Motorbikes (but not self-drive cars) are available for rent, however Phnom Penh traffic is chaotic and dangerous even by Asian standards.
Motorbike-taxis (motos) are everywhere and will take you anywhere for a small fare. A trip from Sisowath Quay to Central Market costs about 2,000 riel (50 US cents). Fares are higher at night, you need to agree on a price before you leave.
As a huge number of scarred or maimed locals can attest, motorbikes - either as rider or passenger - are an extremely risky form of transport. On a motorbike you are exposed to the worst consequences of the city's dreadful pot holes, bad drivers and appalling accident rate.
Please be careful, especialy traveling with children. Watch the traffic, because the traffic won't watch you.
The country is developing at a fast pace, people can buy cars, but sometimes don't even know how to drive. It is obvious! They do not drive with high speed, but do often drive reckless. When they pull up, they don't look if anything is coming, they just go. If you decide to rent a motorbike or bicycle, be aware of that.
There are a few traffic lights, but don't count on every vehicle to stop, especialy motordrivers like to ignore the lights. At a crossing there are no rules, there is no politeness, you go as far as you can, and when you get stuck you have to find a way out again.
When you look at a main road like Monivong Boulevard near an intersection you may get a bit confused about how many lanes of traffic there are supposed tio be and which side of the road do they drive on. The answer is simple. there are two lanes more than are indicated and the lanes by the sidewalks are used by bikes and motorbikes which would be turning across the traffic at the next turn. What seems to happen is that you get into a flow (for example) for a right turn about 100 metres before the turn. Obvious !!!
You have to be careful about crossing the road, and you will ride on Moto's. They have an interesting left turn. Your go against the flow of traffic until you can get into the other lane. There is a wide varity of transport such as Pedicabs, Cars, Tuk Tuks, and some bicycles.