Yoğunluk arttıkça trafik akışı yavaşlıyor.

-dikçe is an adverbial clause-forming suffix that means “as/when(X) happens” or “the more X… the more…”. You attach it to a verb stem (here art- from artmak, “to increase”). So arttıkça literally means “as it increases.” In English we render the whole clause yoğunluk arttıkça as “as density increases.”

This is due to consonant and vowel harmony plus assimilation:

1. The verb stem is art- (ending in voiceless t).
2. The suffix in its basic form is -dikçe (with voiced d).
3. Voicing assimilation turns that d into t after the voiceless t of art-, giving arttikçe.
4. Vowel harmony then adjusts the vowel i to ı (back vowel after t), so you get arttıkça.

In Turkish noun-noun compounds expressing “A of B,” the second noun takes a third-person possessive suffix. Here akış (“flow”) becomes akışı (“its flow”), so trafik akışı literally is “the flow of traffic” (i.e. “traffic flow”).

Yes, both akış and akım can mean “flow” or “current,” but there’s a subtle nuance:

• trafik akışı focuses on the continuous movement or passage of vehicles (“flow”).
• trafik akımı can sound more technical or like an electrical/water “current.”
  In everyday traffic contexts, trafik akışı is more idiomatic.

yavaşlıyor is the third-person singular present continuous (progressive) of yavaşlamak (“to slow down”). In Turkish you often use the present continuous for ongoing processes or general truths. Here it emphasizes that the slowing-down happens as density rises. You could also say trafik akışı yavaşlar (aorist), but yavaşlıyor highlights the dynamic nature of the process.