Posted by Jeffrey R. Parenti, P.E. on December 3, 2009
Sammy Hagar made it officially OK in 1984. His rebel anthem “I Can’t Drive 55″ blared from the stereo speakers of Camaros and IROC Zs all across the USA that summer, giving countercultural permission to all 17-year-old boys to do whatever they wanted, even with cops around.
Twenty-five years later, we are still a nation of bad-asses, even in our mini-vans and Honda Fits. We can’t help ourselves. It’s the only law God-fearing PTO mothers and mousy middle managers break every day. For some of us, it’s the only thing that makes us feel alive:
Breaking the speed limit.
Speeding is either fun or necessary, depending on your point of view. But it’s also dangerous, wasteful, and stupid. Driving the speed limit has many pleasant benefits. I can hear you snickering. Hear me out.
The Numbers
The National Highway Traffic Safety Administration believes that one-third of all fatal crashes are caused by speeding. This amounts to around 14,000 people killed in the US every year. By way of comparison, a total of about 5,000 US soldiers have been killed in Iraq and Afghanistan combined since 2001.
About Speed Limits
Speed limits are not set arbitrarily nor randomly. They are not set by police to maximize revenue from tickets. They are not set by the government to annoy you.
Speed limits are set instead by highway engineers. Limits are related to the road’s “design speed.” When designing a highway, the engineer must assume a speed for all calculations. Higher-speed roads must be straighter, flatter, and wider. The speed limit is set below the design speed as a “factor of safety.” In other words, if you design a road for 70 MPH and set the limit at 60 MPH, a car that slightly exceeds the limit can still safely use the road. (Similarly, a building or bridge is built stronger than how it’s expected to be used — just in case.)
When the rubber hits the road, however, people drive at a speed at which they feel comfortable. This comfortable speed varies by driver and by road. So if we know that many people are comfortable driving closer to the design speed than the posted limit, why don’t we just set the limit higher?
Because the gap between the posted limit and the design speed is our safety net. Imagine how many more speed-related deaths we would have if we cut this safety net and pumped up all posted highway limits by 10 MPH. I don’t even want to think about it.
Speed limits on rural, suburban, and urban streets are set in similar ways. Additional factors are involved, such as population density, proximity to school zones and playgrounds, and roadside objects like trees and utility poles.
You have your reasons for speeding and why you think it’s perfectly safe.
“If I don’t speed, I will be late for work”
Does speeding save you time? Sure! Makes sense, doesn’t it? But how much time does it really save you? Let’s do the math. We’ll assume you are going to Boston from Billerica, a 20 mile trip. And we’ll also assume that all of those 20 miles are on the highway with a 65 MPH speed limit. Finally, we will assume you can sustain your speed for the entire trip. Here are the results:
| Speed (MPH) |
Travel Time (min.) |
| 65 |
18.5 |
| 70 |
17.1 |
| 75 |
16.0 |
| 80 |
15.0 |
So under ideal conditions, driving 15 MPH over the speed limit saves you a robust three-and-a-half minutes. The real number is much lower, of course, since some of those 20 miles are not highway miles. Plus, it would be impossible to sustain 80 MPH unless the highway were completely empty — you would be constantly slowing down for other cars.
In other words, on this trip you would be saving about three minutes by speeding, and probably much less than that. Meanwhile, you are putting yourself (and others) in a greater danger for a crash — no to mention an expensive speeding ticket. Not worth it. Besides, if you get in trouble with your boss for being three lousy minutes late, you need a new job.
“If I drive the speed limit, I feel like I’m crawling”
Every car I have ever owned has been quieter than the last — and I’ve never owned anything fancier than a Chevy Celebrity. Even today’s economy cars are loaded with insulation and have excellent aerodynamics compared to just ten years ago. These features cut down on wind, engine, and tire noise. Noise is one of the “cues” that gives you a feel for how fast you are going.
If you’ve ever gone 30 MPH downhill on a bicycle — with the wind in your face and the unforgiving pavement roaring past inches beneath you — you know that feeling. And going that fast on a bicycle is not a comfortable feeling because of those cues.
Inside our cozy car, we are mostly removed from the blazing speed at which we are traveling, especially when the rest of the cars on the highway are going about the same speed and there aren’t any objects right on the side of the road whizzing by. But I assure you — at 65 MPH, you are not crawling.
How fast is 65? At that speed, you are going 95.6 feet every second. At 80, you are going 117.6 feet per second. Walking pace is a little more than 4 feet per second.
For a true feel, walk to the edge of your favorite freeway and stand as close as you feel safe doing so. Watch the cars go by at warp speed for a few minutes. Feel uncomfortable? Of course you do. Because 65 MPH is fast.
“I’m a great driver. I have great reaction time. If something happens, I can swerve or brake in time.”
Are you sure? Your life depends on it.
Highway engineers have studied human reaction time extensively. The American Association of State Highway and Transportation Officials (AASHTO) has settled on an assumed perception-reaction time of 2.5 seconds. It’s true that your pure reaction time might be between 0.15 and 0.20 seconds, but this is when you know the stimulus is coming. Researchers call this the “alert condition.” Under normal conditions, such driving, reaction time is around 1.5 seconds. Then you must add on the time to physically move your foot from the gas to the brake. So when a barrel rolls off the truck in front of you, two-and-a-half full seconds elapse before your brakes even engage.
Then, from 60 MPH, most vehicles with brand-new tires need about 140 feet to stop on dry pavement. So the total distance required to stop from 60 mph is 360 feet, or more than a football field! And that assumes the pavement isn’t wet, your tires don’t have 30,000 miles on them, and that your eyes are on the road and not your cell phone. At 80 MPH, total braking distance is a whopping 545 feet.
So how close is that car in front of you that you are tailgating?
You may consider yourself a world-class driver, but you can’t defeat physics. Slow down.
“But I have a high performance car”
Good for you. Yes, a more expensive car might have a tighter suspension, but that does not help you in a panic stop. Anti-lock brakes (ABS) can cut down on braking distance considerably on snow, ice, or rain and a few percentage points on dry pavement. But your car can’t help you react faster. Even if your special high performance car can cut your total braking distance to 450 feet at 80 MPH (which I seriously doubt), this is still much more room than you would need with a regular car at 60 MPH.
“My car has lots of safety features”
Congratulations. You are more likely to survive a crash than if your car were not equipped with them. However, in a high speed crash, your body (including your very fragile brain) will be stopping very, very quickly, meaning you will be subjected to extremely high “G” forces. There isn’t a safety feature you can buy that will save you from a 20-G crash. And the faster you are going before the crash, the higher the G forces will be during the crash.
“Everyone else does it”
They sure do. I have a friend who says people speeding are late for their accident. “Everyone else” is going to be involved in a crash someday soon. It is only a matter of time — ten million cars are involved in crashes every year. If you choose to engage in a high-risk behavior such as speeding on a freeway among trucks 10 times heavier than you, your chance of being in a crash goes way above the average. Your best chance of staying out of the that group of 10,000,000 wrecked cars this year is to reduce your speed.
“But driving fast is fun”
Driving on a road with other people is not a video game. Your vehicle is very heavy and in one instant, it can cause injury or death to yourself, someone in your car, or a stranger outside your car. If you kill someone because you were having “fun” speeding in your fast car, you will live with it for the rest of your life.
You want thrills? Play a video game or drive a real mini race car at F1 Boston.
Advantages of Driving the Speed Limit
I drive the speed limit, and I live a fulfilling, happy, meaningful life. So can you.
Better fuel mileage
Driving 80 MPH versus 65 MPH results in much higher air “drag” — a force that pulls your car backwards. The engine must work harder to counteract this drag, using more gas. Driving the limit will save you money on gas. How much you save depends on the kind of car you have, but you will notice a difference.
Better chance of getting home alive
The safety benefits of driving slower are immeasurable. I’ve covered just a few above. The exact numbers are hard to calculate, but I can assure you that your odds of not being involved in a crash go way, way up when you are traveling at the speed limit.
Lower stress level
If you drive 80 MPH, you are constantly changing lanes to weave around slower cars, right? Changing lanes is not a cinch at this speed, either. But at the speed limit, few vehicles on the road are slower than you, except for the odd semi on an uphill. You can set your cruise control at the speed limit, sit back, and enjoy the ride. No more lane changes. Let “everyone else” pass you.
Never get a speeding ticket
So you think the police are out to get you. Or they have quotas. Or they target you because you have a red car. Whatever you believe, they can’t pull you over if you aren’t speeding. This will save you real money in fines, court costs, and insurance costs.
Before you laugh, try driving the speed limit for a week. If you feel ridiculous driving “slow” and following the law, all I can say is it takes some getting used to.