Friday, 18 March 2016

Sustainable Safety: A thorough look into each of the 5 principles. Part 2: Homogenity

The second (out of 5) core principles of Sustainable Safety is that to the greatest possible extent, the differences in masses, speeds and directions on any particular thoroughfare whether it be road, cycleway or footway, be as close together as possible.

It is fairly intuitive as to why. You know by intuition that a large object heading your way at high speed is not going to result in a good outcome on your end and probably the other object's end as well.

It is literally not the speed that kills, but the crash, or at least one that causes problems. So preventing the crash is the ultimate goal, and the secondary desire is that any crashes that do occur, occur at low speed low conflict environments.

I know technically it's acceleration, but when you have a net acceleration (which oddly enough also includes slowing down) of 0, you can be said to effectively have speed rather than acceleration.

Let's see what we can do to make the roads homogeneous in terms of mass.

We can look at each type of road user and see what masses they have. Pedestrians have around 50 kg of mass, give or take about 45 kg depending on the person. Cyclists have a mass of around 100 kg, depending on the bicycle, it's rider and whatever the cyclist is carrying with them. Mopeds and scooters have a maximum mass in terms of the machine of 55 kg, and the rider is the same as with a normal person, for about 150 kg or so. Mobility scooters have up to about 410 kg depending on the rider and model. Cars can have a mass of anywhere between around 750 kg and 1250 kg depending on the model. Newer cars tent to be heavier. Light trucks and vans can be up to 3500 kg. A bus or truck may be 12000 kg or more.

So the ideal road should put pedestrians and mobility scooters (up to a certain speed) on the footway, cyclists, mobility scooters being ridden above the sidewalk speed, and mopeds. The roadway should combine anything larger, although in some cases buses may have their own busway due to the fact that a 12 tonne bus and a 1.2 tonne car means that due to the squaring law of magnitudal increases in kinetic energy, that a collision is going to be 100 times worse with a bus than with a car. This also is often why buses and trucks tend to have lower speed limits on freeways and expressways in many countries. Buses and trucks should not be present at all if possible on a road where small road users are present without physical protection and thus are vulnerable.

To look at how to homogenize speeds, we can look at actual travel speed for the various types of road users and see how they may conflict. Pedestrians and cyclists while they can in low enough volumes and speeds mix reasonable well, at least in terms of how they travel when going parallel to each other (IE not at pedestrian crossings), they do not mix well at normal travel speeds of cyclists and in the normal volumes you are likely to find them. Thus they must be separated except in the pedestrianized zones and low volume access roads.

The difference in mass and speed between cycles and cycle like objects (mopeds, scooters and mobility scooters) and motor vehicles must now be considered. A number of studies have shown that the maximum safe speed for mixing (assuming you have low volumes, thus low amounts of mass, of motor vehicles) is 30 km/h. More and the differential rapidly and predictable causes many more casualties, because of the fact that doubling the speed squares the energy.

Swedish and other research has shown that the maximum speed that a well designed vehicle is capable of handling in a T bone crash is 50 km/h. As you may expect, the Swedish default speed limit for their version of distributor roads is 50 km/h, due to the number of side roads, including other arterial roads.

Also by Swedish research, the maximum speed for a head on collision to be taken safely by a well designed vehicle is 70 km/h. As such, modern practice indicates that a road should have no more than a 70 km/h speed limit without a divide between the two directions. Of course this doesn't apply on a one way road.

But back to cyclists and pedestrians. In the rural area, access roads are limited to 60 km/h. The roads where cyclists and pedestrians share with motor vehicles are very low volume. They don't provide any use at all except by those who live on that very road. They are usually 4 metre wide asphalt or concrete roads with some space on either side as a clear zone. Or they may not even be paved at all, just some crushed seashells or grass, as this prevents mud from forming so easily when wet. On the busier access roads, like the route between two villages or small towns, there is usually a separate cycling path. One way or two way, it doesn't matter so long as they suit the functional requirements of that very road.

On roads where the head on and side crashes are not a problem and are prevented by physical design, the speed limit can be higher. On roads where there is no hard shoulder and not at least 2 lanes per direction, and interchanges cannot be assured on every junction (or no direct access to the through road), there is a maximum of 100 km/h in the Netherlands. These roads are called autowegen. On roads where there are at least 2 lanes per direction, controlled access, hard shoulders at least on the right side and a crash barrier between the two directions, then it can be called a full motorway. Due to the problem that motorways have very few ways to enforce their speed limit by design as things to physically prevent speeding are unheard of, nobody ever puts a bump on a motorway for example, the speed limit is hard to enforce without the speed being selected by the road users themselves. This doesn't apply to vehicles with limiters in their engines/motors.

So what can be done about this situation? Well, as best as can be done, the 85th percentile speed standard can be brought back as conflicts are already removed, and the problems arise when you have vehicles traveling at different speeds, some at the legal speed limit, some going at their comfort speed, which tends to be between 125 and 135 km/h. In Germany this is usually around 180-240 km/h. But we aren't ready to autobahns if we ever will be, and 240 km/h uses a lot of power. So a speed of between 120 and 140 km/h is a good speed. 130 km/h is the normal motorway speed limit throughout much of Europe. And if it isn't, it's usually 120. So adding these to the speed limit options will ensure that speed is not a problem on full motorways as much as possible.

The final major conflict has to do with direction. If you have a different direction to that of anything in your path, then there is likely going to be a big problem because you will suddenly change their direction as well.

The Dutch, and a considerable chunk of Europe and Australia (and NZ), even Carmel Indiana have found out that roundabouts are great at reducing the direction problem. Especially when you combine it with the lower speed. It also makes that you can easily predict the direction that other traffic will come from, the opposite of the driving direction (from the right in the UK, from the left in the Netherlands and here in Canada). They have been found to be very good at reducing crashes where side conflicts are possible. The Dutch even use them on 100 km/h autowegs, although they are less preferable to full interchanges.

Roundabouts of the annular ring with priority design have also proven dangerous due to the poor sightlines, the inability of cyclists to assure their own safety and drivers having to do too many things at once. The non annular, and unless experiments prove otherwise, non priority, designs are the way to go, unless you grade separate the roundabout, in which case at grade crossings are not any problems as they don't exist.

When this is not possible, on non through roads it is generally better to have intersections controlled by yield signs (and sometimes stop signs), with generous divides between the different things you need to do, (let cyclists go first, then traffic from the left, then the right, etc). I covered this in a blog post about doing one thing at a time. Something like 4-6 metres will do the job.

On access roads, no control is required. The volume will ensure that there is no gridlock by having to yield to the right, and the low speeds mean that crashes are less likely and bad results even less likely.

Uncontrolled crossings where there is no divide between the things you must do, as in you must do everything at once, but one road has legal priority over another, has proven to be very dangerous, often the site of fatal crashes and injuries, especially when cyclists and pedestrians are involved. So these designs should be minimized to the greatest possible extent.

Interchanges have the highest capacity, and the highest speed allowable, but they also are costly and need a lot of room, and thus should be limited to through roads as much as possible on limited corridors. However they are among the safest designs when roundabouts are not used, as they have similar speeds between vehicles and similar angles, with no crossing traffic. They are safest when traffic does not weave, which is why cloverleafs tend to not be used anymore for systems interchanges.

The next topic is predictability. Stay tuned for that.

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