Would it make sense to mix some TiO2 or other whiteners into the asphalt and concrete to increase their albedo?

Edit: Looks like LA is doing something like that https://www.mnn.com/earth-matters/climate-weather/blogs/how-...

I‘m imagining that Texas has far wider roads than Europe. Would it be possible to plant street trees or other tall native vegetation to provide shade?

Houston does just that. Streets in Houston are lined with live oak, providing shade to much of the street.

Have you been outside the loop? There is no culture of shade protection in Texas the way there is in other not places like Australia or mideast. Massive concrete car parks with no shade structures in sight. No solar panels on the roofs of big box stores. It's pathetic.

It's a shame we can't convert that heat to electricity in some efficient way.

Oh we can convert high grade heat to electricity with amazing efficiency. Problem is, this is super low-grade (low temperature) heat. Darn you, thermodynamics.

PV solar roadways are a total bust, but I do wonder about liquid loops under the asphalt. If you need a source of cold to create a temperature gradient vs the road, you could always pull from geothermal next to the roadway collector.

Provide shade by putting up solar panels above the roads.