The reason bridges form ice before roads is that they are exposed to cold air on all sides and have lower total thermal mass, so conduction from the bridge to the air allows the temperature of the bridge surface to drop faster. The ground has nearly infinite thermal mass, and it takes a long(er) time for ambient air temperature to affect the surface temperature.

When you say “the ground holds more heat” you’re talking about that thermal mass. The temperature of the air is colder than the temperature of the ground, so yes from that perspective it “holds more heat.” But the temperature of a human is much much higher than the ground, and conduction is an extremely effective way to pull heat out of a human.