I've just done a consulting job that involved a great deal of travel time, back-and-forth, to the company's offices. The company has paid all the expenses, but as I've spent a full 3-4 days of actual travel time, I wondered whether to bill at least for part of that time? What do you all think?

Best to make this clear before you start work i.e. include in the contract that travel time is chargeable at an hourly rate of X over 60 minutes per day.If you haven't then I think it is still reasonable if you have done excessive travelling. If you were a lawyer you would be billing by the hour, It amazes me that consultants are happy to work a 12 hour day but only get paid for 8 hours.I am quite bad at this. Charge a day rate and regularly do 12 hour days. However once I got up at 6am and got home at 2am the next day. Billed 2 days for that.