Road transport
What if I told you 480 kilometers? This is the total road network (of which only 743 kilometres are motorways), to which I should add that 1/3 are not paved. In short, it could be better, because it is below the average for Southeast Asian countries. But it certainly does not hinder visiting the essential part of the country (for comparison, France, a country 4x smaller, total 1 million kilometers).
The buses
Except for the Papua part of Indonesia and the Moluccas, the bus will probably be your mode of transport of choice.
On inter-city routes, you will only benefit from air-conditioned buses if the roads are wide, therefore mainly between large cities. Otherwise, it will be more like local buses, often narrow with little space for luggage that you will have to fit somewhere under your feet or knees.
Knowing that in Indonesia, people smoke everywhere, I hope that the idea of traveling with the constant smell of cigarettes does not bother you. (I know not sure if this is also the case in air-conditioned buses, because we did not have the opportunity to test any).
Another peculiarity, hold on tight, Indonesia has quite the reputation of "fast & furious" for its bus "pilots".
We can distinguish 3 classes:
- Economy class (economy) : cIt is the cheapest, so the conditions are proportional to the rate. No air conditioning, no spaces (possibility to travel standing up), it's slow because it will stop to pick up people all along the way, but it's picturesque!
- Express (patas) : cIt's the same as in " Economic " except that theoretically, they don't stop at the side of the road recover people passing by, but only at city bus terminals, hence the more “express” aspect. Some journeys will even have the luxury of air conditioning.
- Air-con nozzles (executive): if you know Thailand, it's similar to the VIP buses. You can choose between different price categories, depending on whether the equipment includes reclining seats, toilets, TV, karaoke (better not have it working...) or small snacks. These buses must be booked in advance and you can choose your seat.
Economy fares are particularly "cheap". Even though my trip starts on that date (it was in 2011), we had local bus journeys costing no more than €2 per person. From what I've read, we always get away with quite low amounts (even if it's more than at the time of course).
What is common, however, is the change of buses. The tourist sites are far from the cities.

The guy on the roof is there to tie down luggage if needed.
An example, go from Dieng Plateau to Yogyakarta : you must first take a bus to go back down to Wonosobo, the nearest city (1h30, 20 Rp). The bus does not necessarily go to the city terminal, so it may be necessary to get dropped off at the nearest one and then take local transport (000 Rp) to the Wonosobo terminal, then reach Magelang (3h000, 2 Rp) and finally finish the journey to Yogyakarta (another 30h, ) i.e. 25 different buses and 000-1 hours of driving in total to cover barely 3 km.
Despite all these changes, the total cost is normally between 60 and 000 Rp (it depends on how you get ripped off, because that too is still common on the bus to pay for something extra than you knows not why), which is barely €5.
Afterwards we haven't tested it but in local transport, there is also the famous Bemo, a sort of rustic minivan.

An old Bemo who deserves his retirement.
Taxis
We didn't take many taxis during this stay, especially because when we could, we rented a scooter on site and also because we were really on a budget, so we preferred to walk if possible if the distances weren't too great (despite my 20 kg bag at the time).
From my calculation, I see that the rates were around 3 500 Rp per kilometer, according to the table below, I was right. While the pick-up and per km rates vary by city, my example is for Surabaya, which is 3 Rp the km.
To cite an example, to get from the airport (Juanda International Airport) to our hotel (The Family Guesthouse), there were 16 km, we paid 150 Rp…
If you do the math, we should actually have paid less than 60 Rp, or 2x less… But it was our second big trip so not yet well prepared… We didn't think too much about it, because we hadn't checked the distance and it was about 500 Baht at the time, which didn't seem huge to us either. (in Thailand, many people get scammed with airport-hotel trips for 600 or even 800 Baht when in reality it costs half as much have).

Taxis waiting for customers at Ubud market.
Conversely, to get to Surabaya station the next day, we had a little over 6 km and this time we paid 20 Rp, which is good.
In Indonesia too, transport applications are becoming more widespread and to make your taxi journeys easier, you can download:
- Grave, popular in Thailand and also available in Indonesia
- Duck, purely local app of the same style (link for the android version)
There is also the possibility, as everywhere, of renting a car with a driver. It is useful for simplifying part of the journey since it involves letting yourself be transported without worrying about the direction, but more quickly than by public transport.
It is rare that we use it for budgetary reasons, firstly, but also because we always prefer our independence. And to this day, Indonesia is one of the only countries where we tested part of the route with this method, in order to finalize our stay in Java. (to visit the volcano bromo et Ijen, to the ferry to Bali).

Our 4x4 taking a mechanical break as we descended from the Ijen volcano.
Becak (rickshaw) etc.
Local transport par excellence, still very present in small towns or certain districts of large cities. From our experience, it was still quite expensive compared to a taxi. On every journey (often around a kilometer) we paid around 20 Rp.

Becaks parked on the street in Yogyakarta.
More precisely, to give an example again, Poor, had paid 20 Rp to go from the bird market to a Chinese temple (Eng an Kiong Vihara, 1,2 km).
On the way back to our hotel (near the train station roundabout), we had 1,6 km and we paid 25 Rp (around €2).
In several cities in Java, we also came across horse-drawn carriages, this was the case in Yogyakarta and Borobudur in particular. No idea of the prices, the only time we used them was to go from our hotel in Borobudur to the bus terminal next to the market, like barely 1 km, offered by the hotel.

Horse-drawn carriage parked next to Borobudur Market.
Vehicle rental (car or scooter)
True to my desire for independence, I tested driving two-wheelers in Dieng Plateau on Java and to gravitate around Bali.
Already used to driving in Thailand, I already had some experience in Asia, although at the time I was still a "beginner". As much as I had no particular worries on the Dieng plateau, I had a few scares on the mountainous roads of Bali, returning to Ubud at night on a poorly marked road with potholes.

Mountain road in the north of Bali, a few minutes later it was pitch black.
Be careful! Here are some rules regarding motorcycle and scooter rental :
- Small motorcycles and scooters can be rented for around 100 Rp per day (€000).
- Wearing a helmet is mandatory, don't let the "holiday" aspect tempt you to want to imitate the locals.
- In Bali in particular and other places suitable for surfing, you will often have a vehicle equipped to transport a board.
- A license is mandatory, although it is rarely required by rental companies, it is not necessarily required by the police and even less by the insurer.
- Some insurances simply do not cover you in the event of an accident involving a two-wheeler (as a driver) or do not cover you if you do not have the appropriate license, read the conditions carefully (yes, yes, the small print).
Gasoline is found everywhere along the roads, the small shops offering one-liter bottles (sold 10 000 Rp or, about 60 cents).
An international license is required for all rentals., it is free and must be requested from the prefecture (requests are now made online). Be careful because the waiting times for obtaining it are sometimes very long (more than 2 months!).

On my “bolide” at Borobudur.
If you rent a scooter, the B license alone or the A1 is not enough, to my knowledge, just like in Thailand, it is a valid A license that is required to be legal in Indonesia.
For car rental, we find prices from 300 Rp per day (18 €) in tourist areas with limited insurance.
If you prefer not to drive yourself, it is possible to rent a car with driver from 600 Rp, it remains a good compromise in terms of freedom of movement and cost, especially if you are with family or in a group.