medic 21 engine 21 HYW 30??

[midnightcaller]

I was listing to Snohomish County East, and I heard some tropo, they had a medical call for med 21 and engine 21 on talk group 720 on HYW 30 at mile post 57.5 the control channel was 868.925 and the talk group was 720

I know where state route 30 is,but where abouts is mile marker 57.5?