There are a few things you should know before riding a bike in Belgium. Although the rules of the road are pretty similar to those in the US there are a few important distinctions.
One is the bike lane ("fietspad"). Nearly all roads have one or two bike lanes and cyclists must use the bike lanes. If the road has a bike lane on either side, you simply ride in the direction of traffic. If there is only one bike lane, you must cross the street and ride on that bike lane. These bike lanes are two way by default and that is often indicated by paint marks. Remember that mopeds must also use the bike lane so watch out for fast traffic. Always ride single file on a two-way bike lane.
The bike lane is usually separated from the main road by a section of cobbles, hedges or plants, or a small neutral zone. Sometimes it is simply carved out by broken white lines. Near city centers it is often a part of the sidewalk that is painted red.
One of the most tricky traffic rules, and one that fools even the locals and leads to many accidents, is the "voorrang van rechts" (yield to the right) rule. It is the default at any intersection that those coming from the right have the right of way, regardless of the size of the road you are on. This rule applies to all intersections, unless overruled by a specific sign. Most city centers have no signs and so the rule applies there.
Furthermore, the law states that those who stop when they have the right of way, loose that privilege, so drivers are inclined not to stop or slow down too much when coming from the right. Whenever you approach an intersection in Belgium, always look to the right and unless you know you have the right of way, be prepared to stop.
The triangular traffic sign with an X warns road users that the voorrang van rechts rule is in effect.
There are specific signs to indicate the voorrang van rechts rule does not apply. These signs let you know that you are on a main highway and that you have the right of way. The older sign is a triangle that shows a tick arrow (signifying the road you are on) with one or two small side-sticks (signifying the minor roads you are crossing). It applies to the upcoming intersection only.
The newer sign, that is also used in other European countries is a yellow diamond. It stays in effect until another sign showing the same diamond with diagonal bars appears.
There are very few stop signs in Belgium, and stops are rarely used to slow down traffic as is common in the US.
Also be aware that Belgian traffic lights are positioned before the intersection and not across from it as is done in the US. Many intersections with traffic lights have cameras and you will get a fine when you run the light. Automatic cameras are also used to enforce speed limits. There is no right on red rule in Belgium, ever. You cannot move beyond a red or yellow (orange) traffic light placed to your right. You need to wait for a green light. If you run a yellow light you will get a ticket too.
Traffic circles or roundabouts are another feature you need to watch out for. The general rule for a traffic circle is that those on the circle have the right of way. As a biker you are riding on the outside of the circle and so you have to yield to those on the circle who want to leave the circle -unless there are shark's teeth as in this photo below.
Also note that on Sunday mornings the roads belong to cyclists. So many amateur (and pro) riders are out, riding in large packs with one or two follow vehicles, that they take up the entire road. On Sunday afternoon, those same roads will likely be blocked of for a bike race, so be patient when driving.