I am allowed to wear reflector band on shabbos because I cross big roads with 6 lanes on each side?
Yes, this is permitted where necessary.
There is a preference to sew it onto the garment, where this is possible.
The fundamental question is whether an armband can be considered a “garment” of sorts (or a tachshit, a decoration which effectively has the status of a garment), which is not carried but rather worn, or whether this is considered “carrying” the armband which is forbidden.
An important source for this matter is the Talmudic case of wearing an amulet, whereby it is permitted to wear an amulet (which has been tested and is known to be reliable) whose purpose is to heal or protect a person. This is ruled by the Shulchan Aruch (see Orach Chaim 301:26).
Clearly, the amulet is not an item of jewelry or clothing, for otherwise it would be permitted even if it wouldn’t be healing or protecting its wearer. Rather, we see that something protecting the body, when worn on the body, is considered as a garment (since it, too, protects the body) and is not considered carrying on Shabbos.
It is true that the amulet provides constant protection, whereas the armband only protects when crossing busy streets. Yet, this distinction does not appear to be significant, seeing as the armband is at the end of the day a requirement to protect its wearer.
The reason why it is preferable to sew the armband onto the garment is that we find in the Rema (301:23) that a mandatory badge that Jews had to wear can only be worn if attached to the clothes (based on Or Zarua). Although the badge does not have to be sewed on, the Mishnah Berurah explains that this is because they were worn all week long, and it is not clear that this will apply to the armbands.
The Rema writes that sewing is preferable, yet this, too, helps only for something which is generally worn in this manner.
See also Lehoros Noson 8:18; Shemiras Shabbos Kehilchasa 18:16 (concerning reading glasses); Yalkut Yosef 301:32 (also concerning reading glasses).