To expand a bit on the question.

If you had to resort to walking out of an area.

How many miles an hour do you think your group can make and over what kind of ground?
How about in a day or a week?
Are you counting time for feeding and sleeping?
Are you counting the weight of the packs slowing you down?

How about for a person you are looking for after they became lost?

I am like a snail.

Not being a bug out kinda guy, I would have to say zero...

Bug out on foot? Not going to happen. It's too far to any kind of better shelter than what I have right here and both directions necessarily cross the major earthquake faults so the damage would increase.

If a bad earthquake hit while I was at work I'd figure 2-3 mph depending on the amount of damage/obstacles en route on the trip home, assuming I had to walk. I have a light pack that's just perfect for such a trek. The walk will take 8-12 hours, but before I did that I'd hunker down for a few days and camp out with the supplies in my truck. Fix a few roads and I wouldn't need to walk home after all.

This is where practice comes in to play. Get yourself a handheld GPS, doesn't have to do mapping or be color or anything like that, just needs to be able to log. Go find a park with a trail, turn on your GPS, walk until tired then turn off the gos and go home. Now download/export the log file and use any of the dozens of programs or web sites that can plot and analyze your log to see how how far you went, how fast you did it, your average speed, the elevation, etc. Now compare with the route you may need to work someday to get some benchmark to compare to.

Projecting 3ish map miles per hour. I say map miles because of the upping and downing- driving my primary route, the map says 96 miles, but my odometer thinks more like 110. I'm not going to push it- if you push it, you are more likely to hurt yourself. If I'm on foot, odds are 3mph vs 5 mph isn't going to mean diddly squat.

Unless they are 4 mph zombies. In which case... ah, man....

All of the above - you'll only move as fast and as far as the slowest person in your group. For me that probably means my teen age daughter and wife, my teenage son I know could handle at least 7 miles a day with a full pack based on Scout hikes, my daughter not so much, my wife least of all. If we were hiking out of the area together with full packs we might expect to make as little as 5-7 miles a day, which would call into question why we hit the road in the first place. Like alot of folks I'm probably biased towards sheltering in place, and evacuating from an area as early as possible, and always via a car.

This all depends on the terrain, the weather, the pack weight and the level of fitness.

You may find the following interview entertaining, the experiences of the Maggie Hems about the enjoyment of the TGO challange crossing (A walk across Scotland in Spring), getting her pack weight down to 15kg and walking as a solo lady.

http://backpackinglight.audioblog.com/deluge/167d028d-85f3-c98d-73b5-ef1ca08588a0.mp3

Think we would be capable of 20-25 miles the first couple of days leaving DC. It's pretty level for 90 miles west. We routinely walk 10 miles on the C&O Canal, in 3 hours, carrying daypacks.

3 miles an hour on flat ground.

1 mile an hour on a steep climb (such as a mountain pass).

That's with a backpack and a gallon of water.

Have done day hikes up to 13-miles gaining and losing 2500-feet of elevation. Carrying a gallon of water-Gatorade (it's humid here) but not as much gear as I would in an evacuation scenario (for one thing I'd have to carry dog food in an evacuation scenario).

There are many variables, of course: motivation (radiation or bioterror behind me would provide adrenaline), pack weight, weather, terrain.

I have three mountain bikes and two bike trailers. Those would certainly be preferable to walking.

Your dog needs a pack so he/she/it can carry his/her/its own food.

The type of terrain, genetics, fitness and mental level of indidviduals, footwear, the weather and many more variables need to be factored in.

For me personally, I know that I can walk 7.7 kms (4.6 miles) in 1 hour 5 minutes on concrete with carrying a pack that weighs anywhere from 6 to 20 lbs. I do this at least twice a week walking to and from work which makes a single day total of 9 miles.

Even one way at 4.6 miles, I know that I could not keep this pace for hours on end. At best, perhaps 3-4 hours broken up over a day while the rest of time at a slower pace of perhaps 3 to 4 miles per hour. At this rate, figure on about a 28-30 mile day. Keep in mind that this would be the extreme and that I am better shape then 99% of my contemporaries and I also cycle over 2000 miles per year. How many days could I do walk at this rate? Maybe 3-4 days in a row before the feet and legs start to have problems as walking on concrete with a heavy pack is extremely hard on the bones and muscles.

To take into consideration other people who may be with me. My partner (ex marathoner / tri-athlete / adventure racer), she can keep up with me for a few hours but certainly not for many hours or days. If it was just her and I carrying 20 lb packs, 20-25 mile days are doable on concrete. If the weight of the packs were not a factor then this distance could be exceeded....believe me, I would be on the lookout for the first available baby stroller etc to carry the packs in.

As for other people in the group, my son and daughters who are in their mid teens are no slouches, both are good athletes and I know from previous experience, they are very capable of walking long distances and could probably outwalk us both in the long run (no pun intended).

For others who may be in a group, this includes other fmaily members or most friends, the distance would not be measured in miles, rather in hours. I would suspect that most would be done in by hour 3...even at a slow pace.

Walking offroad on mountain trails, foresty roads etc, this is a totally different matter. Depending on the terrain and weather, the days walking distance can be measured in single digits.

For example, a hike we regularly do in summer takes 2 hours each way over an 6 km (3.6 miles) distance and over some steep terrain. This same hike yesterday took almost double the amount of the time due to the warmer then expected afternoon sun heating up and melted the previously walkable on snow crust that had us falling through up to our knees into slush below. By time we got down off the mountain, my partner and I were tired, wet, sore and hungry. There is no way that this type of walking can be sustained for hours...let alone days on end.

In summer, the most I have covered in a single day in rough mountainous terrain over an elevation gain of 6200 feet is 17 miles...and it just about did me in. On less steep ground, 2 miles an hour is about our average and over a very long day, 20 miles has been done on good trails with pack weights between 25-35 lbs. Over a number of days, these distances would be much shorter with each passing day as this pace cannot be substained carrying any amount of weight.

Lastly, the above is based on our experiences and our physical conditioning. YMMV (Your mileage may vary)....really.