RE:view - Fall 1989
Report from the Field
To Cross or Not to Cross: Objective Timing Methods of Assessing Street Crossings Without Traffic Controls
DONA SAUERBURGER
My client, who was nearing the completion of her orientation and mobility program, asked me, "Can I cross here safely or should I walk a couple of blocks to the nearest traffic light?" I realized then that neither her program nor other standard orientation and mobility programs gave sufficient instruction on how to know when it is safe to cross a street without traffic controls.
To answer such a question, mobility instructors need to consider two distinct issues: (a) the possibility that a person's ability to detect vehicles is too limited to allow the safe crossing of a given street and (b) the need for the person who is able to detect vehicles from a distance to judge when the oncoming traffic is far enough away or approaching slowly enough to allow a safe crossing.
This paper presents a method of assessing a crossing in each of the two situations above. The Timing Method for Limited Detection (TMLD) addresses the first issue, and the Timing Method for Unlimited Detection (TMUD) addresses the second. The discussions pertain to streets with two-way traffic. Adjustments can be made for one-way streets.
Timing Method for Limited Detection:
To Determine if the Person's Ability to Detect Vehicles Is too Limited to Allow Crossing a Given Street Safely
To cross a street safely a person, hypothetically a Ms. A., must be able to finish the crossing before she can be reached by a vehicle that she had not been able to detect when she started the crossing. Anyone's ability to detect approaching vehicles may be limited by factors such as a hill or bend in the street that obstructs the view, impaired hearing and/or vision that limits ability to detect vehicles or noises that may mask the sound of approaching vehicles.
To determine whether Ms. A's ability to detect vehicles is too limited to allow her to cross a street, one must compare the time she requires to cross the street with the time in which an undetected vehicle might reach her. Table 1, the Timing Method for Limited Detection, analyzes this situation by measuring the shortest time in which a vehicle can reach Ms. A once she has just barely detected it. An illustration of the use of TMLD follows the table.
Table 1. Timing Method for Limited Detection
The first step is to find out how long it takes Ms. A to cross the street. At a
break in the traffic the instructor tells Ms. A to start crossing the street. As she steps
into the street the instructor should start the stopwatch and should stop it the instant
that Ms. A has stepped out of danger, either onto the curb or out of the moving lanes
of traffic. The instructor should record the time it takes Ms. A to cross. The instruc-
tor and Ms. A should repeat this step more than once. The "required crossing time"
should be the longest time recorded.
Step 2. The instructor should ask Ms. A to stand on the curb where she intends to
cross. At the first instant she detects what might be an approaching vehicle, she
should start the stopwatch but remain standing on the curb. If she has misinterpreted
a sound or movement as an approaching vehicle, or if a noise masks the sound of the
approaching vehicles, have her wait for the next quiet time and start to listen or watch
again. If she would normally cross when such a noise was occuring, she should time
her detection of the approaching vehicles to determine if she can detect cars ade-
quately under those conditions.
She should stop the watch the instant the vehicle arrives in front of her at the curb.
The instructor should record the time that elapsed from when the vehicle was
detected. If the vehicle is passed by a faster car, she should stop the watch when the
faster car arrives and the instructor should record that time.
Step 3. Ms. A should repeat Step 2 several times until the instructor feels certain of
having timed at least one of the vehicles that arrives in the shortest time.
Step 4. Compare the shortest time recorded in Step 3 with the time Ms. A required to
cross in Step 1. She can cross safely only if the time in Step 1 is less than the shortest
time in Step 4.
After a person has learned to do the above procedure, he or she can then use it independently to assess future situations.
Mary Smith who is totally blind needs to cross a busy two-lane residential street to go home from her bus stop.
To her left, the road is straight for three blocks; about three fourths of a block to her right the road turns.
Using the TMLD process outlined in Table 1 the situation is assessed as follows:
When the instructor says it is safe, Mary crosses the street several times. The instructor records that the crossings take 10.1, 9.8, and 10.5 seconds. Thus, the time required to cross is 10.5 seconds (Step 1).
Mary holds the stopwatch, stands at the curb, and waits until all sounds have died down. When she thinks she hears a car coming, she starts the stopwatch. When that car (coming from the left) reaches her, she stops the watch. The watch reads 11.3 seconds (Step 2).
Mary continues to time approaching vehicles from both directions, each time waiting until the sound of the receding cars disappears before timing the detection of the next one. The instructor records that the times of the cars from the left are 11.8, 14.7, and 10.8 seconds. Cars from the right are 5.2, 12.1, and 7.6 seconds (Step 3).
Mary needs 10.5 seconds to cross, but several cars coming from the right reach her much sooner than that. She concludes that it is not safe to cross there under those conditions (Step 4). However, she could cross safely from the opposite direction, with the curve on her left, because even the fastest car did not reach her before she could finish crossing half of the street. She considers going farther from the curve to find an intersection where it is safe to cross. If that location also proves to be unsafe, she plans to ride the bus another 15 minutes until it lets her off on the other side of the street.
If a Street Is "Not Crossable": Some Alternatives
If, after using the TMLD, a person decides it is not safe to cross a street at the point selected and considers it impractical to cross at a safer intersection, there may be other alternatives:
Shortening the crossing time. If it is not too risky, the pedestrian may determine that there will be enough time to reach the other side safely if he or she crosses at a narrower part of the street or walks faster or runs across the street.
Starting and returning to the curb. When the time needed to cross is slightly more than the time in which an undetected vehicle might reach the pedestrian, he or she might be able to cross safely, if there is the option of returning to the curb after the crossing is started.
For this possibility, assume a pedestrian can cross a particular street in 8 seconds. Some vehicles, once detected, can reach the pedestrian in only 6 seconds. If he or she starts to cross and detects no approachng vehicle during the first 2 seconds of crossing, he or she can safely complete the trip because in 6 more seconds (the shortest time in which an undetected vehicle might arrive), he or she will have reached the other side. However, if during the first 2 seconds after entering the
roadway, the pedestrian hears a vehicle approaching (especially if it is coming from the right), he or she returns to the curb and waits for the next opportunity to cross because that approaching vehicle might reach the crossing path before the person can reach the other side.
Correcting the timing. The arrival times of approaching vehicles (Steps 2 and 3 of Table 1) may be erroneously short because the pedestrian waited to start the stopwatch until he or she was absolutely certain of what had been heard or seen. The watch should be started the instant anything that might be a vehicle is detected.
Improving the ability to detect vehicles. The ability of the visually impaired person to detect vehicles may improve. One instructor taught a person with macular degeneration who learned to use eccentric viewing to detect vehicles better. In another case, a person with impaired central vision discovered that she could detect vehicles from farther away if she held her gaze for a few seconds, rather than glanced.
Another person who relied on hearing to cross discovered by using the TMLD that many sounds she had not noticed, for example, airplanes, trees rustling in the breeze, and receding cars, effectively masked the sound of approaching vehicles. She learned the importance of waiting until all was quiet because the TMLD dramatically showed that only then could she consistently hear the vehicles from far enough away to cross safely.
Changing locations. If the crossing is determined to be unsafe because of an obstruction to the sound or visibility of the vehicles, such as a hill or curve in the road, the person may be able to cross safely farther from the obstruction. In some situations the person may be able to cross the street from the other side, placing the obstruction to the left. This is possible only when (a) the obstruction is far enough away so that the person can still detect vehicles from the left with enough time to cross half the street safely, and (b) when vehicles from the right can be detected from far enough away to cross the entire street safely. Under these conditions, by the time undetected vehicles from the left reach the pedestrian, he or she would be on the other half of the street (and the vehicles would have passed to the rear).
Riding to the end of the bus line. If the pedestrian needs to cross a difficult-to-cross street after having alighted from a bus, he or she can ride to the end of the line and continue until the bus comes back on the opposite side of the street. The commute will be longer, but the necessity of crossing at a dangerous intersection is eliminated.
Soliciting aid. The pedestrian may be able to solicit aid from passers-by at the intersection.
Timing Method for Unlimited Detection:
To Assess a Person's Ability to Judge the Speed and Distance of Approaching Vehicles