The intent of this phrase is to designate zones in which people are permitted neither to get in a taxi nor to get out of one.
However, I think “no-boarding and no-alighting” is a whopping long phrase to use as an adjective in front of the noun “zones”. It’s so cumbersome that my initial inclination was to read it as an elliptical formation designating two different kinds of zones:
[Be aware of the] no-boarding [zones] and [the] no-alighting zones.
This would be analogous to a sentence like:
If the medium-size shirt doesn’t fit, let me know; there are bigger [sizes] and smaller sizes available.
Obviously there are no sizes each of which is both “bigger and smaller”; the adjectives are separate, and there’s a noun implied but omitted after the first one.
I’m not even sure the intended reading of “no-boarding and no-alighting zones” is syntactically possible, unless you hyphenate the whole thing, which would be ugly and probably violate most style guidelines:
No-boarding-and-no-alighting Zones
Since there’s a set of illustrations below the text, I think probably I would write it as:
No boarding or alighting in these zones.
Or, even shorter:
Do not board or alight in these zones.
