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The Four-way Stop
by Lionel E. Deimel

Our world is less well regulated than we would like to believe. That is the thought that came to me after an encounter at a four-way stop the other day. I thought I knew how to behave at such an intersection, but neither I nor the other drivers seemed to have all the answers.

A four-way stop, of course, is an intersection of two, usually minor, two-way streets, controlled by four stop signs. Proper driver behavior, as I understood it, is described in the Pennsylvania Driver’s Manual:

A FOUR-WAY STOP sign means that there are four stop signs at this intersection. Traffic from all four directions must stop. The first vehicle to reach the intersection should move forward first. If two vehicles reach the intersection at the same time, the driver on the left yields to the driver on the right.*

Any logical thinker should immediately recognize that these instructions cannot possibly be adequate to cover all circumstances, but I had not realized that, not having thought much about four-way stops before the aforementioned incident.

A Thought-provoking Incident

On a school day morning, I approached a four-way stop adjacent to an elementary school. When I arrived at the corner, a crossing guard was escorting a student across the street in front of a waiting vehicle that faced me with its left-turn signal blinking. I stopped just after the driver to my left did the same. To my right was a school bus discharging passengers, its stop lights flashing and its own stop signSchool bus extended from its side. The crossing guard finished her task, and, with her back to the intersection, left the drivers to their own devices.

I was signaling for a left turn, and a quick analysis convinced me that, although I was last to arrive at the intersection, I was, in fact, the only driver who could proceed. The apparent first arrival, the school bus, did not seem to be going anywhere. The vehicle in front of me could not complete a left turn, as the school bus would have required it to stop in the middle of the intersection, which did not seem a proper thing to do. The vehicle to my left could not move through the intersection for the same reason. This seemed to leave me free to execute my left turn.

I moved forward and was nearly hit broadside by the vehicle to my left, whose driver clearly believed I did not have the right-of-way. A quick glance about revealed that the school bus had retracted its stop sign and turned off its blinking lights. The now irate driver likely concluded that he could move before I could because he had arrived at the intersection before me, and he then proceeded to do so as soon as the school bus allowed vehicles to pass. I have no idea what he thought about the vehicle to his left.

Nearly having an accident is upsetting, and I became angry at the driver who had narrowly avoided hitting my car, as well as at the seemingly indifferent crossing guard. As I drove away, however, I came to realize that I was the victim of unfortunate timing. The crossing guard could surely have moved the traffic more safely, but she, no doubt, did not understand directing traffic to be part of her job. Perhaps she should have.

But probably not. A police officer could have done everything I thought was needed at that intersection, but police are paid more than crossing guards because they have more training, incur more risk, and exercise more responsibility. Crossing guards probably need to concentrate on seeing our children safely across the street; damaged automobiles are more easily fixed than damaged children.

The problem, I realized, is that the four-way stop is more complex than the Pennsylvania manual would have us believe. To begin with, timing seems to be critical. I analyzed the situation and decided to move. The bus driver probably retracted his stop sign and turned off his blinking lights as I began to move, and the other driver analyzed the new situation and decided that he should go. That driver was assuredly concentrating on his own analysis, rather than trying to figure out mine. Each of us was surprised by what the other did.

Such problems seem unavoidable, even in much simpler circumstances. Suppose two vehicles arrive at a four-way stop on different streets at approximately the same time. If the arrivals are simultaneous, the vehicle to the right has the right-of-way. If the driver to the left believes he arrived an instant earlier, however—a belief that may or may not be shared by the other driver—he will think that he has the right-of-way, and this could easily contribute to an accident.

In my admittedly peculiar encounter, there was also a problem with the statement that the first vehicle to arrive at the intersection should move first. Does this imply that I should not have moved until the vehicle in front of me did so, even if I could move safely and he could not? (I construed the school bus as essentially parked.) The Driver's Manual appears to be too simpleminded to deal with this case. It should probably say that the first vehicle to arrive should move first (or next) if not otherwise prevented from doing so. (Movement could be prevented by a crossing pedestrian, for example, but pedestrians are not even mentioned in the description of the four-way stop.) Also, it should probably say that a vehicle should not enter the intersection if its movement will block that intersection, thereby preventing movement by other vehicles.

In fact, however, the Driver's Manual inadequately specifies what motorists should do even in less pathological cases. What if four vehicles arrive at the same time? Does each driver yield to the vehicle to his right forever (or until someone backs up, thereby ceding the right-of-way to the vehicle to the left)? What if vehicles traveling in opposite directions arrive simultaneously at a four-way stop indicating intentions to turn in opposite directions? It is not clear which vehicle should move first, and no obvious rule-of-thumb comes to mind that could guide motorists safely and reliably in such situations.

What, in fact, do drivers do in cases of right-of-way ambiguity? They tend to wait an arbitrary few seconds, then proceed slowly, prepared to stop if other vehicles move at the same time. This procedure usually works fine, though it may take a few tries before all vehicle clear the intersection. (Readers familiar with networking protocols will recognize a striking analogy here.) Barring changes to the rules governing four-way stops, such behavior is prudent. Unfortunately, some aggressive drivers seize the right-of-way and try to intimidate others into letting them keep it. Public policy would do well to structure rules so as to minimize the temptation to employ such antisocial strategies. One friend of mine says that she slows down if she sees that she will arrive at a four-way stop at the same time as someone else. She thereby avoids the ambiguity and attendant anxiety, as long as the other drivers are not pursuing a similarly deferential strategy!

A Better Approach?

The four-way stop is clearly useful for controlling intersections of streets with traffic that is significant, yet insufficient to justify the use of signals. Could altered regulations make such intersections safer and more efficient? Perhaps. Timing problems, almost certainly, cannot be eliminated, as they necessarily involve subjective judgments of motorists, but the problem of tie-breaking at a four-way stop surely is solvable when the drivers agree on who arrived when.

If two or more vehicles arrive at an intersection at once and the yield-to-the-vehicle-on-the-right rule (or some other factor) does not decide right-of-way, we simply need to impose a scheme that assigns movement priorities that is straightforward and clear to all. I suggest assigning priorities to each vehicle position in a clockwise fashion, since vehicles normally yield to the vehicle to the right in case of ties.

Our task, then, becomes one of picking a favored vehicle position and communicating that to motorists, possibly with a reminder that movement authority proceeds around the intersection in a clockwise direction. The “favored” position is only favored when the conventional rules are ambiguous. If possible, it should be the position that sees the most traffic; otherwise,  the position can be assigned arbitrarily. Obviously, embellishment of the rules for dealing with four-way stops must be accompanied by changes in signage.

To achieve rapid acceptance of any plan that changes the way four-way stops work, we would do well to change signage as little as possible, modifying existing stop signs or adding auxiliary signs to be used with them. Since signs at four-way stops often (and preferably) display a rectangular “4-WAY” placard below the stop sign proper, it is reasonable to try to modify the stop sign itself.

To simplify discussion, I will adopt the military convention of describing locations using an imaginary clock face, in this case, one centered on the center of the intersection. By this convention, we describe the vehicle of primary interest as being at 6 o’clock, and a vehicle on the opposite side of the intersection as being at 12 o’clock. Vehicles to the left or right, respectively, are said to be at 9 o’clock or at 3 o’clock.

Consider the simultaneous arrival at the intersection of four vehicles. If signage indicates that the vehicle at a particular position (say, at 3 o’clock) should move first, the usual impasse is immediately resolved, and the remaining vehicles can proceed according to the yield-to-the-vehicle-on-the-right rule.

My proposal, then, is to use four variants of the conventional traffic control device deployed as follows:

Augmented stop sign, variation 1

Augmented stop sign, variation 4Augmented stop sign, variation 2

Augmented stop sign, variation 3

Here, the top sign is seen by the vehicle at 12 o’clock, the bottom sign by the vehicle at 6 o’clock, etc.

The circular arrows on the signs indicate how movement authority proceeds in a clockwise direction around the intersection in cases where such guidance is needed. For example, the sign seen by the vehicle at 6 o’clock is intended to indicate that a vehicle at 3 o’clock has the right-of-way first, though the one at 6 o’clock may proceed before any vehicle at 9 o’clock. A vehicle at 12 o’clock gets the right-of-way last. In other words, the back end of the arrow shows the position of the vehicle that may move first, and the arrowhead points to the position of the vehicle that must wait for all other vehicles before moving. The arrows are to be ignored if the usual rules are adequate. Thus, if vehicles arrive simultaneously at 12 o’clock and 3 o’clock, the vehicle at 12 o’clock should proceed through the intersection first, irrespective of the fact that the arrow assigns priority to the vehicle at 3 o’clock in other situations. (It would not be unreasonable to make the arrows applicable in all situations, but doing so might slow acceptance and compliance by motorists.)

Analysis

I now turn to additional details and to questions as to whether my proposed solution is practical.

First Try

The signs shown above do not represent my first try at designing an augmented stop sign. The first sign I devised was the following:

First try at new stop sign

The idea here is that the numerals indicate the order in which vehicles should proceed in cases of ambiguity. The numeral at the bottom applies to the vehicle approaching the sign, the numeral to the right applies to the vehicle to the right, etc. (As in my final version, each of the four signs at the intersection is different.)

Suppose vehicle A approaches this sign at the same time as vehicle B reaches the opposite side of the intersection. Suppose further, that one vehicle is indicating a left turn, and the other vehicle is indicating a right turn. Vehicle A should proceed first, as its order numeral is lower than that representing the other vehicle.

Reader David Spector pointed out a problem with this sign, which led me to recognize a second defect. David’s concern was that the numerals would be hard to read, particularly for those having less-than-perfect eyesight. Because one has to take in the numerals and process their significance, the sign cannot be interpreted as quickly as one would like. David also made me realize that my sign should have looked like this:

First try at new stop sign (improved version)

Here, the numerals are in clockwise, rather than counterclockwise, order, which results in the vehicle on the right—as one expects from the “standard” rules—having the right-of-way most of the time. My having originally ordered the numerals in a counterclockwise fashion was unnecessarily confusing and was really just a silly error on my part.

David offered specific suggestions for signs using numerals, but I realized that numerals are intrinsically problematic because of the cognitive processing that interpreting them requires. This insight led me to the signs discussed in my revised essay.

My first attempt at designing an augmented stop sign for four-way stops resulted in signs that were harder to interpret and, for that reason, potentially inefficient, if not actually dangerous. (See sidebar at right.) I chose to use a circular arrow, rather than numerals, because it is easy to “read” and suggestive of movement, in this case, not the movement of vehicles, but the abstract movement of right-of-way from one vehicle position to the next. This graphical element of the sign could have been made more literal by adding, for example, a cross representing the intersecting streets or large dots representing possible positions of vehicles. Such additions would clutter the sign, however, and make it more difficult to interpret from a distance. In highway and street signage, less is usually more.

The need for improved signage is most obvious in the case where four vehicles arrive at the intersection at once, or as close to simultaneously as to leave drivers uncertain as to who has the right-of-way according to the conventional rules. Using my proposed signs, the driver of the vehicle facing the bottommost sign interprets the directed arrow as indicating that the vehicle to the right has the right-of-way. Its driver can proceed first. The driver to the left sees that the vehicle opposite first receives the right-of-way, followed by the vehicle to the immediate right. The driver of the fourth vehicle sees the topmost sign, which indicates that he or she cannot move until the other three vehicles do so.

The augmented signs are also useful when two vehicles arrive at the intersection travelling in opposite directions. If neither vehicle is making a left turn, the vehicles may proceed simultaneously. If one vehicle is going straight and one is turning left, however, the augmented sign should determine which vehicle may move first. Without the new signs, one driver might try to turn in front of the other, which is a dangerous movement. It is safer for the vehicle going straight to clear the intersection first. It is also safe for the turning vehicle to turn behind the other vehicle after the vehicles’ simultaneous movement into the intersection, though this strategy can occasion anxiety on the part of the driver who is not turning, as he may be uncertain of the other driver’s intentions.

If both vehicles are turning left, my experience suggests that drivers usually move together and pass one another on the right. If the street onto which they are turning is wide, they may pass one another and turn behind each other. In neither case do the new signs improve on current practice.

Four-way stops where divided streets meet or where there are multiple lanes in each direction can be very confusing to motorists. These are not common; such intersections are usually signaled or are signed as two-way stops. A signaled intersection with non-functioning signals—due to a power failure, for instance—can present a real challenge to drivers and is difficult to analyze into a small number of simple cases.

It is worth noting that similar augmented stop signs could be devised for three-way stops, that is, at “T” intersections controlled by three stop signs. Such intersections are actually much simpler, however, and, since a vehicle capable of going through the intersection and having only a curb at the 3 o’clock position never has to defer to a vehicle to its right, it is reasonable to assume, even without an augmented sign, that it always has priority over other vehicles that have arrived at the intersection at the same time. Augmented stop signs could clarify this interpretation, however.

Is my proposal practical? I think so, although my first try at solving the four-way stop problem may have been intellectually interesting without really being effective or safe. Augmented stop signs could not have resolved my encounter with the school bus, but, together with additional regulations, I think they could make four-way stops safer and less frustrating. Here is a suggested rewrite of the Pennsylvania instructions:

A FOUR-WAY STOP sign means that there are four stop signs at this intersection. Traffic from all four directions must stop. The first vehicle to reach the intersection should move forward first. If two vehicles reach the intersection at the same time, the driver on the left yields to the driver on the immediate right if there is one. If a vehicle cannot move because of some impediment—a pedestrian crossing in front of it, for example—or, for some reason, cannot clear the intersection unimpeded, it must await a change in status and should be treated by other motorists as though absent from the intersection until circumstances make it eligible to proceed. If the intersection is controlled by augmented stop signs (see figure —) and the foregoing rules do not determine who has the right to move first through the intersection, priority among the vehicles that might have a claim to it according to the above rules is determined by their positions at the intersection and the circular arrow displayed on the signs. A vehicle may not proceed until all vehicles that might have priority by the foregoing rules at locations corresponding to positions on the arrow to the right of the 6-o’clock position have first cleared the intersection.

Admittedly, this is long, tedious, and, perhaps, not crystal clear. I do not doubt that its content could be better expressed, but I leave that as an exercise for the reader.

My new signs would cost money to deploy, of course, and they would demand driver education and require a bigger inventory of stop signs by departments of transportation. They could, however, be worth the trouble if drivers could adapt to them.

Postscript

I have received a fair amount of correspondence concerning the above essay. A crossing guard in Toronto informed me that, in Ontario, crossing guards are prohibited from directing traffic. I suspect that this may be the case everywhere. I probably should not have been upset with the crossing guard in the incident described above, so I apologize to crossing guards everywhere for my misunderstanding.

Another correspondent, Alex Henkel, suggested that my semicircular arrows should not appear on the stop sign proper but should be on an auxiliary sign below it. This idea has many advantages over my plan for modifying the design of the stop sign itself. According to this idea, a signpost at a 4-way stop would hold a conventional stop sign, the usual “4-WAY” auxiliary sign below it, and a sign like the following below that:

Auxiliary sign to be used with stop sign

Advantages of this scheme include the following:

  1. The familiar stop sign remains unchanged.

  2. The secondary nature of the semicircular arrow is made clearer by removing the arrow from the main sign.

  3. Existing stop signs do not need to be replaced.

  4. Departments of transportation need a smaller inventory of signs. Only one stop sign (of a given size) is needed, and the new auxiliary sign could be installed in different orientation, obviating the need for four different signs.

— LED, 11/29/2001 (last revised 1/18/2010)

____________________
(Pa.) Department of Transportation Driver and Vehicle Services, Pennsylvania Driver’s Manual, English Ed. (Pub. #95, May 2000), 10.

 
Readers of this essay may enjoy “Poems of the Open Road,” one of whose poems addresses the four-way stop.

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