Focus on showing the effect the noise and hassle is having on your characters. How do they feel, what are they doing? In what way is the traffic impeding their task. Maybe they peeled their fingers from the wheel one by one only to find their hands still ache from clenching it so hard. maybe they rest their head against the cool glass of the window for a moment to try and relieve the noise-induced headache only to flinch as the car right next to them honked.

This is in addition to the suggestions others have made. Although I'd advise against too much onomatopeia as it doesn't usually induce annoyance in the reader so much as make the story unreadable.

And don't forget plenty of paragraph breaks. Every time someone has to stop talking and start again, start a new paragraph. Too often writers try to keep all the speaking to a single paragraph until the speaker changes, but feel free to interrupt. It makes the dialogue flow better and shows the feeling of tedium the characters are feeling with being constantly interrupted.

If you can, find someplace noisy to hang out for a bit. Doesn't have to be traffic, just someplace loud and busy. Then imagine your characters there while you're hearing and feeling all the noise and bustle. make some notes about how that situation feels. You might also try to look up some traffic sounds on youtube or wherever and play them at a high volume while you write that scene to help set the mood.