What is "Quiet"? -- Effect of slight increase of ambient sound

I noticed something strange while teaching people to recognize if they are in a Situation of Uncertainty.

In some places, a slight increase in background ("ambient") noise such as from distant lawnmowers and airplanes seemed to have little or no effect on how well we could hear the vehicles approaching. But at other places, the slightest noise drastically cut back on our ability to hear the vehicles, so that we were unable to hear them until they got much closer. A good example of this is the crossing where Sue Etters was hit.

As a result, I began to teach people that when they are in situations where they can hear traffic with enough warning when it is quiet, they should notice whether they can still hear it as well when there is a little background noise. In other words, evaluate how much ambient sound they can tolerate (how "quiet" it has to be) and still hear well enough to be in a Situation of Confidence.

The research that Rob Wall Emerson and I did seemed to validate my observation:

The chart below shows that for every approach except the hill, increasing the ambient noise as much as 5 db(A) didn't have much effect -- people could still hear the cars just as well. That is, as the ambient noise went from 36 to 40 or even 42 db(A), detection time fluctuated up and down and back up again before it started to go down permanently.
It was a different story at the hill approach, however. When the ambient sound level was 38 db(A), the vehicles approaching from the hill were heard an average of 13 seconds away, but when the ambient sound was increased only 2 db(A), rather than fluctuate up and down, the detection time was reduced to 9-10 seconds, and then it continued to plummet as the ambient sound became louder than 42 db(A).

straight

Photo shows the same street looking to the right. It is lined with trees on both sides and houses on the left. The road is going slightly uphill and disappears about 200 feet away.

severe bend
We are standing at a street and looking to our right. The street goes slightly uphill and crests about 50 feet away and disappears down the other side of the hill.

hill

We are looking to the left along a straight two-lane street with a curb and grass on both sides, houses on the right side and woods on the left. A woman is sitting in a lawnchair on the grass about 4 feet from the left curb, facing the street. A few feet beside her is a grey board suspended on two poles. The board is 8 feet wide, the top is 5 feet above the ground, the bottom is about 3 feet above the ground, so that the board is beside the woman's head and shoulders.

baffle

We are looking to the left along a two-lane street with curbs, grass and trees on both sides. The road is going slightly downhill and curves gently to the left, disappearing about 100 feet away. About 20 feet from us is the corner of another street. A tripod about 2 feet high and some equipment lie on the grass.

minor bend
We are looking to the left along a two-lane street with curb, grass and sidewalk on the left and thick trees on the other side of the sidewalk. On the right side are thick trees starting at the edge of the street. The road is going slightly downhill and curves gently to the left, disappearing behind the trees about 100 feet away.

trees

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