A monthly allowance may be given to designated employees who use their private automobiles on a frequent basis for official authority business in and about the municipality in which the project is located, provided the authority has ascertained that such travel and its frequency are necessary, that other means of transportation are not available, or if available, their use is not to the best interests of the authority; and provided, further, that the Division of Housing has approved such allowance. Such employees must have the required insurance coverage on their car. Payment of such allowance can only be made on or after the last business day of the month for which the allowance is being paid.