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What is a gap in traffic?

Long gap in traffic

Short gaps in traffic

A gap in traffic is the time between vehicles, OR the time between YOU and an approaching vehicle.

Gaps in traffic can be very short, such as the gaps between vehicles in the first photo, where there may be only a second or two between the time that one vehicle passes a point and the next vehicle arrives. And they can be very long, such as the gap in traffic in the second picture, where we see no vehicles for a distance that is the length of several blocks. Heck, gaps in traffic can last for hours or even days, for example on isolated roads or at night.
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"Crossable gaps" in traffic" are gaps that are at least long enough to allow time to complete a crossing before a vehicle arrives. For example, if your crossing time is 9 seconds and the nearest approaching vehicle is at least 9 seconds away, you have a crossable gap in traffic. Traffic engineers, researchers, and O&M specialists may vary in whether or not their definition of crossing time includes a safety margin and if so, how long it should be.

"Acceptable gaps" in traffic are gaps that a pedestrian or driver thinks is a good opportunity to start crossing. Some people accept a gap and start to cross when there is not time to finish their crossing before a vehicle will arrive, perhaps because they misjudged it and thought they'd have more time than they did, or maybe they are more willing to accept risks.

We know that drivers vary in their gap acceptance (click here if you want some fun facts about how the gap acceptance of drivers varies). Meanwhile, research suggests that pedestrians who rely on hearing not only miss more crossable gaps, they also accept gaps that are more risky than sighted pedestrians do. For example in one study (Ashmead et al, 2005), blind participants waited three times longer to cross than sighted participants, and about 6% of their crossing attempts were considered dangerous enough to require intervention (none of the sighted participants required intervention).

This may help us understand why it is so important that our students learn to determine when they are able to detect crossable gaps reliably, and when they are not.

Ashmead, D.H, Guth, D., Wall, R.S., Long, R.G., Ponchillia, P.E. (2005) Street Crossing by Sighted and Blind Pedestrians at a Modern Roundabout. Journal of Transportation Engineering Volume 131, Issue 11