Saskatchewan Driver's Handbook

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Estimation of safe passing time

Using time to estimate when you can pass is complicated, but it is well worth the effort required to master it. You can practise passing safely and once you can judge time well, you have a good method of judging when it is safe to pass.

You can get all the information you need to make a passing judgment from two estimates: the estimate of your safe passing time and the estimate of the closing time between you and the approaching vehicle.

The first step in developing your new technique is to find out how long it normally takes you to pass. This varies from 10 to 40 seconds depending on your vehicle, your technique and the difference in speed between you and the vehicle you are passing. To find your safe passing time, choose a divided highway or a twoway highway at a time and place when traffic is light.

First, determine your passing time as follows:

  1. Follow the vehicle ahead at the same speed, but three seconds back. Note its speed.
  2. Check that you can see at least 40 seconds ahead, that there are no vehicles coming the other way and that there are no intersections or solid lane markings.
  3. Begin counting "Thou - sand and one, Thou - sand and two" from the point you decide to pass and begin to accelerate.
  4. Signal, and at a point where you still have good vision and can smoothly change lanes around the vehicle ahead, move into the left lane.
  5. Continue in the left lane until you see the vehicle you have just passed in your inside mirror. As you pull back into your original lane, signal right and continue your acceleration. Do not decelerate as you change lanes.
  6. Continue counting until you have completed your pass. Stop your count when your left rear wheel crosses the lane line (just as you are returning to your original lane in front of the vehicle you have passed).
  7. The last number you say after "Thou - sand" is your measure of passing time in seconds.

Next, build in a safety margin that you are comfortable with.

Five seconds is a minimum safety margin. For example, if your passing time is 15 seconds, add another five seconds for possible error. This will give you a cushion of five seconds if the vehicle coming towards you is approaching faster than normal or if your estimation is not accurate.

Your passing time + your safety margin = your safe passing time

Your safe passing time in this case is 20 seconds.

If you think about it, your safe passing time will also tell you the minimum sight distance needed for safe passing, even if there is nothing coming the other way.

Try this several times at highway speeds varying the difference between you and the vehicle ahead until you get a handle on what your safe passing time actually is.

Watch Now: Estimation of safe passing time

Estimation of the closing time of two vehicles

This is similar to judging the time from fixed objects except now you have a vehicle coming towards you. The gap between you and the approaching vehicle will close about twice as fast as if you were approaching a fixed object.

Steps to estimate closing time:

  1. Position your vehicle three seconds behind the vehicle in front.
  2. Determine the point at which you could safely return to your lane if you were to pass the vehicle in front - the safe return point. For example, watch a vehicle pass and note where it pulls back into the driving lane.
  3. When an approaching vehicle appears, start counting and continue counting until the vehicle reaches the safe return point ahead of you. The last number you say after "Thou - sand" is your measure of closing time.

The safe/unsafe judgment

When you feel confident that you can estimate closing times reasonably well, practise judging when it is safe to pass, then check by counting time. When a vehicle is approaching, decide if it is safe to pass or not. Say "safe" or "unsafe," then count "Thou - sand and one, Thou - sand and two" – check the actual closing time against your safe passing time to see if you were right. Keep practising until you can make the safe/unsafe judgment with 100% accuracy.

There are two advantages to practising your passing judgment in this way. First, it is completely safe because you never actually pass the vehicle. Second, your timing will be based on the actual speeds of vehicles using the highway.

Be sure to keep the type and size of vehicle you are passing in mind. For instance, when passing a motorcycle make sure you leave a greater distance between your vehicle and the motorcycle you are passing, before pulling back into the driving lane. It's easy to underestimate how much room you should allow due to the smaller size of a motorcycle.

The actual passing judgment

Now practise what you have learned. When you judge that it is safe to pass, carry out a passing manoeuvre.


  1. You have to build a minimum of a five-second safety cushion into your safe passing time. Do not give this up under any circumstance.
  2. When you have enough experience with this method of estimation to be skilled and comfortable with it, extend your range. Determine the safe passing time for different situations, such as different speeds, surface conditions and longer vehicles such as tractor trailers. The passing time will vary.
  3. Your safe passing time is also a measure of the shortest sight distance you need to pass, even with no oncoming traffic.
  4. Your passing time will vary with the speed difference between you and the vehicle you are passing.

Being passed

When you are being passed:

  1. Maintain your speed; do not speed up.
  2. Slow down, move to the right of your lane and let them back into the lane.
  3. If the driver passing you has misjudged, slow down and let them back into the lane.
  4. When being passed by large trucks, you are likely to be pushed around by the wind. Just look where you want to go and your steering will take care of itself.


Rev: 2017