Exceptions overruled. Subject to the defendants’ exception the judge permitted an eyewitness to testify that the vehicle of the defendant Malvina Sulkoski, while backing up eight to ten feet, was moving at a speed of ten to fifteen miles an hour. There was no error. It was for the judge to determine, as a preliminary matter, the capacity and opportunity of the witness to make the observation, and for the jury to weigh it if admitted. Davidson v. Beacon Hill Taxi Sen. Inc. 278 Mass. 540, 541-542. Logan v. Goward, 313 Mass. 48, 51. Giles v. Barbosa, 338 Mass. 793, and cases cited. The exception is frivolous. Double costs are to be paid by the defendants. G. L. c. 211, § 10.