The city?s preferred design for the Rainier crosstown connector and interchange may have ?adverse affects? on Highway 101, the state?s transportation agency says.

In a letter to city officials, Caltrans casts doubt on whether the diamond-style interchange favored by Petaluma will work as part of a Rainier Avenue extension under the highway.

The comments are based on ?preliminary information? contained in a draft report on the project, the Oct. 30 letter states.

Furthermore, the question of whether Caltrans will allow a ?design exception? so that a Rainier interchange can be built less than one mile north of the freeway access at East Washington Street is unresolved.

The city wants Caltrans to build auxiliary lanes between the two interchanges so entering and exiting drivers have more room to merge, avoiding a conflict between vehicles at different speeds.

City Councilmember Mike Harris, Petaluma?s representative at the Sonoma County Transportation Authority, said despite the letter, nothing is final until a Project Study Report is completed and the question of the design exception is resolved.

?I view that as more of a status update,? Harris said of the letter, written by Caltrans Deputy District Director Lee Taubeneck to Vince Marengo, Petaluma?s public works director.

?I?m still going to be pushing for the design exception,? Harris said. ?We have a lot of momentum behind us and we?ve come a long way to make this a reality.?

The city is awaiting the release of a Project Study Report that will evaluate five possible designs for the Rainier project. They range from doing nothing to building an undercrossing only, with no freeway access.

The city prefers a diamond-style interchange with ramps in all four quadrants of the city, at an estimated cost of $75 million.

But that option, or a modified diamond interchange, ?would involve technical and operational challenges and may have more potential adverse effects on the mainline,? Taubeneck?s letter said.

?Local improvements can proceed on the route 101 mainline as long as they do not jeopardize the tremendous investments and benefits of current and planned improvements along the corridor,? the letter said.

Either a ?split diamond? interchange ? which essentially replaces the East Washington southbound offramp and planned northbound onramp with frontage roads that lead to a Rainier interchange ? or a Rainier extension running under the freeway are consistent with Caltrans? ?requirements and standards,? the letter said.

The split-diamond option is estimated to cost $109 million and the undercrossing only is pegged at $38 million.

The city believes its preliminary studies show that with the use of auxiliary lanes, highway operations will not be affected by a diamond-type interchange at Rainier.

Traffic studies presented to the council in May showed that Rainier would create ?no discernible change? to rush-hour traffic on Highway 101.

In fact, the project would ease congestion for rush-hour drivers heading north in the morning and south in the afternoon, according to the studies.

Final traffic projections will be presented to the council as part of the PSR, which could be released in January, Harris said.

The city and Caltrans are ?going back and forth right now,? he said. ?It?s good to have dialogue.?

(Contact Corey Young at