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You could just, and I mean just, make it from the DC area to Denver in three days via either I-70 or I-80, but that would be three days of fairly heavy driving on flat and smooth high-speed highways with only minimal stops. If, on the other hand, you can afford a fourth day then all sorts of great RoadTrip possibilities open up. The primary one I'd suggest is splitting the difference between I-70 and I-80 (with its heavy tolls until well past Chicago). Such a route would utilize many of the great four-lane surface highways of the 'US' system to both maintain decent speed while also giving you the chance to see something of small town America.

Here is one such possible route of the many available to you. Leave the DC area on I-270/I-70 to Hancock MD and then continues westward on I-68 to I-79 north and I-70 west past Wheeling. At St. Clairsville OH take OH-9 and US-250 to New Philadelphia. This will take you through Ohio Amish countryside. Continue on US-250 (it will spend a bit of time duplexed with I-71) to Wooster and US-30 west. That will take you to Fort Wayne IN where you'll switch over to US-24 west. Use I-74 to get through Peoria IL and then pick up US-34 at Galesburg. Enjoy US-34 all the way across Iowa to Omaha and then use I-80 for a bit to Grand Isle NE. From there westward US-30 parallels I-80 along the Platte River and the old Oregon Trail with several historic sites and geologic landmarks. Finally, from Big Springs NE to Brush CO you have a similar situation with US-138/US-6 running parallel to I-76 allowing you to jump back and forth depending on whether you want to mosey through towns or cover some ground. I-70 would then get you the rest of the way into Denver.

Such a route offers you the chance to take a break in the many state parks or even small town playgrounds whenever your son needs time out of the car. It also offers you a break from the monotony of hour after hour of straight, flat, constant speed driving. It will take more time, but that can be mostly time well spent doing something other than just driving for three days straight. There are similar options built around routes like US-36 across northern Kansas and Missouri, but the main point is that you are by no means limited to just I-70 or I-80.

AZBuck