OpenHistoricalMap

This has I-90 coming into existence in 1996, 40 years after it was designated as part of Interstate 90 (https://en.wikipedia.org/wiki/Interstate_90_in_Washington#Interstate_designation_and_early_construction)

I have specifically told not to use Wikipedia for the source of data. Instead I am in the process of working with WSDOT to acquire exact build dates for sections.

The main reason you see a date much later is that the creation of I-90 as you see it today was not created until much later. Instead I-90 was a label that was added to much of Highway 10 also known as the Sunset Highway until the modern construction. So the data is partially right. Additionally I can confirm from that the road was in place from person experience in 1996.

Last sentence is a bit wonky. I know from personal experience were the road was in 1996.

Still wonky. I know from personal experience where the road was in 1996.

You can see that the sunset highway has been added. And I spent some time tonight adding highway 10.