You need the singular impact in this sentence.

Hello Narges! I suggest using the singular impact instead of the plural to make it sound like there is only one major impact but multiple areas are affected. "The major impact of traffic jams..." In addition, I suggest saying it using the passive form: "The health and behavior of the citizens are considered to be heavily impacted by the traffic jams." Or; "The health and behavior of the citizens were considered to have been heavily impacted by the traffic jams." -- If it happened in the past and is no longer happening. "Behavior" is usually an uncountable noun. In this case it's better to use singular. I hope this helps. Cheers!