It looks like a dying profession ... from many Asian cities trishaw or samlor have already vanished largely because they have been replaced by much faster motorbikes (and you won't even have to work that hard with your legs then). Now moped taxis and tuk tuks compete with samlor and they are winning - in places where this is allowed or prefered.
You could actually had pretty rides in past, but now it is more hazardous due terrible traffic and pollution in Phnom Penh. Feels vulnerable. Only fewer and fewer tourist even do that ... mostly because of safety concerns, yes. Another issue is also that you could be (as well) easy target of a robber if you display your flashy property and money. The roads have become busier than ever before... virtually all life happens next to the road.
... and still, for a trishaw man his bicycle often means his only property. In his cycle he lives. Therefore I would still go for a ride or two in Phnom Penh sometimes, as you never know when this will be finally gone from the streets forever. Things change.
Whenever you like a quiet way to look around, without walking for hours, the rikshaw is your transport way. They don't make noice, nor smoke, and you will have all the time to look around and to take videos or pictures.
You will find them on every corner of the street and around market places.
Prices for some km fare are about 2$.
In Phnom Penh hundreds of motor bikes roam the streets. The best and most inexpensive way to get around is to take a tricycle (rickshaw). Around Central market there were many of them waiting to take fares from both locals and tourists.
There are several tour operations in PP who off tours to other parts of the country also - the transport was fairly old and rattly. Most of them provide cars and mini-vans for rent.
Samlors are a popular non-motorised way of getting around Phnom Penh at a slower pace. I had a chuckle when i saw this one sitting in the street - love to know the origin of the seat cover!