An unsafe driver is someone who operates a vehicle in a dangerous or careless manner. 

Just some examples of unsafe driving include:

Speeding

Reckless driving

Improper changing of lanes

Following too closely

Not using blinkers

Turning improperly

Failure to yield to right of way

Railroad grade crossing violations

Failure to obey a traffic light

Unsafe Driving BASIC Factsheet

https://csa.fmcsa.dot.gov/documents/fmc_csa_12_008_basics_unsafedriving.pdf