A reply from Paul Barter to my comments about cycling within NUS led me to www.bikely.com, where I found a not-tested route posted on www.bikely.com, and commented on it. Let me also share here how I would ride from Clementi to Toa Payoh ... .
My current preferred route is by Commonwealth Avenue West, North Buona Vista Road, Holland Road, Farrer Road, Dunearn Road, Chancery Lane, Thomson Road, cross at junction on to the footpath to cross the PIE, into Toa Payoh.
Other alternatives that I have tried and my findings are:
a) Using the Ulu Pandan Park Connector, entering from Clementi Avenue 4 or Clementi Road. There is a need to cross at the Ulu Pandan and Clementi Road traffic junction. I will use it if I have time to relax and enjoy some fresh air. I don't use it often because I dislike mounting/unmounting during my journey.
b) By Adam Road, Lornie Road, Thomson Road, Toa Payoh rise. This way seems longer as it leads to a bit too North relative to Toa Payoh. However, the main reason is the high degree of difficulty and risk in riding over an entrance into and an exit from PIE along Adam Road. The advantage is the ride along the wide footpath besides Macritchie Reservoir Park, which has a lengthy down slope, and easily make a short visit to the park.
c) In the earlier days when I was mainly familier with roads traveled by buses I took, from Lornie road, I would turn into Bradell Road instead, which is even further, as I didn't know that I can enter Toa Payoh from its west edge.
d) Sometimes, probably bored of the same roads, I would continue on Holland Road to Napier Road, Tanglin Road, Orchard Road, Scotts Road, Newton Road, Thomson Road. Usually in the evening/night time when there are people and happenings to watch and make the ride less monotonous. Also, these roads are less stressful except a little tricky to continue on Scotts Road instead of riding into Stevens Road, or to cross the traffic light at Stevens Road, which I dislike.
e) From Dunearn Road into Whitley Road and Mt Pleasant Road. The disadvantages are the part across the PIE exit along Whitley Road, and the Mt Pleasant Road is too quiet and thus dangerous for night riding, while the alternate short stretch of PIE seems safer but it is illegal to ride on. On Whitley road, I had to wait patiently for a no-traffic slot, otherwise it is very dangerous. Also need to beware of the Merryn Road opening. By Mt Pleasant Road is scary in the dark and dangerous too as drivers seldom expect cyclists on this quiet road.
f) For a couple of times, from Chancery Lane, I continued on Dyson Road, which is very steep, cross Whitley Road, which is risky due to fast traveling vehicles speeding up for PIE, or those that just exit from PIE, across the quiet heavy vehicle carpark below PIE, and turn right towards Thomson Road.