduced fuel prices
Pretty much like the above. There might be a modest slowing in price increases.
4. Safety
There probably is some effect, but it's difficult to quantify - it's not the sort of thing for which you can ethically set up a controlled experiment.
The Transportation Research Board's Managing Speed report examines the issue in some depth in Section 2, Effects of Speed.
quote:
Although the evidence is not conclusive, speed appears to contribute to crash occurrence. Theory, empirical data drawn from correlational studies, and causal analyses of crashes provide evidence that both speed and speed dispersion are associated with crash involvement.
Basically, there is evidence that faster speeds lead to more accidents, especially single vehicle ones, but there is considerable uncertainty about the exact effect. There is also evidence that speed dispersion, i.e. cars traveling significantly faster or slower than the flow of traffic, leads to accidents, so concerns about slowing down to 55 while others continue to go faster are justified.
While the relationship between speed and accident frequency isn't strongly established, the relationship between speed and accident severity is. From the same report:
quote:
Fitting curves to crash data ... the probability of a fatality is related to Delta-V to the fourth power.
...
In summary, all of the studies that have investigated the link between vehicle speed and injury severity have found a consistent relationship. As driving speed increases, so does the impact speed of a vehicle in a collision. Increased impact speed, in turn, results in a sharp increase in injury severity because of the power relationship between impact speed and the energy released in a crash.
So, hitting something solid at twice the speed increases the chances of dying roughly by a factor of 16.
With that said, the effect on safety from lower speed limits in practice is debatable. When the National Maximum Speed Limit was implemented in 1974 highway fatalities fell by 15%, but at least some of that was attributable to less driving and some, like the rather anti-Federal Cato Institute, argue that the decrease can be explained entirely by a combination of other factors and chance - see Speed Doesn't Kill: Repeal of the 55-MPH Speed Limit.
The Transportation Research Board's 1984 report, 55: A Decade of Experience, concluded otherwise.
quote:
The review concluded that the unprecedented decrease in highway fatalities, over 9000 lives, immediately following the enactment of the NMSL resulted from many factors including reduced exposure and reduced discretionary driving. However, taking into account other variables that may have contributed to the safety benefits, the report concluded that the NMSL continued to save lives. They estimated that, in the early years of the 55 mph NMSL the lower speeds saved about 3000 to 5000 lives annually.
Since the rollback of the national limit was gradual, especially when factoring in increasing noncompliance, studies on the effect of raising the limit are rather equivocal. Improvements in car safety, emergency response time, and medical care during the period in question also complicate time series analysis, as injuries and fatalities were decreasing independent of speed limits, as shown in Bureau of Transportation Statistics.
I tend to find the TRB's conclusion more persuasive than the Cato Institute's, but quantifying what the improvement to safety would be now is difficult, to say the least. The Institute of Transportation Engineers's Desktop Reference for Crash Reduction Factors indicates that if the mean speed of vehicles can be lowered, safety improves. However, they make a point of distinguishing between merely posting lower limit signs (something that has, ironically, empirically increased accident rates in certain situations) and actually getting cars to slow down.
I personally wouldn't mind lowered speed limits, but lowering speeds is not a panacea. Overstating the benefits doesn't benefit anyone. Another key point is that the merits of reduced limits could be substantially undermined by noncompliance. Considering the lack of adherence to existing limits, I'd be pretty happy if those were better observed.
The minutes / Some folks / Save through speed / They never even / Live to need / Burma Shave
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