交通 and 渋滞: transport and congested flow
Separate a transport system from congestion and read its stated effects.
The transport domain
交通 (こうつう) refers to traffic or transport. 「この町は交通が便利です」 says transport in this town is convenient.
「交通手段を調べました」 means researching transport options. It does not identify a mode actually used.
A congested flow
渋滞 (じゅうたい) means congestion that slows vehicle flow or similar movement. 「道路が渋滞しています」 says the road is congested.
「渋滞で到着が遅れました」 attributes a late arrival to traffic congestion. で marks the cause of delay.
Congestion versus lateness
「渋滞を避けて早く出ました」 says the speaker left early to avoid congestion. It does not establish a late arrival.
「交通量は多いですが、渋滞はしていません」 says traffic volume is high but it is not congested. Volume and flow conditions are not identical.
Does a large number of vehicles necessarily mean 渋滞?
Show the answer and why
No. High traffic volume and blocked flow are different information.