They say the incident involved a white BMW and grey Peugeot 3008 on the northbound A22 at Little Horsted, south of the A26 junction, just before 6pm on Wednesday, February 14.
The cars were close together driving along the road, in an apparent dispute over their respective manner of driving, although there was no collision at any point and nobody was injured.
At one point the driver of the Peugeot felt a bang and saw that his rear windscreen had been smashed. The cars continued north along the Uckfield-By-Pass until the BMW turned off and disappeared.
The Peugeot driver reported the incident but the BMW occupants have not yet been traced.
If you drove along that stretch of the A22 at the time and have any information about what happened, please contact the police online or by calling 101, quoting serial 904 of 14/02.