OLYMPIA—The Senate’s new transportation package is being hailed for its bipartisan support, while some critics find aspects of it troubling.

The proposals were revealed on Feb. 12 after 22 months of negotiations and would mostly fund various highway projects in the state’s more congested areas. The package would raise $15 billion over a 16-year period mostly through an 11.7 cents-per-gallon gas tax implemented over the next three years.

Washington’s current 35.7 cents-per-gallon gas tax would increase by five cents in July, 4.2 cents in July 2016 and another 2.5 cents in July 2017....