These traditional transportation vehicles became popular in India where they were developed with Vespa and later imported to and built in Indonesia. Similar vehicles are known as rickshaw in Africa, Tuk-Tuk in Thailand and MotoTaxi in Peru.
Bajaj seat two passengers comfortably and up to five passengers - depending on the size of the passenger of course. Their areas of operation are limited to one mayoralty in the city. On the side of the driver's doors you'll see a big circle in which the area is designated ... Jakarta Barat, Jakarta Pusat, etc., with a different color for each mayoralty. The drivers are not allowed to go out of their area and aren't allowed onto many main roads, so routes may be a bit circuitous. But I guess that may depend on the driver too!
Fare determination is by bargaining. It's always best to ask an Indonesian what they would pay for a trip to a particular destination from your point of departure, and then bargain and pay accordingly.
A ride in a bajaj is hot, utilizing AC alam - or nature's air conditioning. The ride will also be noisy, smelly (car and bus fumes), bumpy, harrowing, and a grand adventure. One of the favorite maneuver's is when the bajaj driver decides to flip a u-turn in the middle of the road.
There is some protection from the rain, unless it's blowing hard. You'd think you'd have to be careful about robbery since the vehicle is so open - but it's not as common as robberies in buses. Having said all that ... bajaj are extremely convenient in many areas of Jakarta for a short drive.