While driving, the manual use of a mobile phone is heavily restricted due to the interference with the primary driving task. An alternative would be the use of speech interface. The current study aims to provide a guideline to implementation of a speech interface to the mobile phone. To do so, an expert evaluation was made and it revealed that a speech interface requires less workload, less performance degradation of the driving task than that of the keypad interface. To make speech interfaces more usable, new improvements are suggested. Subjective workload can be reduced and user satisfaction can be improved without degrading the primary task performance, for instance, by letting the user interrupt the speech of the phone, eliminating the repetitive words, letting the user know clearly what makes an error, providing a way to go back to the previous state, reducing the usage of keypad buttons and reducing the amount of the information on the screen.