Does the word "decaled" here refer to having a logo on the wind shield?
Yes, though it need not be on the windshield. Just as you say, it means that the vehicle displays an appropriate decal indicating that it qualifies for low emissions designation.
MW, Oxford and Collins only list decal as a noun, but reference.com contains a verb listing. Conjecture: reference.com and similar strictly internet based sources (as opposed to traditional print dictionaries like the first three) are more representative of "new English", meaning you'll find more modern or recently coined definitions in them, but they may be contentious or contain things which never make it into standard parlance. The usage of decaled in the quote follows the common pattern of using the past participle of a verb as an adjective. Here, it means bearing a decal.
And why the writer has put HOV ( high occupancy vehicle ) lane beside an on ramp?
I assume you mean grammatically (you may use a carpool/HOV lane or on-ramp)? Because the special lanes and ramps function in the same way and serve the same purpose. Thus, the author wants to talk about them together. The text that follows applies equally to both, even though it only says lanes in the interest of brevity. Any time in the paragraph you see lane, you can mentally add or on-ramp.
In case you didn't mean that question to be about grammar: these ramps and lanes are standard construction on many highways or freeways; it's an accurate description of the author's part of the world. They create an incentive to use transit falling in the designated vehicular classes. They yield a faster commute because they have much less traffic and also avoid (or reduce) tolls.