Federal funding for interstate highways may be a more direct subsidy that promotes the high per capita car ownership in the US, but it's not the only one. Indirect federal subsidy – low taxes – on fuel and fuel consumption by cars & trucks contributes heavily to this. If SUVs were taxed by weight, engine size, or fuel consumption, as they are in other countries, they would be a minor fraction of the vehicles on the road.
In addition, the comparable lack of federal funding to support rail transport, both local & long distance, can also raise the high per capita of car ownership in the US. In many parts of the country, there is no alternative.