According to LTA, road hogging is an offence.

Vehicles with speed limit of < 60km/h failing to drive on the left-hand lane of expressway, and vehicles at slow speed failing to keep as close to the left-hand side of the road as possible will get a maximum of $1,000 and/or 3 months jail for first-time offenders.

Vehicles obstructing other vehicles moving at a faster speed will be fined a maximum of $1,000 and/or 3 months jail, with 4 demerit points for first-time offenders.

www.onemotoring.com.sg/content/onemotoring/en/on_the_roads/educational_materials/_jcr_content/main_par/tabctrl/tab-5/download_1073148309/file.res/NoRoadHogging_Brochure.pdf