Tolls to cross Lake Washington could cost commuters $6.85 a round trip, be levied on both the I-90 and 520 bridges and change people’s decision to cross the bridge itself, according to a new study unveiled Wednesday in Bellevue.

The 520 Tolling Implementation Committee presented four scenarios which showed how much money would be raised by tolling to replace the current 520 bridge and what driving changes might result if different tolls were put in place. The scenarios are not proposals, but material for discussion and analysis.

The actual toll rates will depend on a final finance plan and will be set by the State Transportation Commission with approval by the state Legislature.

State officials envision tolls raising $2 billion, about half of the estimated $3.7 billion-$3.9 billion cost to replace the aging 520 bridge. Engineers say the bridge is in danger of sinking if battered by a major windstorm.

Two of the scenarios raise less that the $2 billion goal, two raise more. However, officials cautioned that many assumptions are built into the scenarios and all are subject to change.

Gov. Chris Gregoire wants a new 520 bridge opened by 2014.

The four scenarios are diverse.

The first assumes tolling would start on the new bridge in 2016 when the corridor is complete and that tolls only would be collected on 520. It included bridge and segment tolls and produced the highest toll rate for analysis purposes. Segment tolls would be those from just beyond the end of the bridge to either I-405 or I-5. The study estimates $835 million would be raised from tolling.

The second scenario would start tolling in 2010, again only on 520. There would be no segment tolls. It produced the lowest toll rate of the four. The study estimates $900 million would be raised from tolling.

Scenario three would put tolls on both 520 and I-90 in 2016. It, too, would include segment tolls on both bridges. Mercer Island residents would face a half toll to go from the island to Seattle or the island to Bellevue. It produced a moderate toll rate. The study estimates $2.3 billion would be raised from tolling.

The final scenario would start tolling the 520 bridge in 2010 and I-90 in 2016. Segment tolling would be adding to 520 in 2016 (when the corridor is complete) and on I-90. The study estimates $2.5 billion would be raised from tolling.

There would be no toll booths in any scenario. Money would be collected electronically, as it is now on a portion of Highway 167 in south King County.

The study also assumes toll amounts would vary by time, giving people options when they would use the bridge or it they would switch to public transportation.

However, officials note that they have little real world experience in this region on how people will respond if there are tolls on 520 or I-90. The four scenarios are seen as comparing alternatives rather than precisely estimating behavior.

What does seem certain is that the higher the toll, the more people change how they travel. Some people are expected to change to carpools and transit. Others might use alternative routes to and from Seattle and the Eastside. However, the largest change, the study suggests, is that people may choose not to cross the lake at all, in short, changing their destination.

Under scenario 1, the one with the tolls starting on 520 in 2016, 15.5 percent of people are expected to choose not to cross the lake at all.

Scenario 2, adding tolls to 520 in 2010, but with lower tolls (commuter costs at $3.45 round trip), 1.7 percent would choose not to cross the lake in 2010, but 8.3 percent choosing to not cross the lake in 2016.

In both scenarios, no tolls would be on I-90.

Scenario 3 would put tolls on both bridges in 2016, with a commuter paying $4.55 round trip. In this scenario, 22.3 percent of people would choose not to cross the lake.

Scenario 4, with tolls on 520 beginning in 2010 and on I-90 in 2016 also assumed commuter tolls of $4.55, with between 19.6 percent and 22.3 percent of people choosing to not use the bridge.

The next step will be a series of public open houses to hear from the public. In Bellevue, the open house will be on Aug. 6 at Bellevue City Hall from 5-7:30 p.m. with a presentation at 6 p.m.

All comments are due to the tolling committee by Aug. 31 at info@build520.org or by mail to 520 Tolling Implementation Committee, c/o Puget Sound Regional Council, 1011 Western Ave., Suite 500, Seattle, WA 98104-1035.

Note:

Bothell on July 29 at UW Bothell North Creek Events Center.

Renton on July 31 at Spirit of Washington Events Center.

Kirkland on Aug. 7 at Kirkland performance Center.