Belgium has a very extensive motorway system, one of the densest in the world. Most motorways are 2x3 lanes, and the amount of trucktraffic is incredible. However, traffic jams do occur often around Brussels and Antwerp.
In the French-speaking Wallonie region are less traffic jams, but mostly the right 2 lanes are taken by trucks, so traffic can be busy, especially around Liège, Namur, Charleroi and Mons.
The map above shows the A and R-3 numbers. A means Autoroute/Autosnelweg (sometimes called Autostrade), and R-numbers represent the Ringroads, which isn't nessecarily a motorway standards road.
However, in daily use, the European E-numbers are used. A numbers only appear on signs when there isn't an E-number, and A-numbers can be found on every milemarker in Belgium, despite being an E-route.
The Brussels ringroad is the R0, this is the busiest road in Belgium. Fast after that, we see the Antwerp R1, which is very busy as well, because of the lack of better ringroads around the city.
The E313 has only 2x2 lanes, and is crowded with trucks, so traffic jams in excess of 20km occur often here.
This is a map of the more daily used E-numbers.
Some Autoroutes in Wallonie do have names, like the E42 is the Autoroute de Wallonie and the E411 is the Autoroute des Ardennes. The A54 is named La Caroloregienne.
The Belgian non-motorway network is quite extensive too, sometimes with 2x2 lanes and a 120km/h limit. Some are very motorway-like, like the N25 between Louvain-la-Neuve and Nivelles, or the N97 near Dinant.
Unlike the Netherlands, Belgian cities tends to have a lot of suburbs, which create a lot of traffic. The busy Antwerp port generates a lot of truck traffic along the E19, E34 and E313.
Belgium is one of the busiest transitcountries in Europe. The quality of the roads was extremely poor a few years ago, with almost every Autoroute filled with potholes and trackformation, however, most of that has changed, but there are some worse motorways still leftover. Maintenance wasn't much of an issue the last 30 years, so there is a lot of construction to do. Motorways like the E25 and the E411 to the south are completely renovated, and of good quality now.