King County has developed six alternative alignments for a future Mount Si Bridge. Four of them would occupy roughly the same location as the current span, while two alternatives would place the bridge farther upstream. However, those two alternatives would require major improvements to 436th Avenue Southeast and Southeast Tanner Road. The alternatives are as follows:

Alternative 1 – Consists of new alignment with straight bridge connecting Southeast Mount Si Road to 436th Avenue Southeast and to the Exit 32/Interstate 90 interchange. One reason for: best access to downtown and I-90. One reason against: requires new road. Estimated cost: $8.49 million. Construction duration: two years. Bridge length: 400 feet.

Alternative 2 – Consists of new alignment with straight bridge just upstream of existing bridge. One reason for: shorter construction time. One reason against: requires large number of property acquisitions. Estimated cost: $7.32 million. Construction duration: 11U2 years. Bridge length: 320 feet.

Alternative 3 – Consists of replacement bridge on existing bridge alignment with revised northern approach curve. One reason for: keeps bridge in current location. One reason against: requires most complex construction staging. Estimated cost: $12.76 million. Construction duration: two years. Bridge length: 300 feet.

Alternative 4 – Consists of new alignment with curved bridge just downstream of existing bridge. One reason for: least amount of private property acquisitions. One reason against: eliminates part of existing Little Si trailhead parking lot. Estimated cost: $7.66 million. Construction duration: two years. Bridge length: 270 feet.

Alternative 4A – Consists of new alignment with straight bridge just downstream of existing bridge. One reason for: lowest construction cost. One reason against: crosses mature, second-growth forest. Estimated cost: $6.47 million. Construction duration: 11U2 years. Bridge length: 300 feet. (This alternative was recommended to the county by its engineering firm.)

Alternative 5 – Consists of new alignment connecting Southeast Mount Si Road to Southeast Tanner Road. One reason for: allows best emergency access for police and fire, especially if old bridge is left in place. One reason against: requires major improvements to Southeast Tanner Road. Estimated cost: $10.83 million. Construction duration: two years. Bridge length: 350 feet.