One in three optometrists say they have seen patients in the last month who continue to drive with vision below the legal standard, their association said.
Motorists must read a number plate from 20m (65ft) in the practical driving test, but there is no follow-up check.
The Department for Transport said changes to eyesight should be reported by motorists to the DVLA.
"All drivers are required by law to make sure their eyesight is good enough to drive," a spokeswoman said.
