Shalom! This question touches on technical details of the Laws of Shabbat and invites an interesting consideration about how we deal with halakhic situations which are less in our own control. Your question has similar applications in hotels and other places where one might need to stay on Shabbat (and to a lesser degree in walking at night on Shabbat - there usually one can walk outside the area of a motion-triggered light). To address it, we need to consider the relationship between your act and the light going on, as well as the nature of the prohibition of fluorescent lights.
The work that is prohibited on Shabbat is classified as "melekhet mahshevet" (thoughtful/constructive work). Your walking by to cause the light to go on hardly seems like that kind of work. In fact, it (the causing the light to go on) can be classified as a davar she-eino mitkaven (an act/consequence which one did not intend, precisely as you wrote in your question). It is an inevitable consequence, however, called in halakhah a psik reisha (from the illustration of inevitability of cutting of a chicken's head and its subsequent death!), which makes it more severe, and halakhically mitigates your claim that you don't have the intention to illuminate the hallway. But then we have a final component - your relationship to this outcome: is it good for you, not good for you, or are you indifferent? (Note, before we move to the application to your question, the beautiful "swinging pendulum" of these categories - we are lenient when you have no intention for this outcome, stricter again when it is unavoidable, and then ultimately dependent on how you benefit from that outcome. I think of these as logical and wise evolutions of the Laws of Shabbat.) If it is good for you, this is a category which is hard to permit in Orthodox halakhah. If it is neutral or not good for you, it is easier to permit (especially with the addition of other factors - see below). It's hard to say the lighting is not good for you - it helps you navigate the hallways. In cases like this, though, Orthodox authorities have written that if there is some light already so you could see your way to your room without it, you can be considered more indifferent to the new light, and this is more easily permissible. If there is no other light and you really couldn't see without this light, the only permission I am aware of (The 39 Melochos, Rabbi Dovid Ribiat, p. 1215) is the suggestion to shut your eyes just as you enter the hallway, so you do not benefit at the time that you cause the lights to go on. Once they are own, you can proceed. As silly as this may sound, it has the spirit of helping distance yourself from this act and certainly doing it in an uncommon way - part of the way we deal with non-ideal Shabbat situations.
How about the fluorescent light? Whereas an incandescent light works by heating a metal filament, and is (widely considered) a Biblical prohibition, a fluorescent light is simply electricity exciting the neon gas in the tube, not any specific Biblical prohibition and widely considered to be a Rabbinic prohibition (unless there is a 'starter', which most fluorescent lights don't have - see details on the Tzomet webpage here). This combines with the unintentionality of your action and not needing the outcome, to get to a permissive response.
So: for fluorescent lights, if there is some light already in the hall, you may walk the halls. If there is very little or no light, then using the eyes-shut method is advisable. I am guessing this is a rare scenario: I imagine if you are on an active hall, the lights are often on anyway and you might be able to get in or out while they are already on from someone else's motion.
Finally, this is a fascinating instance where you feel at the mercy of a system beyond your control, but you may wish to seek accommodation from your dormitory. It can be a beautiful teaching moment and a chance to exercise your right to ask for recognition of your religious practice. If some arrangement could be made to disable the motion sensors on your hall (even just yours) on Friday and Yom Tov nights (in the daytime the halls are presumably naturally lit sufficiently already), while it would expend a little more energy, it would alleviate the halakhic challenges here.
I hope that within these minutiae of halakhah, you are able to find meaning and think about the impact of our actions, and in this case to consider the ways we do and don't have agency in the institutions we participate in. Shabbat shalom!
Your question presumes that you do not use electricity on Shabbat, and for such people the prevalence of such devices that turn on and off lights automatically has become a real problem. One might take refuge in the talmudic principle of lo neikha lei, it is not what you want, and therefore if it happens against (or, in this case, beyond the control) of your will, you are not responsible for it. The problem is that the Talmud restricts that principle to cases where you cannot know ahead of time that it will happen, for, as it says, p'sik reishai v'al yamut, can you cut off the head of a chicken and it will not die? In this case, you know full well that if you enter the hallway, the lights will go on. Given the prevalence of these devices, though, it seems to me that one simply cannot live in modern society if one follows the p'ski reishei limitation, and so we all must simply recognize that one cannot be held responsible for what one cannot avoid.
This affects not only lights. Every time you enter a building these days you are being recorded on tape or some other digital device -- and that happens even when you walk on the street. Does that violate the Shabbat prohibition against writing or making other permanent images? Clearly, Jewish law cannot be reasonably interpreted to prevent us from leaving our homes or dorm rooms on Shabbat.
So no, in my view, entering the hallway to your dorm room on Shabbat is permissible even though you know that that will automatically turn on the hallway lights. So too is it permissible to walk into buildings and on the street on Shabbat even though you know now that you will be recorded doing so.
This is a question that touches on a two areas of Jewish law and values. First, your school should be commended for promotion of the value of Baal Taschit (do not destroy) in its effort to conserve energy. Even if the motive was to save money, the positive result of less energy use is admirable.
The challenge here, is when Baal Tashchit conflicts with the concept of Shomer Shabbat, especially when the benefit of following Baal Taschit is very minor. From a Reform perspective, this really isn’t much of an issue. Even if you have made the choice to not consciously turn on and off lights on Shabbat – a valid Jewish choice without a doubt – you are simply within a process started long before you arrived when it comes to the lights going on and off.
While I can see how it can be argued that by entering the hallway you are the one turning on the light, in this particular case, because this is a decision made well out of your control that as long as you are not physically flipping a switch with your hand, that this is no different than using a timer to turn off lights at a certain time of the day on Shabbat (which is completely acceptable by traditional practice as long as the timer was set prior to the start of Shabbat).
If possible, I would recommend that you speak with the dorm manager and maybe the lights can be set to stay on from the start of Shabbat to the end each week for your particular floor or other areas where you are likely to be the only one present. Given the amount of activity on a typical dormitory floor on a Friday night, I would guess that it would only be an hour or two of extra time on for the lights anyway!
If that is not possible, then the situation is out of your hands and you have done everything possible short of staying holed up in your dorm room on Shabbat – which I believe would be a far greater violation of the spirit of Shabbat than whether or not lights are automatically turned on because you enter a hallway.
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