Chart for How to Calculate Distance Per Second at Different Speeds
In a motor vehicle collision where the parties dispute liability, they usually are disputing the time, speed and distance of the vehicles. So you need to be able to calculate how far a vehicle will travel in a second at any given speed.
As any accident reconstruction textbook will underscore, people usually overestimate the time it took for a car accident to unfold. In one study, participants who viewed a thirty second event gave an average estimated duration of 150 seconds, 500% longer than it actually took.
You can also figure this out by just asking someone to estimate how many feet a vehicle travels in one second while driving 65 mph. The range of responses is laughable and this is what lawyers get from fact witnesses.
This typically works to the victim's advantage, particularly with respect to speed. Why? Because the longer a visual estimation takes, the slower the vehicle must have been traveling.
Here is a sample cross:
Q: When you saw the Plaintiff, how far away was he from your vehicle?
A: I'm not sure. I did not have a tape measure.
Q: So you have no idea. You can't even offer an estimate?
A: Maybe 40 yards.
Q: Forty yards? Could it have been 50?
A: I guess so.
Q: Could it have been 60?
A: I don't know. Again, I didn't have a tape measure.
Q: How long was it until impact after you saw the vehicle?
A: Maybe a second.
Q: Then you have to close in and underscore how the defendant is contradicting himself/herself; the court has taken judicial notice of how far vehicles can travel over time at given speeds. You would agree that at 30 mph a vehicle is traveling at 2 miles per minute.
Q: And there are 1760 yards in a mile? (The witness is using yards here. If feet, there are 5,280 feet in a mile?)
Q: So at 30 mph, a vehicle is traveling less than 15 yards per second?
A: I suppose.
Q: So you had maybe four seconds before the impact?
A: No matter what the answer is to this last question, you have underscored that the defendant's story is woefully inconsistent which substantially diminishes defendant's credibility.
Certainly, you want to look for opportunities to exclude the argument of plaintiff's speed because it was not causally related to the accident or because the lay witness testimony is inadequate to support a finding of contributory negligence because of excessive speed. Argue Myers v. Bright but be prepared for argument that the Maryland Court of Special Appeals opinion in Romero v. Brenes is controlling.
This chart is a good cheat sheet for time, speed, distance calculations for how fast a vehicle is going per second.
- 1 mile per hour = 1.4667 feet per second
- 10 miles per hour = 14.7 feet per second
- 20 miles per hour = 29.3 feet per second
- 25 miles per hour = 36.7 feet per second
- 30 miles per hour = 44.0 feet per second
- 35 miles per hour = 51.3 feet per second
- 40 miles per hour = 58.7 feet per second
- 45 miles per hour = 66.0 feet per second
- 50 miles per hour = 73.3 feet per second
- 55 miles per hour = 80.7 feet per second
- 60 miles per hour = 88.0 feet per second
- 65 miles per hour = 95.3 feet per second
You can roughly calculate the speed of a car or truck if you can measure the skid marker. The formula is the S² = Es² + 30fd. S is for the speed of the vehicle, Es is the ending speed, f is the drag factor and d is the length of the skid.
The formula is easy enough. Applying it is another matter. Calculating the ending speed, if the vehicle does not come to stop, is a challenge as is estimating the drag factor.
How long tire marks remain on the road after a car crash is dependent on a lot of variables. These variables include the tires, the weight of the vehicle, the wear on the asphalt or concrete, the type of braking system, the weather, and so forth. Tire marks can give you estimations of speed so you can answer the "how far do you travel at X mph in Y seconds?" questions.
The take-home message is if you are a car accident lawyer and you want to preserve physical evidence from a crash scene, you want to secure that evidence sooner rather than later.
It takes the average driver between 2.3 and 2.5 seconds to hit the breaks in a sudden emergency. There is data that suggests the average time to break is lower than 2.5 seconds if the driver perceives a critical sudden emergency. Younger drivers typically are quicker to the break than older drivers.
The braking distance increases fourfold when you double the speed.
- Here's a good time, speed distance calculator
- Good post on how to calculate speed at time of accident
- Cross examination of the Plaintiff's expert on bias
- Sample cross examinations and other examinations of witnesses in personal injury case
- Maryland jury instruction on speed
- Getting ready for trial: more samples
- Overview of Maryland car accident lawsuits (what our accident lawyers do)