It largely depends on what events might be going on during your visit…

Rocky Mountain National Park can be crowded during Memorial Day weekend and Red Rocks can be inaccessible if a concert is going on that day…but well worth seeing…

Downtown and Garden of the Gods is no problem…

You’ll have a lot of fun…

I don’t do much driving over Memorial Day weekend normally. However I do know google can give you a prediction:
Go to the following url:
http://maps.google.com/?ie=UTF8&ll=39.73993,-104.992218&spn=0.466751,0.933838&z=10&layer=t
there should be a box that says live traffic, click the change link next to it. You can view traffic averages for Denver and the surrounding area.

From a personal point of view, driving south of Denver is always backed up. Red rocks shouldn’t be too bad as there are a few routes to the general area. I really can’t even begin to speak on Rocky Mountain as I have never been there.

where in the rocky mountains will you be? where you go exactly what the traffic will be like.

i-70 and i-25 will probably be fairly congested, and you have to take i-25 to get to garden of the gods down in colorado springs. i think the earlier you start driving the better. memorial day weekend there is the bolder boulder 10k race (which i am running in!) up in the city of boulder, so traffic and parking in boulder and up to there is going to be insane. that will be highway 36 and 93 that will be clogged mostly. if you are going to red rocks you will probably have to take c470 to a nearby town called morrison. c470 may also be a bit congested, an alternate route would be 285. not sure of what other memorial day events are going on, i’m sure downtown has something. just in general count on things being a bit nutty.

If it is a nice day Garden of the Gods will most likely be packed but it is worth it, as soon as you find a parking spot take it! Mid-day traffic going south on I-25 should be fine, it is always more congested in both directions early in the morning and in the early evening no matter what day it is. Red Rocks is beautiful but I personally think Garden of the Gods has more to offer. And you can check out Manitou and if you really want you can head up highway 24 and there are some really nice sights up that way including driving up the Pikes Peak Highway. Territory Days will be going on in Colorado Springs so there are road blocks and people everywhere on the west side of town…just a heads up.

Generally traffic isn’t a huge problem in the outlying areas. I-25 and I-70 in the metro area can get a little congested during rush hours during the week. I-25 south of Denver and I-70 West of Denver can be bad on Friday afternoons and the opposite directions busy on Sunday afternoons. Memorial day may shift that second part to Monday afternoon. The one thing that you’ll want to avoid is the Boulder area on Monday morning. Bolder Boulder is one of the largest 10k running races in the country and is run every Memorial day causing huge traffic issues early in the morning and to some degree the rest of the day.