In the Tacoma area, WA 512 and 167 is alternative to I5.
Currently on Google Maps, this times the same as I5 the whole way.
On GM, don't just look at the routes it suggests - add some other waypoints to test alternatives.
Given the layout of the Seattle/Tacoma metro area there isn't such a thing as a fast outer beltway. You are stuck with going through on I5 or 405. And with east side development, traffic is similar. Think of Bellevue as the east 'downtown' with rush hour traffic focus.
WSDOT also has a good traffic map website with lots of traffic sensors and webcams. But I suspect Google Maps is making good use of that information when making real time suggestions.
https://wsdot.com/travel/real-time/From the travel time page, the I5 v I405 times from where they split to where they rejoin:
Southcenter to Alderwood via I-405
Distance: 29.62 miles
Average time: 31 minutes
Current time: 35 minutes
Last updated: 07/08/2022 1:00 PM
Southcenter to Alderwood via I-5
Distance: 27.05 miles
Average time: 32 minutes
Current time: 32 minutes
Last updated: 07/08/2022 1:00 PM
