1. A method for determining a level of degradation of a road marking, as executed by a determination unit comprising a processor, a database, and an image capturing device having a plurality of shutter speeds, wherein the method comprises: generating a plurality of reference images of a reference road marking by the image capturing device photographing the reference road marking with each of the plurality of shutter speeds; selecting a reference calculation area from each of the plurality of reference images by the processor; generating a plurality of reference data by the processor performing a first image analysis procedure on the reference calculation area of each of the plurality of reference images; storing the plurality of reference data in the database by the processor; generating an image of an examined road marking by the image capturing device selecting one of the plurality of shutter speeds according to a light intensity of an environment and photographing the examined road marking using the selected shutter speed; selecting a target calculation area from the image of the examined road marking by the processor; generating an examined data by the processor performing a second image analysis procedure on the target calculation area of the image of the examined road marking; retrieving one of the plurality of reference data from the database that has a same one of the plurality of shutter speeds as the selected shutter speed of the examined road marking, as performed by the processor; determining a color difference between the retrieved reference data and the examined data by the processor performing a difference determination procedure between the retrieved reference data and the examined data; and outputting a determined result of the level of degradation of the examined road marking according to the color difference, wherein the plurality of shutter speeds comprises a first range, a second range and a third range, wherein the first range is from 1/200 to 1/800 sec, the second range is from 1/800 to 1/1200 sec, and the third range is from 1/1200 to 1/1800 sec.