A secure toll payment system is realized by transmitting a changeable encryption code from roadside equipment at a toll plaza to a moving vehicle. Thereafter, the moving vehicle uses it to encrypt payment information according to the Data Encryption Standard algorithm. The moving vehicle transmits the encrypted payment information to the roadside equipment which performs a credit or debit transaction. Because the encryption code changes from time to time, so too does the nature of the signal which is transmitted by the vehicle; and fraud, based on electronic eavesdropping, is substantially eliminated. The encryption code comprises an 8-bit random number and a time/date number. Vehicle-mounted apparatus includes a transponder unit and a portable smart card which inserts therein. The roadside equipment includes a pair of spaced-apart antennas that are sequentially located along an express payment lane at a toll plaza and a computer (Plaza Server) which controls them.