Section 4: Teaching to determine crossable gaps --
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to determine there is a crossable gap in approaching traffic
In the last page, you learned that Determining Gaps in Approaching Traffic requires that you define a length of time, which we will call "X" seconds.
This page has more information about how to determine how many seconds is "X" seconds.
How to determine how many seconds is "X" seconds?
"X seconds" is typically:
The crossing time, the preferred safety margin, and the error margin should all be considered when determining how many seconds "X" will be. For example, if you need 6 seconds to cross and want a safety margin / clearance time of 3 seconds, you want to be sure the traffic is at least 9 seconds away when you start to cross. Thus your "X" should be 10 seconds, so that if you misjudge the arrival time by one second less than it actually was, you'd still have the full 9-second gap that you want.
the time needed to cross, plus
- a preferred safety margin ("clearance time"), plus
- an extra second to allow for error.
- The time needed to cross is the time it takes to get across the street or lanes and be safely out of the path of traffic.
For two-way streets in countries where vehicles travel on the right side of the street, the first lanes that you cross have traffic from your left.
Therefore your crossing time for traffic from your left would be the time needed to cross the first part of the street (all the lanes with traffic from your left),
and crossing time for traffic from your right would mean the time needed to cross the entire street.
- The preferred safety margin / clearance time) is the preferred minimum time from when you complete the crossing until the first vehicle passes your crosswalk.
Most people think it's cutting it too close if they finish their crossing just as the traffic reaches them - yikes!
So they prefer additional time for a safety margin, which is why we add this to their crossing time.
How many seconds are needed for the safety margin may vary
-- people usually want more clearance time for fast-moving traffic than they do for slow traffic, and cautious people may want a longer clearance time than high risk-takers do.
One way to find out how much safety margin a person needs is to use a stopwatch and say something like,
"We need to figure out how much time you need for your safety margin for this crossing.
To help you decide, I'll demonstrate a two-second safety margin.
I want you to imagine that you're crossing the street and you know a vehicle is coming but you're feeling comfortable with the clearance that you have.
So imagine that you finish the crossing and step onto the curb NOW" . . . [wait two seconds and then say] "NOW is when that vehicle would reach you.
Does that safety margin seem to be enough time for you?"
If that doesn't seem long enough, demonstrate a 3-second safety margin and ask if that is enough and if not, demonstrate a 4-second safety margin and ask again.
Be prepared to change the length of the safety margin during the training because sometimes people change their minds as they practice with real traffic, deciding that they need either more or less time for their safety margin.
- The one-second error margin or "fudge factor" is needed because most people can learn to make this judgment accurately within one second or less.
That is, they can determine when the vehicle is X seconds away, give or take a second.
I am concerned about the possibility that when they apply this skill in real situations, they will misjudge it by a second less than they need, so I like to add a second to take this margin of error into consideration.