1. Determine how much time the student needs in order to cross with enough clearance or safety margin, and add a one-second "fudge factor" (this is explained in more detail on the next page). We will call this time "X."
2. Have the students stand at the curb and, as a vehicle approaches, start a stopwatch when they think it's the last possible moment that they could start a crossing with enough time to get out of the path of the vehicle with a safety margin.
3. Stop the watch when the vehicle reaches the students, and report to the students how accurate they were.
- If the lapsed time is within a second of "X," it would be considered acceptable.
For example, if "X" is 9 seconds and the student started the stopwatch when the vehicle was between 8 and 10 seconds away,
that would be considered acceptable.
If the lapsed time is significantly longer or shorter than X seconds, the students can use this feedback to modify when they think there is still time to start crossing.
Some instructors are tempted to consider anything longer than "X" as being acceptable, but if your goal is to help the students develop good judgment of the speed and distance of the vehicles (get that "whoa!" feeling in their gut when it's too late to cross), then they need to know that they were incorrect if they started the stopwatch when the vehicle was significantly longer than "X" seconds away.
NOTES:
- If a vehicle changes speed while being timed, disregard that trial, since in that situation it isn't possible to know whether the student accurately judged its speed and distance as it approached.
- This exercise can be complicated when there is more than one lane of approaching vehicles, as the students need to judge when they think they still have enough time to cross before any of the vehicles will arrive (assuming that the vehicles could change lanes and arrive in the nearest lane).
IMPORTANT!:
Students who use vision to cross should learn to make this judgment in a variety of lighting conditions, including at night. It may be that they can do it well in certain lighting conditions and not others, and they need to know that. For example, it was at night that Lorraine Evensen misjudged how close and fast the traffic was coming -- if she had realized her judgment was not good, would she have chosen other strategies for getting across the street?
CAUTION ABOUT USING LANDMARKS TO JUDGE GAPS:
Students will NOT develop and improve their judgment of the speed and distance of vehicles if they use a landmark along the street to help determine when the approaching vehicles are "X" seconds away.
Adriana Hermosillo, an O&M specialist who trained people to judge gaps as part of a research project, explains the problem as she experienced it. When she started learning to judge gaps in traffic, she noticed there was a prominent landmark -- a dip in the road where the median strip and lane markings ended, as shown in the photo to the left (a car is passing that landmark). She said:
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When I first attempted this exercise I was looking at where the cars were in relation to various landmarks to determine when they were "X" seconds away.
The dip in the road was significant enough to draw my attention.
With the feedback I received, I realized that if we started the stopwatch just as the car approached the dip, my timing was perfect.
All I was doing was using that landmark, not taking into account the actual vehicle or its speed.
I didn't even realize I wasn't learning the skill of judging gaps until we moved to a place that had no prominent landmark, and then I was clueless about judging the vehicle's speed or distance. That is when I actually practiced the skill of judging gaps based on the car's speed and distance -- I isolated my attention solely on the car, which is critical to gain the ability to judge gaps in traffic."
Doesn't seem to be working?
If the students' judgment does not begin to improve after about 10 minutes, it may be because they are unconsciously using a landmark, as Adriana was - try doing the training somewhere that doesn't have the natural landmarks.
If the students' judgment still does not improve, perhaps they are unable to perceive the speed and distance of vehicles accurately, at least in that situation, either because of visual limitations or the student is unable to cognitively process it. When this happens, the student could consider an alternative for determining when there is a crossable gap in approaching traffic.