What is the easiest place to cross the US Rocky Mountains, going to Oregon, in a large truck?

Is your concern traffic, snow, difficulty of the road, ...?

Google Maps thinks that I-80 and I-84 will be your best routes to Portland from, say, Kansas City (other parts of Kansas won't have a terrible time reaching I-80). US Interstates are almost always designed well, with reasonable curves. If you are concerned, you can check the height of the mountain passes on each route; the higher the passes compared to the base elevation, the more up and down driving you have to do.

Secondary highways are often less well engineered. I'd stick to the Interstates.

One of the gentlest routes through the Rocky Mountains is the Crowsnest Highway (Alberta and British Columbia Highway 3), and your question originally didn't specify that you wanted an American route through the Rockies, but I'm assuming that with a truck, you don't want the hassles of clearing customs and immigration twice, so I'd stick to I-80 and I-84.

Check out Google Street View from a computer to get a sense of what your'e going to face.

Be sure to avoid riding your brakes on downhills; this can cause your brake fluid to boil causing brake failure. Use engine breaking wherever practical, and stop to cool off your brakes occasionally if you need to use them frequently. Whatever you do, don't ride your brakes constantly during a downhill, as this heats up the fluid significantly.

The answers by Jim MacKenzie and Harper are both full of good advice, and the suggestions to use I-80/I-84 are great. I have taken that route with a 16ft box van.

I just want to note one more thing you should be aware of. Based on your description, it sounds like you might be going to the Portland/West Coast area. If so, you are most likely going to take I-84 from Boise. Along that route, you will cross another series of mountain ranges. So, just preparing for the Rockies isn't enough. The area that is most likely to give you trouble in my experience (and my extended family who lives in the area) is known colloquially as "The Blues" in eastern Oregon. A series of steep grades, high traffic, and a penchant for bad weather make it a particularly dangerous section of highway.

I can't really suggest more than to follow the advice that has been given; drive slowly, stay calm, avoid overbraking, and be willing to use the escape ramps if you do lose your brakes. It is just one more obstacle to be aware of.

If you would like some more information on what to expect, this link explains some of the dangers of that section of I-84. The information is somewhat outdated, but still gives a decent description of what to expect.

And welcome to the Pacific Northwest!

Something other answers haven't gone into details about are how the interstates are designed through most of these passes. On the steeper uphill sections, the interstate expands into extra lanes to allow for big trucks that have trouble moving at reasonable freeway speeds (you have to move all the way to the right). Typically, if you are slow on an uphill road, you are required to turn on your hazard lights.

On the downhill portions, as long as you are using your engine to slow you down, you should be fine. If you have a runaway/brake failure situation, there are pull-offs at regular intervals that are made of large gravel/stones which will stop you pretty quickly. I have lived in the Rockies my whole life and have only ever seen one truck that needed to use a runaway lane. It is really a last resort because it is very difficult to get your truck out of one once you are in it.

If you take I-84, the worst spot is probably going to be Deadman Pass before you get to Pendleton, Oregon. There is about 2000 ft in elevation change over a few miles. This is totally doable and most drivers, even out of state ones, have no problem navigating it. Be careful with your brakes for descending and heat for ascending and you will be fine.

Also, enjoy the mountain drives. They are very beautiful and should be a treat for a Kansas-dweller like yourself.

As others have noted, the U.S. Interstate Highway system is well built and likely your best choice. Given that you are driving in June and the weather ought to be nice, my biggest concern would be long downhill stretches where riding your brakes can result in brake failure (long before you run out of downhill).

If you were thinking of heading west on I-80 then making a right turn onto I-5 North in Sacramento, you would face two stretches of downhill driving (coming off Parley's Summit approaching Salt Lake and west of Truckee in California) that have a bad enough history for there to be dedicated Runaway Truck lanes.

If you can cobble together a specific enough search to find out where brake failure is a real danger on the candidate roads for your journey, I would weight such info highly in my choice of routes.

Happy travels and safe trip!

Having actually done exactly what you're describing, I'll agree with what others have said: in mid June, any Interstate highway will be fine as far as road conditions are concerned. When you consider the number of commercial "eighteen wheelers" that cross the Rockies per day, it has to be this way.

I will also emphasize that brake fade is your biggest number one enemy. Period. There are four ways you can help mitigate it

1. Downshift to a lower gear and let the engine do as much of the braking as you can get it to do. Preferably all of it.

2. If you just can't manage on engine alone and have to use the brake pedal, do not "pump" your breaks, instead keep them on with a light touch continuously, till you can get your speed back under control and revert to engine braking alone.

These two are directly from the truckers my wife and I were talking to via C.B. radio as we crossed over the Rockies.

3. Slow down. This is basic physics. You're trying to get rid of your trucks kinetic energy with your brakes. The slower you go, the less energy per second you need to get rid of. See below for a discussion of the details of this. Less energy per second means less brake heat up.

4. Take a break from time to time. This is a good idea anyway, and by taking a break, you get a rest yourself, and let the brakes cool down.

Good luck, and drive safely!

Detailed physics discussion. Ignore this if you don't care. A moving vehicle has two forms of energy: potential and kinetic. Kinetic is energy due to its movement. Potential comes from a lot of sources, we care about mechanical potential energy due to being at the top of a hill.

Left to its own devices, as the truck rolls down the hill, potential energy is converted to kinetic, and you lose altitude but gain speed. You convert the same amount of energy for every foot of altitude you lose.

Therefore if you lose fewer feet of altitude per second by slowing down, you lose less potential, and gain less kinetic per second. Hence your brakes have less kinetic energy they need to convert to heat per second, and thus they stay cooler.

All answers have given great suggestions, so I will suggest something quite different. If you are not pressed for time you could head south and then west through Albuquerque, or you could even go more south, in any case you can then take I-40 heading toward Bakersfield and catch I-5. You can't completely avoid the mountains but by going this route the slopes are much more gentle than crossing the Rockies and going through NV, CO, UT. If you leave from southern Kansas you are not adding too many extra miles.

I drove the exact route, other direction, from Portland, OR to Overland Park, KS, in a 26-foot long U-Haul towing a compact car behind it.

The route I picked is here on Google Maps.

The steep parts were outside of Salt Lake City, near the Wyoming border, but everything else was quite easy. The Wyoming section is at very high altitude, so the engine seemed to slow down there.

A view from the UK. Here we use 'stick shifting' as the norm (only a few have automatic gearboxes), so down shifting is well understood in terms of 'clutch control' (we even have a separate pedal for clutch control!). However many still are not good at down shifting to control speed as an alternative to braking.

On simple techinque for practice is the "No brakes game" - with the vehicle (your normal one) in good working order, drive around your local area, including places with a few rises and falls, and USING THE GEARS ONLY (and accelerator/gas pedal), try and control your speed to smoothly proceed through the route. (touching the brakes isn't a failure, just an indication that road conditions needed it at that moment... should have seen it coming... etc. hey-ho, better next time.)

Obviously allow plenty of space and anticipation (look ahead) so that the engine braking is given time to work and you get the time to get used to it.

The value of the anticipation (looking ahead part) is probably of most value as it provides safety in depth. In the UK, with the stick shift, we can also get into 'double de-cluching' but that's really for old vehicles which don't have synchro-mesh gearing (you needed to make the engine revs match the geared-wheel revs, so that the different parts of the gears would slot together smoothly without 'crashing' - historically brakes were very ineffective!).

The key is that you can practice gear shifting without any significant hills, which should give extra confidence that you at least have the mechanics in place.

Enjoy the trip.

I90 through Montana/Idaho/Washington has a couple of mountain passes but they aren't as treacherous in the summer. Then you drop down through Kennewick/Richland/Pasco to the I84, the route is flat once you hit Coeur D Alene. Or just take the I80 to I84 in Ogden and head through Boise , its pretty flat all the way to Portland once you get to Ogden.