Over a thousand learners have registered for the driving test trials according to the latest figures from the Driver and Vehicle Standards Agency (DVSA).
As of 17 June, 1050 learners and 430 ADIs had registered for the trial and, so far, 267 tests have been conducted.
Out of those, there had been 77 passes and 57 fails of the trail sat nav test with 63 passes and 70 fails in the control group.
This gave pass rates of 57% for the trial group and 47% for the control group.
Although the DVSA said it was progressing well with numbers, it needed more volunteers to ensure its researcher, TRL, was able to come to valid conclusions.
To this end, it is adding Bedford, Bournemouth and Ipswich test centres to the trial "to help with this and may add other centres if necessary".
DVSA chief examiner Lesley Young said the agency was "aware that there have been a few issues for ADIs booking tests as part of the trial".
She added: "Based on a review of the test booking process and feedback from ADIs, we have put in place a process so that ADIs can request a test slot be converted from a standard test to a trial test if it is more suitable for them and the learner driver. In most cases this can be accommodated provided the examiner is trained to conduct the trial test."
In addition, the DVSA has taken steps to ensure learners on the trial were not disadvantaged when booking a driving test.