Assume a lawyer charges a client for the time it takes to travel somewhere. Then while traveling there the lawyer does work for a different client. In an older opinion, the ABA Standing Committee on Professional Responsibility held that the attorney would be acting unethically if he or she charged both clients the full amount of his or her fees. The lawyer, according to the opinion, did not earn a full fee for either client. The lawyer's efficiency should benefit the client, not the lawyer.
In a new opinion out of the 5th Circuit, the Court held a firm should be awarded only 50% of its usual hourly rates for time its lawyers spent traveling, but not working, in a bankruptcy proceeding. See In re Babcock & Wilson, 526 F.3d 824 (5th Cir. 2008).
Would it all be okay if the lawyer gets clients to consent? There is a interesting discussion on this topic in Legal Ethics Forum (once you get to this link make sure you scroll up to see the original post and then read the comments).