Is driving slower better?

In October of 1973 a group of nations got sick of the US “foreign policy” of military intervention, and, knowing we had developed a lifestyle totally dependent on oil, they all agreed not to sell us any more.

This caused massive and immediate affects throughout the US economy. Buying fuel, at any price, meant waiting in long lines – on those days you were even allowed to buy gas at all.

The government took steps to encourage conservation – which (unlike sourcing new oil) could be done immediately – such as banning Christmas lights.

Another major step they took was to enact a national speed limit of 55mph.

The reason for this is that at higher speeds air resistance increases exponentially relative to speed. Going twice as fast requires 4 times the energy. This is as true of modern vehicles as it was in 1973. It will always be true, because it is due to fundamental physics. All vehicles, small or large, gas or alternative fuel, use more energy to travel faster.

In fact, going from 55 to 70mph typically uses between 20% and 25% more fuel to go the same distance. 25% is a massive amount more fuel to use – at our current national usage, that amounts to approximately 5 million barrels of oil every single day!That’s 175 million gallons.

That just happens to almost exactly equal the amount we purchase from the Middle Eastern OPEC nations.In other words, if we still had (and actually enforced!!!!) a 55mph national speed limit today, that alone could completely eliminate any need for importing oil from the Middle East.

Similar to the relationship between wind resistance and speed, kinetic energy varies with the square of speed.

Energy=1/2 mass X velocity2

This means that if you are going twice as fast, it will take four times as much force to stop. In other words, it will take four times the braking distance to stop in an emergency.

If it takes four times longer to stop, you are much more likely to slam into something full force which, at a lower speed, you could have come to a full stop in time to avoid hitting at all.It also means that if you do end up in a crash, at twice the speed you will have four times the impact force. At four times the impact, crumple zones and airbags are much less likely stop your organs from hitting your ribs hard enough to literally explode.

The energy in the previous equation dictates how hard you hit. Whatever energy isn’t absorbed by the car and restraints, gets absorbed by you.

A lot of people focus on the size of the car or truck they are in, believing a large amount of vehicle mass will absorb more crash energy.
Look at the formula: that’s half true.

Literally.

The affect of mass gets divided by two. But velocity is not only factored in fully, it is squared. In plain English, this means that speed plays an overwhelmingly larger role in how bad a crash is than mass.

I realize that almost everyone actually feels they are safe when they are driving a car. It is human nature to take anything which is commonplace for granted. We tend to fear things which seem to have an infinitesimally remote chance of happening.

Interestingly the number one cause of death of all young people in the US in car crashes. It causes more deaths among young people than murder, suicide, cancer, and heart disease combined. It is the number one cause of death up until age 40, at which point it is still in the top 3.

We don’t hear about it much in the news precisely because it is so common. There are roughly 16,500 accidents significant enough to be reported in the U.S. EVERY DAY. Of these, roughly 1/3 result in permanent injuries. Every 12 minutes, an American dies in a car crash.

The number one factor in causing all of these deaths and injuries? It isn’t alcohol. It isn’t teen drivers or cell phones. It’s speed.

Speeding is the single largest factor in injury and fatality collisions.

Contrary to popular belief, driving slower is safer even when other cars around you are speeding.

Here are two different studies’ conclusions on the issue, after compiling actual accident data:

“risk of involvement in a casualty crash, relative to the risk for a car traveling at 60 km/h, increased at an exponential rate for free traveling speeds above 60 km/h [37mph]” ”First, the probability of a crash is approximately proportional to the square of the travel speed. Second, in a crash, injury risk is approximately proportional to the impact forces on a person, which in turn are proportional to the square of the impact speed. These two effects can be summarized in a general rule of thumb: When travel speed increases by 1%, the injury crash rate increases by about 2%, the serious injury crash rate increases by about 3%, and the fatal crash rate increases by about 4% ”

And finally, a chart from the DHS defensive driving course:



So, ok, by now you are maybe starting to accept the physics and the statics.

The catch:

There is, of course, an obvious drawback to driving slower: it takes more time to get somewhere. That is, of course, why people do it. Who wants to spend more time than they have to sitting in a car?

Time for just a little more math:

Time = Distance / Speed

What does this mean for typical driving speeds? Here, I’ll do the calculations for you:

1 hour = (60min x 60sec) = 3600seconds
3600 seconds / 65mph = 55 seconds
It takes 55 seconds to go one mile at 65mph.

3600/55mph = 65 seconds

1 mile at 65mph= 55seconds
1 mile at 55mph= 65seconds

Difference = 65-55 = 10 seconds

Slowing down from 65mph to 55mph means it will take you an additional 10 seconds to go a mile. Ten seconds. That’s it.
Speeding up from 65 to 75 would only gain 7 seconds. Going from 75 to 85 saves 6 seconds. The faster you go, the less time it saves!

When you are tempted to go above the speed limit, remind yourself that the faster you go, the less time you save per increase in speed. You can do the math yourself if you have trouble believing that.

A typical drive is 10 miles. Over a 10 mile drive by driving at the legal maximum instead of 10 under, you will waste 25% more gas (increasing your gas bill by 25%), and increase your risk of death by 60-100%, all to save just a minute and a half. A minute which you may well end up losing again at a single stop light. I’m sure you have been passed by a speed demon only to catch up to them a minute later at the red light they raced to (or are you always that speed demon? Either way, you know what I mean). I find that even at 20mph under the limit, the ETA estimates that Google Maps and my GPS unit give me are always spot on, if not slightly late. Both systems assume I will drive at the speed limit, and calculate how long the trip should take based on that. I often get where I am going sooner than they said I would, even though I drive slower than they expect.

Even on a long drive, say 350 miles from SF to LA, speeding by 10mph the entire way only saves a little over a half an hour.
That half an hour meant using up an extra 3 gallons of gas, or about $11. According to the study quoted above, you also increase your risk of death by up to 100%. And if you have to take additional an stop to fill the tank, you have diminished your time savings to probably 15-20 minutes!

Just some food for thought, and a little incentive for anyone who might be considering driving a little slower

 

I caught myself last night driving 40 mph in a 45 mph zone. Must have been becuase I was in a minivan...


Seriously, good post though.

 

Thanks. Here is what I learned from my UltraGuage recently regarding speed vs fuel economy. These results are instant readout during flat terrain constant throttle driving. All results were taken after the vehicle was thoroughly warmed up on a wind free 70f day.

03-30-2014

Speed MPG Load RPM
45mph 32.7 26.1 1775
50mph 32.2 26.3 1975
55mph 29.9 28.0 2075
60mph 27.3 31.4 2350
65mph 26.5 32.2 2525
70mph 23.9 35.7 2800

 

I can believe it. Driving to see the families, we pass through the Hill Country around Fredericksburg,TX. Not mountains, but the closest thing we have too it. In my forerunner or, I get almost 22mpg vs about 17 by slowing from 75 to 65. It costs me 30 minutes to a 4 hour drive. A moot point as we stop for 1 to 2 hours so the 17 month old can eat and run around. But drive in West Texas, where there is nothing, it's nice to speed up a bit.

 

1) The odds of dying in a crash are higher if you are going faster, but the odds of getting in a crash are actually lower if you go faster. Driving significantly slower than the flow of traffic is likely to get you in a serious crash. Driving significantly slower also makes your more likely to fall asleep at the wheel. So the real life big picture odds aren't as bad as your table suggests.

2) Speeding comparisons on a 10 mile trip are a little dishonest in my opinion. Driving 2200 miles to Florida at 55 takes 40 hours of driving each way. Driving 2200 miles to Florida at 75 takes 29.3 hours each way. With the amount of vacation we get in the USA I can see an extra 21.4 hours of time vacationing being worth the extra fuel consumption to a lot of people.

 

I totally agree with OP. My 2007 Tacoma DCLB Prerunner AT V6 which I just traded on a 2014 Tacoma averaged 23 mpg. Driving 50 mi per day to work on a 4 lane hwy at 55 mph. And thinking about the old 55 mph speed limit we had a lot less Rollover and fatality accidents. We need 55 back. Let people complain they will get over it. I also realize that at 55 mph I am more relaxed and alert to my surroundings.
Note: my 2014 Tacoma DCSB Prerunner AT V6 TRD Sport has only 150 mi on it. Went to visit my grandson 40 mi away on 2 lane roads, drove 55 with A/C on and got 26 mpg both ways per my Scangauge II.

 

So, you're the guy in the slow lane on the interstate going 40 mph?

lol jk. Since getting my UltraGauge for Christmas, my average MPG has gone from 13 to 17 and my average MPH has gone from about 85 on the interstate to 70. The biggest thing that helped me was slowly accelerating from a stop, opposed to gunning it.

 

I usually avoid the interstate if I can. Driving 55 is fine with me and I like the fuel savings.

 

Great post op! Thank you

 

Driving at the posted speed limit or a bit over ( say 70 in a 65 ) as opposed to 75-80 in a 65 is far less likely to get you involved in an accident. You will also realize as you do this that driving is actually much easier. No one is " always in your way " , you can just relax and enjoy the drive. Driving trucks for years , I noticed that it was a far easier trip when I wasn't in a rush..whether it was the hour drive to work...or the 10 hour drive to a customer. And driving 10,000 to 14,000 miles a month , you notice a hell of a lot of things having to do with traffic.

As for rollover accidents...I think the number of larger vehicles on the road today ( think trucks,SUV's and minivans ) is far more substantial than it used to be. Coupled with the fact that most people tend to be rather panicky in an accident situation..more rollovers.

 

I agree that on a cross country road trip it may be worth it for some to go the maximum speed limit. That scenario is by no means common place or what I am pointing out here. For the most part, you are better served in several ways to keep you speeds as low as you can tolerate and that the environment can safely support.

 

Some of our roads here around Raleigh recently got thier limit raised to 70, which means 75-80 in real life.


I don't believe that most people know just how fast shit happens at those speeds, especially with one hand on the wheel and all of the other distractions...

 

Lots of info included here....thanks for posting!

Myself, i rarely speed...I do drive the speed limit (70mph on the interstates I regularly drive).

Certain studies would suggest, as there always is at least another twist, there's more to the story. Regarding safety, comparing country to county (say the US vs. DE) deaths per mile are not proportional to speed limits (or lack there of in Germany) and, closer to home, not proportional on a per mile basis on interstates with differing limits. To me, this suggest driver skill and or methods of road construction and safety content play a roll in the real world.

Further (and regarding economizing fuel usage) being a fan of small locally controlled government, I don't support national speed limits. I also have to ask, why the US makes diesel so difficult to certificate. Add to that the lack of public support for public transportation & alternate means of power I just don't think as a nation, we're that concerned. Let alone we don't maximize our own energy resources.

I'm not disagreeing with you... I'm just saying I don't believe the will of the people are behind you yet. I'd like nothing more than see some of the countries (at least certain families) we've helped make ultra rich be broke.

 

I actually agree about national speed limits and find them to be an infringement upon freedom, I was merely using the example of the national law to show what would be possible if we were willing to travel at 55, and that it didn't used to be all that uncommon for many people to travel highways at such speeds.

I love Diesel, and would love a Tacoma with it (30% more fuel efficient!!!). Sadly, the EPA is bought and payed for. Diesel puts out more "greenhouse" emissions/gallon consumed. Anyone with a brain can see the corruption in measuring it this way when you can go 30% further on the same gallon compared to gas. End rant.

 

I've tried to convince some of the people I work with that traveling faster between two points doesn't save that much time. I gave up. They just like driving fast.

One factor that is not considered. From the table on a 10 mile trip a driver is on the road for 12 minutes when traveling 50 mph, but only 8 minutes when traveling 75. He is on the road for only 2/3 the time when traveling 75 mph compared to 50mph. I would think the risk of death would be 2/3 * 6x = 4x. I would have to have a closer look. But 4 times the risk of death is still worth it to slow down.

Another thing to consider is what is the risk of having a crash. In other words what is the value of x? (I'll make up an example. I have no idea what x is.) If x = 0.000001. What does this mean? On average for all drivers, if this average driver drives 50 mph for 10,000,000 miles the driver will have a crash and die. Remember this is an average. For this example this is 1,000,000 guys each traveling 10 miles for a total of 10,000,000 miles. One of the 1,000,000 guys crashed and died on the 10-mile trip. If you increased the speed to 75 mph, then six (or four if you consider my first commit.) of the 1,000,000 guys will crash and die.

Just something to consider. As I said above I have no idea the value of x. I would guess it is even smaller.

 

I have gotten as low as 15 mpg on the interstate at 80-85 mph, and as high as 25 mpg on two-lane highways at 55 mph. If I upshift at 1800-2000 rpm and don't use the A/C much, I will get 19 mpg in mostly city driving. I consider that pretty damn good for a 4200 lb. double cab 4x4 truck. The utterly gutless 2wd 4-banger Ranger I used to have could barely beat that, and it was a miserable penalty-box to drive.

 

Figuring out the odds on scenarios like this one are based on previously known variables and may not reflect future events. They can give us an idea of what to expect though. Regarding the odds of a crash taking place, there are many factors, not the least of which is the time it takes to stop. Traveling faster takes longer to stop and makes it harder to avoid accidents, logically leading the reader to assume driving slower will make it easier to avoid accidents where stopping distance plays a crucial role. However, this does not account for the situation in which driving faster would have lead to missing the event in which one needed to stop all together. With 2/3 the amount of time spent on the road you are 3x% less likely to be on the road when the event occurs. There is value here, but there are to many variables to know how much. Driving faster may lead to fewer events in which stopping distance will factor into a collision, but that won't be of any solace to you the one time you front end someone going 70mph.

 

The problem here in Raleigh though is that driving that speed can get you run over....I drive from Garner to RTP and back everyday. I always stay somewhere within the 9 MPH they allow over the posted limit and I have folks zipping by or riding my bumper.

Friday morning I was keeping up with the flow of traffic....Looked at the speedo and I was going 87 MPH. Had I slowed down at that point it would have caused a pileup.

 

If you want to see how what you drive fits into the equation.

http://www.iihs.org/iihs/topics/insurance-loss-information

. IIHS has enough data to drive a math nerd crazy.

 

There are a lot of times when you have to go with the flow. I just slow down whenever I can. Mostly on the smaller roads.