Proposition: It is easier to calculate your ability to stop within the assured clear distance ahead by relating your stopping time (seconds) to your travel speed.
When considering safe stopping distances, think seconds not feet.
Time and distance are interrelated. In the collision field, we always relate distance to feet and time to seconds. A basic rule to safe driving is: Always drive at a speed that allows you to stop on your side of the road within the distance you can see to be clear.
Anytime we discuss distance, time is always associated. Even if you have memorized stopping distance for equivalent speeds, how good are you at judging distance? How good are you at judging distance while moving at 20, 30 or 60 mph?
A more practical method is to relate your stopping ability in time (seconds).
If you are traveling at 50 mph hour, you need at least 3 seconds of braking time to stop within the clear distance ahead (see table below for additional times at different speeds).
There are many reference points along the roadway for a rider to calculate time in seconds, especially when following another vehicle.
- Road lines (markings)
- Road patches
- Cracks
- Tar strips
- Shadows
- Pot holes
- Man hole covers
- Pavement changes
Even on cloudy days, vehicles project shadows that traverse roadway points to calculate time.
We recommend fix points on the road surface to count by and NOT landmarks like signs, poles, etc. Landmarks require too much guess work because of the angles involved.
Stopping Time
We propose it is much easier for the rider to relate stopping in time (seconds) as opposed to distance.
The following speed ranges show the braking time to stop on normal, dry, level roads:
- 20 mph and 30 mph = 1.2 secs to 1.8 secs
- 35 mph to 45 mph = 2.1 secs to 2.7 secs
- 50 mph to 60 mph = 3.0 secs to 3.6 secs
Following Time (“Only a fool breaks the 2-second rule”???)
Is 2 seconds enough time?
The 2-second rule is meant as a safety buffer for time to respond. If an event is unanticipated, 1.6 seconds will be consumed in your reaction alone, how much more time will you need to complete an avoidance maneuver? Is the two seconds an adequate safety buffer?
Two seconds may be enough at speeds less than 30 mph. However, remember perception/reaction time is not factored into any of the stopping times for the equivalent braking distances.
The following table shows varying stopping times (equivalent to braking distance) from 10 mph to 60 mph:
Stoping Times
| MPH | Seconds |
| 10.000 | 0.608 |
| 15.000 | 0.911 |
| 20.000 | 1.215 |
| 25.000 | 1.519 |
| 30.000 | 1.823 |
| 35.000 | 2.127 |
| 40.000 | 2.430 |
| 45.000 | 2.734 |
| 50.000 | 3.038 |
| 55.000 | 3.342 |
| 60.000 | 3.646 |
For reference purposes, the equivalent braking distances are:
| MPH | Braking Distance |
| 10.000 | 4.444 |
| 15.000 | 10.000 |
| 20.000 | 17.778 |
| 25.000 | 27.778 |
| 30.000 | 40.000 |
| 35.000 | 54.444 |
| 40.000 | 71.111 |
| 45.000 | 90.000 |
| 50.000 | 111.111 |
| 55.000 | 134.444 |
| 60.000 | 160.000 |
We have seen literature that stipulates “the two-second rule” is always a safe following distance! Here is a quote from Wikipedia:
“The two-second rule is a rule of thumb by which a driver may maintain a safe following distance at any speed. The rule is that a driver should ideally stay at least two seconds behind any vehicle that is directly in front of the driver’s vehicle. It is intended for automobiles, although its general principle applies to other types of vehicles.”
For obvious reasons the “safe following distance at any speed“ cannot be true. While reaction time may remain constant, at speeds greater than 30 mph you need more distance = time.
Side-note: A key element in advanced riding strategies is to improve your ability in anticipating hazards to allow you more time to react. This will certainly involve giving yourself additional time to match your speed or less speed to match the available time.
The PennDot Motorcycle Operator Manual (Pub 147) on page 12, states, “In traffic, motorcycles need as much distance to stop as cars. Normally, a minimum of four seconds distance should be maintained behind the vehicle ahead. A four-second following distance leaves a minimum amount of space to stop or swerve if the driver aead stops suddenly. It also permits a better view of potholes and other hazards in the road. A larger cushion of space is needed if your motorcycle will take longer than normal to stop. If the pavement is slippery, if you cannot see through the vehicle ahead, or if traffic is heavy and someone may squeeze in front of you, open up a three second for more following distance.”
The Observational Advantage
The more time you allow for following the better your view of the road ahead and any other traffic conditions (withing the limits of road configuration) you may need to respond to.
Larger vehicles will limit your view ahead (18 wheelers, box trucks, buses, etc.) and will require more following time, even at the lower speeds.
4 responses so far ↓
Kathy // 05/26/2009 at 7:48 pm
Here in South Florida, aggressive drivers routinely cut into my 4 second cushion. If barely a car length is there, they will cut in – it still amazes me. And so I work at improving my ability to anticipate hazards and stupid drivers. I find that I constantly size up drivers – asking, what might they do and what would I do to avoid them? Playing out these scenarios has come in handy a few times already.
Robert // 05/27/2009 at 5:42 am
What exactly is wrong with someone changing lanes and getting in front of you?
All most every time I bring the subject of following distance someone will say, “But, if I do that someone will get in front of me.” If you think that way your part of the problem.
If you ride using the two second rule you either haven’t thought this through logically or you have a death wish. A two second following distance only works under ideal conditions where you are focused on on the vehicle in front of you. Get distracted for a fraction of a second and have the car in front of you hit the brakes and your in a panic braking situation. Coming to a tire smoking stop half an inch short of a rear bumper can be a real eye opener.
Also note that riding this close cat get oil and transmission fluid all over your riding gear.
Northernrider // 05/27/2009 at 12:30 pm
This was an excellent update to your blog. Here in Northern Alberta, Canada; we suffer every spring with broken pavement and the residue of gravel and sand on the road. It takes discipline to maintain safe following distances in city traffic as cars will continually cut-in to the spaces between your bike and the leading traffic. The only answer is patience – or sell the bike.
Canajun // 05/27/2009 at 7:45 pm
Good post and good point.
The 2-second rule is better than nothing, if for no other reason than it’s easy to remember. But if most of that is used up in reaction time, you’re in big trouble if the vehicle ahead stops instantly, for example because of t-boning a driver pulling out from a driveway.
If you only gave yourself 2 seconds you will be hitting the brakes at about the same time as you hit his rear bumper. Not good.
So on a bike, 3 or 4 seconds always in my opinion. A crumpled front bumper and a deployed airbag may be the worst you get from cheating the time/distance gods in a car; on a bike it will be much worse.
As for drivers pulling into your buffer space, that seems to be a universal problem. Grin and bear it because you aren’t going to change it.