Mythbusters! Factors to identify Situations of Uncertainty

Myth #1: If there is a sharp bend near the crossing, it's a Situation of Uncertainty

Fact: That may often be true for pedestrians using their vision because they can't see beyond the bend, but experienced O&Ms and blind travelers know that there are many crossings where you can hear vehicles well beyond the bend, and that there's no way to predict where that is true.

• In the examples here, it was not possible to hear vehicles beyond the bend in Silver Spring but in Crofton, the vehicles could be heard long before they were seen.
  And conversely, in Freeland there was no bend in the road and yet it was not possible to hear the vehicles until they were about 5 seconds away.

Myth #2: Criteria that can help determine if you can be confident that you can have time to "cross when quiet" include narrow street (no more than two moving lanes), low traffic speed (25 mph) and volume (no more than 10 vehicles/minute).

Fact: Volume of traffic can help determine how many crossable gaps in traffic there are as well as how often it gets quiet, but it has nothing to do with how well you can hear the approaching vehicles when it is quiet. And our research indicated that the speed of an approaching vehicle has very little to do with how well you can hear it (that is, how much warning time you have that it's coming).

• In the examples here, the crossing with the fastest vehicles and the only one that didn't have low traffic volume, Crofton, is the only one where you get enough warning that you can be confident that you have time to cross whenever it's quiet.

Myth #3: If the vehicles can be seen from a distance (that is, if there is a good line of sight), they can be heard from a distance as well.

As I explained, I used to think this was true, and learned an important lesson when I selected a site for training where the vehicles could be seen for several blocks, but not heard until they got close.
Fact: The only way to determine whether approaching vehicles can be heard with enough warning is to . . well, what do you think? YES -- by listening!

• In the examples here, at Freeland you can see the vehicles very far away, but can't hear them till they get close. And at Crofton, you can't see them till they get close but you can hear them quite far away.

And remember, the situation CAN CHANGE!

• Sometimes conditions can change, so that at one time, whenever it's quiet you can hear the vehicles with enough warning, but that's not true at another time. which makes these mythical criteria even less reliable. For example:
• When the roads are wet, vehicles can be heard better than when they are dry.
• Installing a speed hump at a crossing where the vehicles couldn't be heard with enough warning made it possible to be confident that there was time to cross whenever it was quiet.
  
• Later in this Section you'll learn that at the crossing where Gordon Parks was killed, there was a Situation of Uncertainty during the day but not in the evening.
• Parked vehicles can create a Situation of Uncertainty when they block the sound of traffic. And yet the sound isn't always blocked -- in our research, when we placed a barrier between blind participants and the traffic (as shown in the picture to the right), it didn't seem to impair their ability to hear the vehicles much (Wall Emerson & Sauerburger, 2008). So the presence of something that might block the sounds is not a reliable indicator as to whether there is a Situation of Uncertainty either.