28 C.F.R. § 21.4
The fees and allowances of fact witnesses, other than those covered by § 21.2, attending at any judicial proceeding, shall be a follows:
(b) Allowable transportation expenses. A witness shall be entitled to transportation expenses based on the means of transportation reasonably utilized (based on the nature, duration, location and distance of travel) and the distance necessarily traveled from and to such witness' residence by the shortest practical route and the fastest means of transportation available in going to and returning from the place of attendance. Additional costs incurred (including attendance fees and subsistence allowances) because of a slower means of transportation must be justified for consideration.
(c) Subsistence allowance. A witness (other than a witness detained in custody) who is required to be away from his or her residence overnight is entitled to a subsistence allowance. A witness who is not required to be away from his or her residence overnight is not entitled to a subsistence allowance. The witness' subsistence allowance shall not exceed either the per diem rate or the actual subsistence allowance rate prescribed for Government employees for the place of attendance. These rates are established by the Administrator, General Services, for areas within the conterminous United States; the Secretary of Defense for areas of the United States other than conterminous; or the Secretary of State as published in the Standardized Regulations (Government Civilians, Foreign Areas) for foreign areas. The witness' subsistence allowance shall consist of a meal and miscellaneous expense portion and a lodging portion. When an overnight stay is required, the witness shall be entitled to:
(2) The lodging portion for each night the witness is required to incur a lodging expense.
The meal and miscellaneous expense portion shall be 50% of the authorized subsistence allowance rate rounded to the next whole dollar in an actual subsistence rate area, or 45% of the per diem rate rounded to the next whole dollar in a per diem area. The lodging portion shall be the difference between the meal and miscellaneous expense portion and the authorized rate.