Text messages are being used in the fight against sex attacks on women in London taxis.
The creators hope the system will give women confidence
The system allows passengers to text details of the taxi and the driver's licence number while in the car.
The details are stored by Hail-Safe, the firm behind the scheme, so that in the event of an attack they can be used to trace the driver.
Police say more than 200 women a year are sexually assaulted in minicabs but illegal touts are often responsible.
The firm is urging women to text the details for friends who may be "a little worse for wear" when they say goodbye.
The driver should also be told that the system has been used in order to deter an attack.
Ceri Allen, the founder of Hail Safe said: "It will give women back their confidence in licensed cab travel, which is being systematically undermined by attacks.
"I don't want people to get these crimes out of perspective as 'taxi crimes' are rare, but this is no consolation if you become a victim yourself.
"We aim to encourage women to reduce the risk of becoming a victim of such crime, just by taking a few simple steps to increase their own personal safety."
The company says many victims of assaults in cabs or private hire vehicles do not report them as they did not get a good look at the driver's face or the vehicle make and registration.