First, let's identify Sustainable Safety. First, functionality. Roads are through roads, access roads or distributor roads. There are two main kinds of through roads: expressroutes, or what the Dutch call autowegen, limited to 100 km/h, in Canada that would mean a 2 lane highway with a shoulder, control of access, no access from 40 km/h roads in particular, grade separation to the greatest possible extent, a divide between the two directions if possible, roundabout control if grade separation is not possible if possible, and when not a divided roadway, having a painted buffer between the two directions, and freeways, limited to 130 km/h, with at least 3.7 metre wide lanes, a right shoulder, a left shoulder when possible, a divide between the two directions, a vertical barrier too when possible, 2 lanes per directions (except on interchange ramps), full control of access, no at grade crossings and no traffic lights or stop signs. Note that the limit can change if it's needed for safety.
(I boiled, I mean changed (GMM viewers know why this is funny) the rural access road limits for safety, 40 is a much more survivable speed in a crash with pedestrians or cyclists, and is equal to or less than the 50 km/h cap on T bone crashes, plus it is easier to come down from when crossing cycle paths or footways midblock or at crossings, and the sightline distances don't need to be as long)
Access roads are 40 km/h roads, with no traffic that doesn't need that route to get around. They have lower volumes, are not divided roads, have no centre line for the most part, either have dashed white lines on the edges of the road or no markings at all, and only provide the roads that you might use to get from a farm to a county highway, or are the main routes between very small communities. 40 km/h roads are also usually the more direct route between 2 communities, however it is traffic calmed to ensure that it does not become the through route. 40 km/h roads also are used to bypass villages and small towns.
And distributor roads are 70 km/h roads, with a centre line, usually a double dashed centre line, a dashed right edge line and sometimes a divide between the two directions. Intersections are where possible under roundabout control, otherwise by using traffic lights or yield/stop signs facing the minor road. Four way stops often get congested because the traffic from the right rule often can't even be observed, let alone encouraging the traffic to stop in the first place. A few 70 km/h roads have interchanges but that's pretty uncommon. 70 km/h roads also tend to preform the function of bypassing smaller cities and towns, if that job is not taken by a through 100-130 km/h road.
So that is functionality, and I covered some of the basics around homogeneity of traffic flows, but let's take a look at it in more detail.
In urban areas directional conflicts are slightly less of a problem because of the lower speed. In rural areas the speed is usually between 40 and 130 km/h, so it is not good to have directional conflicts. The most obvious directional conflict is the head on kind. This can be solved on some roads with a divide, a verge or vertical barrier. As long as there isn't too much of a need of overtaking, even on 2 lane roads this can work ok. The other main directional conflict is the side conflict, or T bone. Roundabouts work extremely well at preventing this crash. In fact it's nearly impossible to have a head on or T bone crash at a roundabout.
Speed differentials are a big different among the different kinds of motor powered vehicles in rural areas. Many agricultural vehicles can't go freeway speeds. Thus, they should be required to go no more than 40 km/h and use a 40 km/h local access road and not use a 60 km/h minimum speed limited freeway or autoweg. Mopeds too. They use either a low volume 40 km/h local access road or a cycle path designed for 50 (the limiters on mopeds should make this required). Whoever came up with the idea that you should be able to use the shoulder of a freeway to cycle on, I challenge you to do that very action. Cycle on the shoulder of a freeway, and come back and tell me how stressful and dangerous that felt. Shoulders are not suitable cycling facilities, and without a shadow of a doubt, not next to freeways or on autowegs. Not even on 70 km/h roads. Cycling is not allowed on these roads. They are always routed on a separate route or path. The Dutch don't even use the concept for 40 km/h access roads. 40 km/h roads usually either have a cycle path or are low enough in volume to mix safely. A few are 6 metres wide with 1.5 metre wide red asphalted stripes on the sides with a 3 metre wide bidirectional space for motor vehicles to use, but that is fairly uncommon.
Next we have predictability. This can be a challenge in rural areas. Let's make the identification of the roads in question easy.
Starting with freeways and expressways. We can use the European signs for expressway and motorway to fit our definitions. Freeway begin signs are also regulatory, indicating the minimum and maximum speed limits (60 km/h min, 100 on an expressway and 130 on a freeway), the categories of vehicles that are prohibited, bicycles, pedestrians, animals, farm vehicles and mopeds, that stopping, u turns and that reversing are all prohibited (why would you park on a freeway unless your car broke down?). Then we can make expressway classification predictable by always having a hard shoulder, 3.5 metre wide lanes, a green stripe between the 2 yellow ones on non divided expressways.
70 km/h roads are next. Let's make them predictable. Upon leaving the built up areas, you will see a sign like the one to the right, using the one with the red stripe :