Carpool Incentives Act - Directs the Secretary of Transportation to carry out demonstration projects designed to encourage the use of carpools in urban areas throughout the country while not adversely affecting bus and other mass transportation ridership in such areas.

Provides that from proposals submitted by the States to the Secretary, the Secretary shall approve for funding those projects which offer the best prospects of achieving the objectives of this Act and which provide a reasonable balance among cities of various population sizes, traffic patterns, and geographical locations.

States that the Federal share of the cost of such work shall be 90 per cent, except that the Federal share shall not exceed $1,000,000 for any single project.

Directs the Secretary to give permits to demonstration projects proposed for cities or areas in air quality control regions after consultation with the Environment Protection Agency as to the value of such projects as a part of a transportation control plan for such region.

Directs the Secretary to conduct a full investigation of the effectiveness of measures employed in the demonstration projects authorized by this Act.

Authorizes to be appropriated not to exceed $10,000,000 from the Highway Trust Fund to carry out this Act.