Section 2E. Notwithstanding the provisions of section 18 of chapter 90, the department for purposes of public safety and convenience may from time to time by regulations exclude persons and vehicles from state highways or portions thereof for such periods as it may deem necessary. The department may by regulation permanently prohibit or limit the operation of a low-speed vehicle, as defined in section 1 of chapter 90, on a state highway or a portion of a state highway, regardless of posted speeds, in the interest of public safety and convenience where it finds that use of the state highway or a particular portion of the state highway by a low-speed motor vehicle would represent an unreasonable risk of death or serious injury to occupants of low-speed vehicles because of general traffic conditions which shall include, but not be limited to, excessive speed of other vehicles, traffic volumes, use of the state highway by heavy trucks or other large vehicles or if the established speed limit on the way increases above 30 miles per hour beyond the point where a low-speed vehicle could safely exit the state highway. The commissioner of highways shall post signs where necessary to provide notice to the public of such prohibited or limited access to low-speed vehicles. Regulations promulgated under this section may provide for a penalty of a fine not more than $75 for a particular violation.