One of the controversial changes from 3e to 3.5 was giving all monsters square areas on the map. This meant that no longer would a horse control a 5’x10′ space, it controls a 10’x10′ space. This meant many more creatures would have to use the “squeezing” rules (which allow them to move at half speed through an area half as wide as their normal space).
It also means that a typical carriage with two horses running side by side operates as if it were 20′ wide–even though the actual carriage is approximately 10′ wide at most, and a typical road made for a carriage is barely more than 10′ wide.
And if you ignore that 20′ width of the two horses and just treat them as a 10′ wide unit, technically they’re “squeezing” (because the center of each horse is only 5′ apart, their 10’x10′ spaces are overlapping) and should only be able to move at half speed.
Therefore, a carriage pulled by two heavy horses (creatures with speed 50 ft.) moves at half speed, which is speed 25 ft., which is slower than an unencumbered human moves.
Which means I can outrun a stagecoach.