The mention of asphalt roads in pioneer-era America was rather anachronistic. Tarmac (or tarmacadam), what much of the world calls asphalt, wasn't created until 1901 (though earlier variants date to 1834). It's a combination of macadam, or crushed gravel, and tar -- bitumen, or the heaviest components of oil (also what's principally present in the Canadian tar sands). Though yes, asphalt was used locally as early as the 6th century BC.

Tarmac is also a topping material for roads. It stabilizes crushed gravel, but doesn't of and by itself provide much in the way of load-bearing capability. For that, substantial earthworks are required. Roman roads required earth removal, a layer of land, gravel and concrete, crushed stone and concrete, and finally a topped layer of dressed cobblestones, as well as drains to either side.

http://spqrlive.com/communities/8/004/007/457/098/images/452... http://spqrlive.com/roads/4536983176

While that makes for an excellent load-bearing surface, it's a bit rough for high-speed travel. That's where tarmac comes in.

For wagon trains, occasional travel over largely unimproved dirt roads was possible, but any amount of water would result in mud, with wheels and wagons sinking into it. Wheel ruts from pioneer crossings from 1840-1869 are still visible today, 150 years later:

https://en.wikipedia.org/wiki/Oregon_Trail_Ruts_(Guernsey,_W...

This bit on the first macadam road in the U.S. might be interesting:

http://web.en.rmutt.ac.th/highway/eng-macadam.html

The first macadam surface in the United States was laid on the "Boonsborough Turnpike Road" between Hagerstown and Boonsboro, Maryland. By 1822, this section was the last unimproved gap in the great road leading from Baltimore on the Chesapeake Bay to Wheeling on the Ohio River. Stagecoaches using the road in winter needed 5 to 7 hours of travel to cover 10 miles.

It also details some of the intermediate phases between macadam and full asphalt concrete roadbeds.

http://en.wikipedia.org/wiki/Tarmac