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Issues Of The Environment: The Impacts Of Increasing Michigan's Speed Limits

Speed Limit 75 Sign
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Legislation to increase the speed limits on Michigan's highway has been working its way through the state Legislature.  In this week's "Issues of the Environment," WEMU's David Fair talks to Timothy Gates, associate professor of civil and environmental engineering at Michigan State University, about how higher speed limits may impact the environment.


   *   A package of bills percolating in the state House would increase the speed limit to 75, or possibly 80 mph, on some Michigan highways. The proposed limits are highest in the Midwest. The bills have stalled momentarily due to being short a few votes, but are likely to be revisited this summer.

   *   Supporters argue that people already drive this fast and should not worry about penalties for it, but critics say higher speeds correlate with more accidents, decreased fuel efficiency and more polluting emissions, and they claim people will drive 90 mph or more if the limit is raised.

The Bills

A package of bills percolating in the state House would increase the speed limit to 75, or possibly 80 mph, on some Michigan highways.

The bills in the package:

   *   HB 4423 is the bill that actually specifies speed limit changes

   *   HB 4424 makes changes to speed limits in school zones

   *   HB 4425 changes how the Michigan State Police and transportation authorities modify speed limits

   *   HB 4426 amends the Michigan Vehicle Code to make related changes to points assessed on driver licenses

   *   HB 4427 makes related changes in the Michigan Insurance Code

Two of the five bills in the package -- House Bill 4426 and House Bill 4427 -- passed 67 to 39. However, the body moved to reconsider those votes and put them on hold for the immediate future. Representative Bradford Jacobsen said he didn't know when the bills might be up for passage again, but said drivers could still be seeing these faster speed limits this summer.

A state legislative package allowing speed limits to be raised as high as 80 miles per hour on certain highways hit a speed bump as a preliminary estimate showed Michigan’s traffic deaths increased 10% in 2015.  The Republican-controlled Michigan House started, stopped, and delayed voting on a package that would have allowed for speed limits of 75 or 80 miles per hour on rural freeways.

Sponsoring Representative Bradford Jacobsen, R-Oxford, said he was short three to five votes on one of the main bills in the package, prompting motions to reconsider others that had already been approved. “There’s a lot of moving parts here, and we’ll do a little better education before we bring it back,” said Jacobsen, who does not anticipate another vote this week. “I’ve been working on it for two-and-a-half years; I’m not giving up yet.”

Under the bills, the Michigan Department of Transportation and state police would conduct traffic and safety studies before raising any speed limits, which would be set to reflect the rate at which 85% of traffic was traveling on a given stretch of road. While his legislation would allow for freeway limits of up to 80 mph, Jacobsen said experts would be unlikely to approve those speeds for existing freeways, which were engineered to accommodate lower speeds. Still, he said he is willing to drop the 80 mph option to win more support for the 75 mph limit. “I had a number of people who said 80 (mph) is ridiculous, you can’t do that,” Jacobsen said. “I think it seems like that might be something to just take out right away that gets us the extra handful of votes we need, but we’ll see.”

The package would also allow speed limits of 60 or 65 mph on some trunk line highways, up from the current limit of 55 mph.

If a law to increase speed limits on rural freeways in Michigan passes, the state will have the highest speed limits in the Great Lakes region and tie South Dakota for the highest speed limits in the Midwest. 

Pro and Con Arguments


   *   You get to where you are going a few minutes faster.

   *   Today's cars are built safer to accommodate higher speeds.

   *   Faster speed will allow for better traffic flow and more efficient delivery of goods.

   *   Motorists traveling 75-80 mph will be able to concentrate on what's ahead of them rather than in the rearview mirror for flashing lights on a police car coming up behind them.


   *   Fuel economy decreases.

   *   It could mean higher auto insurance rates.

   *   Faster speeds increase the amount of poisonous nitrogen oxide emissions polluting the air, according to the Federal Highway Administration.

   *   Faster speeds can lead to more severe crashes and potentially more fatalities, according to the Insurance Institute for Highway Safety.

Some might consider all those downsides to be flawed in some way because vehicles exceed 80 mph now on roadways in spite of the lower speed limits. But if the faster speeds are legal on stretches of highways, more vehicles are likely to be moving at that speed. 

Supporters of the increased speed limits argue that people should not need to worry about being penalized for keeping pace with traffic on freeways, where most drivers push 80 mph, despite the current 70 mph limit.  However, those opposed to the increase say that if limits are increased people will exceed them just as they do now.  They say that at 85-100 mph roads will be less safe and fuel efficiency will decrease, causing increased emissions and pollution.

Speed Limits Are Tied to Air Quality

To most of us, speed limits seem like an annoying nuisance — a number to keep in mind to avoid getting pulled over.  But speed limits are also an important public health policy. Higher or lower limits, multiplied over thousands of miles of road, can have measurable impact on accident rates and levels of air pollution.

Many states have recently been raising their highway speed limits. But research suggests that, if anything, they should actually be doing the opposite and lowering speed limits on both city streets and highways.

The Wharton School of Business's Arthur van Benthem says that new analysis goes a step further, zeroing in on otherwise similar sets of rural roads.  "Since only rural interstates were affected in 1987 — but not other rural freeways that share many similarities to interstates — I could measure how driving faster on the rural interstates affects accident rates, pollution levels and health," he says.  (Higher speeds lead to increased pollution because cars traveling at more than 50 mph or so burn more gas.)

In his analysis, highways that saw their speed limits raised to 65 mph saw 14%more accidents and 44% more fatal accidents, with an average speed increase of just three to four mph. There was also a substantial increase in pollutants such as carbon monoxide, ozone, and nitrogen oxides, which may have caused an increase in fetal death rates, though that result is less certain.

Still, even without the environmental impacts the costs of the higher speed limits easily outweighed the benefits. Van Benthem calculated that all the saved time would only outweigh the extra deaths if a life was worth $2 million or less — far less than the figure used by the EPA.

Higher Speeds, More Pollutants

The increased speed limit also negatively affected the environment. Prior research concluded that – as cars burn more fuel at higher speeds – carbon monoxide and nitrogen oxide emissions increase rapidly and disproportionately. Measuring carbon monoxide and nitrogen oxide output at a number of recording stations along highways in the three states, van Benthem finds that increasing the speed limit to 65 miles per hour resulted in a 23% increase in carbon monoxide levels and a 16% increase in nitrogen oxide levels.

The relationship between increasing the speed limit and negative health effects – resulting from air pollution – on infant health is less robust. While van Benthem’s research suggests that the increase in air pollution resulted in 17-45 additional fetal deaths, these results seem highly contingent on van Benthem’s model specification. In addition, increasing the speed limit did not seem to negatively impact a number of other commonly used indicators of infant health, such as premature birth and infant death.

Van Benthem moves beyond measuring the effects of increasing the speed limit to compare the private benefits of driving more quickly and the societal costs of higher pollution and more accidents.  Using commonly-accepted measures of the value of human life and the value of travel time, van Benthem calculates and compares the net cost to society of raising the speed limit, suggesting that the societal costs outweigh the private benefits by over six times.

Do van Benthem’s results suggest that recent attempts to raise speed limits are bad public policy? Not necessarily. Increasing speed limits creates private benefits and negative externalities. While van Benthem’s findings rely on data from the 1980s and 1990s, systematic reasons suggest such externalities may no longer be as costly. For example, improvements in car safety and fuel consumption have likely reduced the frequency and severity of accidents, as well as pollution production. As a result, the private benefits of increased speeds may now outweigh these reduced externalities. Future research—using contemporary data or extrapolating from van Benthem’s empirical work—would be helpful in determining whether states should be reluctant to raise speed limits. 

Faster Speeds Burn More Fuel

Driving too fast. Everybody knows that highway mileage is usually better than city mileage. So the faster, the better, right?  Wrong by a mile.  Most American cars operate at peak efficiency—generating the most forward momentum with the least amount of fuel—between 50 and 60 miles per hour.  There's nothing magical about that range, except that the government establishes the city and highway mpg ratings for cars by operating them within certain speeds for a short period of time.  Automakers want to get the highest mpg ratings possible, so they engineer their cars to be most efficient at the speeds at which the government tests them.  If the government tested at 30 miles per hour instead, then no doubt carmakers would engineer their vehicles to be most efficient at 30.

At speeds over 60 mph, gas mileage drops off a lot more than most drivers probably realize. "The aerodynamic drag created by a vehicle moving through the air increases exponentially," says Jack Pokrzywa.  It takes more power to overcome the added resistance, which forces the engine to work harder, burning more fuel.  A lot more.

If your car has an on-board computer that displays your instant gas mileage, the difference between 60 mph and 80 mph will be obvious—and substantial. At 60, a typical four-cylinder car might average about 30 mpg; at 80, it could fall to about 20 mpg. In other words, your gas mileage going 80 miles an hour on an open road might barely be better than the mileage you get navigating stoplights and city traffic.

As a rule of thumb, the best way to determine your car's sweet spot is to watch the tachometer, which measures how hard your engine is working, rather than the speedometer. Aim for the lowest rpm in the highest gear, while still having a comfortable degree of power available if you need to pass or maneuver quickly. That will indicate that your car is doing the least amount of work to stay at the speed you're going. 

Dr. Timothy Gates

Dr. Gates’ interests include the following subject areas: traffic engineering, traffic operations, traffic safety, driver behavior, and transportation economics.  His recent research includes projects related to safety and operational impacts associated with raising speed limits, driver behavior at signalized intersections, safety impacts of intersection sight distance, safety performance and economic assessment of roadway design features, safety and operational impacts of highway rumble strips, highway work zone safety and operations, and pedestrian behavior and safety.  Dr. Gates is a licensed professional engineer in Michigan and Wisconsin. 

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— David Fair is the WEMU News Director and host of Morning Edition on WEMU.  You can contact David at 734.487.3363, on twitter @DavidFairWEMU, or email him at

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