Learn Hardcore Turkish: Bu kestirme yoldan gidince trafik olmuyor; üstelik çok hızlı. - There is no traffic when we take this shortcut; moreover it is very fast.

The converb -IncE means "when/once/whenever/upon" and links a background action to a result. The subject is generic/implicit—often understood as "you/we/one" from context. So gidince = "when (we/you) go (via that route)."

Why is it gidince and not gitince?

The verb gitmek alternates between two stems: git- and gid-. Before many vowel-initial suffixes, it uses gid-: gidiyorum, gidecek, gidince. With consonant-initial past/perfect markers it stays git-: gittim, gitmiş. Both are standard, not a spelling mistake.

How is gidince different from giderken and from gittiğimizde/gittiğinde?

gidince: "when/once (after) going," result happens after completion.
giderken: "while going," two things happen simultaneously.
gittiğimizde/gittiğinde: "when we/you go," same timing as -ince but with an explicit subject/person marker.

Why is it yoldan (ablative -dan) after git-?

With motion verbs, the ablative often marks the route or medium: "via/along/by way of." Hence (X)-den gitmek = "to go via X": otobandan gidince, sahilden gidince, köprüden geçince.

Could we say Bu kestirme yolda… instead of …yoldan…?

Yes, but it shifts the meaning. Bu kestirme yolda trafik olmuyor states a condition on that road itself ("on that road there isn't traffic"). …yoldan gidince trafik olmuyor ties the no-traffic result to the act of going via that route.

What does kestirme contribute? Is it the same as kısayol or kısa?

kestirme (yol) means "shortcut (route)," very common in speech for physical routes. kısayol also exists; today it's frequent for computer "shortcuts," though it can mean a literal shortcut too. kısa yol just means "short road," not necessarily the standard idiom for "shortcut."

I’ve seen kestirmeden and the verb kestirmek. Are they related?

Yes. kestirmeden = "by shortcut/straight to the point." The verb kestirmek has several meanings: "to have (something) cut" (saç kestirmek), "to guess/estimate," and colloquially "to take a nap" (uyku kestirmek). Context disambiguates.

Why say trafik olmuyor instead of just trafik yok?

Both can work, but nuance differs. trafik olmuyor presents "no traffic" as the outcome of those occasions (there ends up being no traffic). trafik yok states a static absence; with -ince many speakers prefer the event-like olmak.

Why not use the aorist olmaz?

You could say trafik olmaz, which sounds more like a general rule or confident prediction. olmuyor (present continuous) is the most natural with -ince to describe what typically happens on those occasions.

The second part is just Üstelik çok hızlı. Where’s the verb?

It’s elided. You can mentally expand it as Üstelik (yolculuk) çok hızlı (oluyor). Turkish often drops obvious elements when the meaning is clear.

Is hızlı functioning as an adverb here? Should it be hızlıca?

Adjectives commonly function adverbially in Turkish, so hızlı is fine as "fast/quickly." hızlıca is also possible but can sound a bit more colloquial or mean "fairly quickly" rather than emphasizing top speed.

What’s the difference between hızlı and çabuk?

hızlı: high speed; suits vehicles, movement, processes.
çabuk: prompt/soon; suits completion time ("It finished quickly": çabuk bitti). For movement, prefer hızlı: hızlı gidiyor, not usually çabuk gidiyor.

How is üstelik used? Could I say ve üstelik or ayrıca?

Üstelik means "what’s more / on top of that," often introducing a stronger or surprising extra point. ayrıca = "in addition" (more neutral); hem de is colloquial-emphatic. ve üstelik is acceptable but usually redundant; üstelik alone already links the clauses.

Why a semicolon before üstelik? Could I use a comma or a period?

The semicolon neatly links two closely related clauses. A comma also works: …, üstelik çok hızlı. A full stop is fine too: … Trafik olmuyor. Üstelik çok hızlı. Style choice.

Can I reorder parts like in English?

Yes, with emphasis changes. Trafik olmuyor bu kestirme yoldan gidince; üstelik çok hızlı is fine, emphasizing the result first. You can also front the additive: Üstelik, bu kestirme yoldan gidince trafik olmuyor.

Who is the subject of gidince and olmuyor? Is it "we"?

The subject is implicit. In context it’s generic "you/we/people." To make it explicit, use person-marked forms: Biz bu kestirme yoldan gittiğimizde trafik olmuyor or Sen bu kestirme yoldan gittiğinde….

Are there more idiomatic alternatives to say the same ideas?

For traffic: trafiğe takılmıyoruz / takılmıyorsun ("we/you don’t get stuck in traffic"), trafik olmuyor/az oluyor/hafif oluyor.
For speed: çok çabuk varıyoruz, çok kısa sürüyor, yol hızlı akıyor.

What are the morphological pieces of the tricky words here?

gid-ince: stem git- (here alternating as gid-) + converb -IncE ("when/once").
yol-dan: yol
- ablative -DAn ("from/via/along").
ol-ma-yor: ol- ("be/happen") + negative -mA-
- progressive -yor (3sg implicit "there is/occurs").