RE:view, volume 21, Number 3, pages 153-161, Fall 1989.
Reprinted with permission of the Helen Dwight Reid Educational Foundation.
Published by Heldref Publications, 1319 Eighteenth St., NW, Washington, DC 20036-1802.
Copyright © 1989.
RE:view - Fall 1989
Report from the Field
To Cross or Not to Cross: Objective Timing Methods of Assessing Street Crossings Without Traffic Controls
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.
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:
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.
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
Visually impaired people who can see far may be able to cross with traffic approaching from a distance if they can accurately judge the speed and distance of the approaching vehicles, and if they can consistently judge how far away the vehicles must be to allow them enough time to cross safely. They also must know under what conditions it is possible for them to make dependable judgments (e.g., in daylight, in the rain, at night).
Precautions When Using the Timing Procedures
The Timing Method for Unlimited Detection (TMUD) can analyze how well a person makes this complex judgment. Table 2 describes how to use this method of assessment with a hypothetical Mr. B. An illustration of the TMUD being used to assess a person's ability to judge approachng vehicles follows the table.
Table 2. Timing Method for Unlimited Detection
Step 1. The first step is to find out how long it takes Mr. B to cross the street. If he is
to be judging vehicles coming from the left, he must determine how much time he re-
quires to cross half the street. If the entire street is to be crossed at once because there
is no median strip, he must find out how much time is required to cross the entire street
when judging vehicles coming from the right. To help Mr. B to obtain this informa-
tion, he and the instructor should follow the procedure outlined in Step 1 of Table 1.
Peter Johnson who has good functional vision wants to assess his judgment about crossing three lanes to the median strip of a busy street during a sunny day. At that intersection there is no traffic light; the street is straight, and visibility is good for about three blocks in each direction.
Step 2. While Mr. B stands on the curb or median strip from which he intends to
cross, he watches for a break in the traffic approaching from one direction or the
other. He starts a stopwatch when he thinks that the nearest vehicle is just barely far
enough away to enable him to finish crossing that part of the street before the vehicle
would reach him.
He stops the watch the instant the vehicle arrives in front of where he would be
Step 3. The instructor records the time that elapsed and compares the length of time
it took for the vehicle to arrive with the amount of time Mr. B needs to cross. Times
of vehicles from the left are compared with the time he required to cross half the
street; times of vehicles from the right are compared with the time he required to
cross the entire street. If the vehicle arrived in less time than he took to cross, then
the crossing would not be a safe one. Conversely, if it took much longer for the vehi-
cle to arrive than Mr. B required to cross, he may be overly cautious.
Step 4. Mr. B should continue to practice judging the vehicles to see if there is con-
sistency and/or improvement in his ability to judge when it is safe to cross.
After a person has learned to do the above procedure, he or she can then use it independently to assess future situations.
Peter determines that his "required crossing time" for the three lanes (from curb to median strip) is 9 seconds (Step 1).
While standing on the curb, he watches the vehicles approaching on his left and waits until he sees that there is a break in the traffic. He starts the stopwatch when he thinks that the nearest car is still far enough away to allow a safe crossing. To his surprise, the car passes in front of him only five seconds later (Step 2).
If Peter had started his crossing when he started the stopwatch, the car would have reached him when he was still in the street. He realizes he needs to start crossing when cars at that speed are further away (Step 3).
At the next break in traffic, he starts the stopwatch when he thinks that the nearest car is still far enough away to allow him to start a safe crossing. This time he allows more of a safety margin, and the car reaches him in 10 seconds. He continues to practice judging the speed and distance of the vehicles until he consistently can judge when the cars are over 9 seconds away from him, regardless of their speed. He then feels assured that he consistently is able to judge when it is safe to cross there on sunny days (Step 4).
Improving Ability to Judge Approaching Vehicles
Using a landmark. Some people, even though they can see the vehicles far away, cannot judge their speed or distance, perhaps because of lack of depth perception or poor acuity. These people can use the TMUD to choose a landmark (a driveway or pole) at a distance so that if they start the crossing before the nearest vehicle reaches that landmark, even the fastest vehicle will not reach them before they have finished crossing that part of the street.
Adjusting the crossing speed. People who initially think it is safe to cross when the vehicles are too close may be judging the vehicles based on a habit of running across these kinds of streets. If this is the case, the orientation and mobility instructor should discuss with them whether in their particular case it is safe to run. If so, the instructor should time the crossing accordingly. If running is not safe, using TMUD and discussing the results should help these people adjust to judging the vehicles based on walking across the street.
TMLD and TMUD need to be done thoroughly and correctly to avoid dangerous and wrong conclusions. An orientation and mobility instructor or a blind person using the procedures independently should keep the following points in mind:
Applications of Timing Methods of Assessment
1. When using the TMLD, one should time enough vehicles to have recorded one that can reach the person in the shortest time from its initial detection. This will not always be the fastest car; sometimes a slow car can get very close before it is detected.
2. Information provided by both timing methods is only true for the conditions under which the assessment was done. It may not be true under different conditions. If the assessment was made in bright sunlight during rush hour on a calm day, it will need to be done again under various conditions such as cloudy, rainy, or windy weather or at night or during midday traffic. By making those assessments under various conditions, individuals can learn which conditions affect their safety and abilities and which have no adverse effects.
3. Be as inconspicuous as possible when assessing crossings, simulating the appearance the traveler will create when actually crossing. The presence of people on the curb, perhaps pointing to cars and timing them, can slow down traffic, providing invalid information about the safety of the crossing.
4. When crossing a street with more than one approaching lane, do not assess the approaching vehicles lane by lane because the drivers may change lanes. One fatal accident occurred after two visually impaired pedestrians had crossed the second of three approaching lanes. The car in that second lane passed behind them, but another driver behind that car, unable to see the pedestrians, pulled into the third lane and struck them.
5. To learn whether an individual is capable of crossing a certain street, that individual must be assessed at that street. Assessments should not be based on the ability of another to cross there. One mobility instructor assessed the ability of four visually impaired individuals to detect vehicles at one intersection. Three people were unable to rely on hearing the vehicles coming from the right; but one man, who was able to hear the cars much sooner than the instructor or the others, was able to cross there safely.
The client quoted at the beginning of this article was a deaf woman with about 5 degrees of good central vision. When crossing at an intersection where her vision was limited by a hill or curve in the road, she initially could not judge where it would be safe for her to cross. After a few learning trials with the TMLD, she could judge accurately when her view of the approaching vehicles was too limited and at which intersections she had a sufficient line of sight to cross safely. In situations where the road was straight, she initially thought it was safe to start crossing when the approaching vehicles were too close. After about 20 minutes of practice in judging cars by the TMUD, she improved so much that regardless of the speed or distance of approaching vehicles, she could tell when the traffic was far enough away to permit a safe crossing.
Another woman with a large visual field did not improve her judgment of the speed and distance of oncoming traffic, even after practice and discussion of the TMUD. She then chose as a landmark a driveway from beyond which, according to the TMUD, even the fastest vehicle could not reach her before she finished the crossing. She would start her crossings only when the nearest vehicle was farther away than that driveway; she then could cross the street safely in spite of her inability to judge the vehicles. Both women can now use the timing methods independently to assess other situations and to assess their own skills as their vision deteriorates.
These examples show the use of the timing methods for assessing or developing general skills; the methods have been used effectively for this purpose with a variety of clients. The timing methods can also be used to help answer specific questions, such as whether an individual can safely cross a street that appears dangerous or where on a twisting, hilly street the vehicles can be most effectively detected. For students (and heedless adults!) who do not understand why they must be cautious, these methods can also illustrate graphically the rationale for safe street-crossing techniques. As an example of using these techniques to answer a specific question, one blind client felt he could cross the busy street near his home on days on which he "felt good." He found out by using the TMLD that it was not safe for him to cross there regardless of how he felt.
Some people, especially very young students may not be able to understand how to use the timing methods independently. However, if they are capable of crossing streets and can indicate when they first notice that a vehicle is approaching, the TMLD can still be used by the instructor or a family member to determine if these individuals can detect vehicles well enough to cross a particular street safely.
Decisions about whether to cross a certain street, are subjective, no matter how much objective information about that crossing is available. Persons who learn that they can detect vehicles just well enough to barely reach the other side will decide it is not worth the risk to cross. Others given the same information will value their independence more than their safety. It is the role of the mobility instructor to provide the visually impaired person with as much objective information as possible, so that sound, informed decisions can be made.
Orientation and mobility programs for visually impaired persons should prepare them to make decisions about crossing streets where there is no traffic control. These timing methods, along with the other training and evaluation methods used by mobility instructors, can enhance the visually impaired traveler's awareness of abilities and limitations, so that he or she can make decisions about crossing these kinds of streets with more confidence and accuracy.
There is much to learn, however, about using these timing methods effectively. As visually impaired people and mobility instructors use these methods, it is hoped that the base of knowledge about their use will continue to expand.
The author acknowledges the invaluable support of orientation and mobility specialist Linda Sussman. Many of her suggestions, based on her use of these techniques with her clients, are found throughout this paper.
Dona Sauerburger is an orientation and mobility specialist at the Family Service Foundation's Institute on Deaf-Blindness, Lanham, Maryland.
Return to Publications regarding crossing where there is no traffic signal or stop sign
Return to home page