The rise in traffic deaths predate the pandemic. I do think that to some degree less traffic does encourage more dangerous driving, but I suspect it's not the whole (or even main) cause.

I suspect a large part of this is the change in the kind of cars that are on the road. Trucks and SUVs predominate in the US now, and sedans are a rare sight these days. More mass = more deadly. Also importantly trucks and SUVs nowadays are obsessed with an "aggressive stance" - i.e., tall hoods which increases impact injuries to pedestrians, and also reduce cockpit visibility.

This has been known to road safety activists for years, but the mainstream media is just starting to catch on. See for example this SUV that has a full 16-foot blind spot in front of the car: https://twitter.com/PickledEntropy/status/155387204384156876...

tl;dr: American car purchases have trended towards heavier cars with deadlier geometry and worse driver visibility, which IMO is a huge contributor to these stats.

[edit] For some additional color, in 2014 SUV sales overtook sedan sales for the first time [1] and by 2019 SUVs were out-selling sedans by 2:1. [2]

[1] https://www.latimes.com/business/autos/la-fi-hy-autos-suvs-c...

[2] https://www.nytimes.com/2020/05/21/business/suv-sales-best-s....

Also, to fully drive home the "deadlier geometry" side of things, see the difference here between a 1994 Toyota Tacoma [3], curb weight 3100 lbs, and a 2022 Toyota Tacoma [4], curb weight 4500 lbs:

[3] https://images.app.goo.gl/rjU2pu3g7Wqv9o6P9

[4] https://images.app.goo.gl/TcKRSYybd1f1NzkR8

The frontend height difference, as well as the sheer mass difference, is stupendous. This is what's causing people to die en masse on roads.