I’m new. I would like to fix the speed limits on a US highway. I can see how to change the tag in http://www.openstreetmap.org/edit but I don’t know to break the road into segments (sections?) so I can set different speed limits in different places. Is there a tutorial on how to do these types of edits? I hate to bother people each time I have a new question… Thanks!
In JOSM you mark the node where you want to split the way and press [P]. It’s likely the same in other editors.
For JOSM you could have a look at it’s documentation, for other editors you’ll likely find something like this linked on their Wiki-Page (z.B. P2, iD). For “new questions” you could simply re-use this thread for your “newbie”-questions, so if someone feels bothered he can ignore it 
How is this done with the Edit mode in the osm web site (iD)? Does adding a node to a way where the speed limit changes what I need to do? How exactly is that done? What type of node? thanks…
Since I’m not using iD I can’t answer that. Maybe someone else?
But I would recommend to switch to another editor, since there are several serious bugs in iD…
If there is no node you have to place one there.
The node may have no tags (except if there’s another reason).
What I have discovered for the iD editor: To enable a change of speed limit in the middle of a segment of road do the following.
Click on the road to select it. If there is a node, small circle, on the road where the speed limit changes, skip the next sentence.
Double click on an empty place on the selected road and a node, small circle, is added.
Click on the node to select it, a “palette” appears with one of the options a pair of scissors. Clicking on the scissors splits the road segment into two segments.
Select one segment of the road and set it’s speed limit.
Select the other segment of road and set it’s speed limit.
That sounds like the correct procedure, yes.
@Kit Richards: I’m sorry that you received a stock answer “Dont use editor X”, and I’m glad you worked out the answer for yourself. The forums are, for reasons I’ve never worked out, not very popular for English speaking users. This sort of question is perfect for the OSM Help site which is more heavily used.
@rayquaza: consider being more helpful to OSM newcomers, rather than telling them they are using the wrong editor: it’s a great way to put people off. Not everyone has the persistence to persevere on their own.