Question: Do I need to cross the road on Shabbos to avoid triggering a motion sensor light?
Answer: The Mishna Berura (277:9) writes that one mustn’t open a door near a candle. As the breeze will inevitably fan the flame, this is considered to be pesik reisha denicha lei as one may benefit from this too. The Piskei Teshuvos (277:9) writes that this halacha applies equally to walking past a light that will be triggered by a sensor.
Nonetheless, R’ Chanoch Padwa (Cheshev Haefod 3:83) and R’ Ephraim Greenblatt (Rivevos Ephraim 7:1231; 8:532:1) argue that when one isn’t interested in triggering someone else’s light it is considered to be pesik reisha delo nicha lei. Thus, they write that while it is ideal to avoid triggering such lights, one may walk past them if necessary.
R’ Ribiat (The 39 Melochos, p1214) differentiates between a regular residential road and a rural area. Only if the street is lit up enough that one can see where they are going clearly is this extra light considered pesik reisha delo nicha lei. Even then, one should avoid triggering this light if possible. Such a light on a dark street would be akin to walking into a building with a light sensor which would be assur (See Orchos Shabbos 26:28).
R’ Shmuel Wosner (Shevet Halevi 9:69), however, disagrees. He quotes the Gemara Yerushalmi (Shabbos 13:6) which writes that one may close one’s house door as normal even though a deer will inevitably be trapped (See Rashba, Shabbos 107a; Magen Avraham OC 316:11). Unlike an automatic door that opens as you walk up to it, when one has no interest in triggering the light sensor, there is no issue with walking normally down the street (See Yechave Daas 5:29). R’ Yosef Shalom Elyashiv, R’ Nissim Karelitz (quoted in Orchos Shabbos 26:31) and R’ Shraga Feivish Schneebalg (Shraga Hameir 8:137) concur.
In conclusion, if one knows that one is going to trigger a light switch by walking past it on Shabbos, one should cross the road to try to avoid it.