How Are Highway Speed Limits Set?

How Are Highway Speed Limits Set?

Laying out a new roadway seems like a simple task. You have two points to connect, and you’re trying to create a simple, efficient path between them. But, there are lots of small decisions that make up a roadway design, nearly every one of which is made to keep it vehicle safe and comfortable.

Although many of us are regular drivers, we rarely put much thought into roads. That’s on purpose. If you’re thinking about the roadway itself at all while you’re driving, it’s probably because it was poorly designed. 

If you put it in the context of human history and evolution, it’s a remarkable thing we’re able to put ourselves in metal boxes that hurtle away at incredible speeds from place to place. It’s not entirely safe, but it’s safe enough that most of the world chooses to do it on a regular basis. And the place that level of safety and comfort starts isn’t immediately evident to the casual observer.

Geometrics and shape of highways.

Designing a road is like designing anything complicated. There are a multiple conflicting constraints to balance and hundreds of decisions to make. In an ideal world, every road would be a straight, flat path with no intersections, driveways, or other vehicles at all. We could race along at whatever speed we wanted. But reality dictates that engineers choose the maximum speed of a roadway based on a careful balancing act of terrain, traffic, existing obstacles, and of course, safety. If you’re going to sign your name on a roadway design, and especially if you’re going to choose a speed motorists are allowed to travel, you have to be confident that vehicles can traverse the road at that speed safely. That confidence has everything to do with the roadway’s geometry. You would never put a 100 kph speed limit on a city street. 

Why? 

Because hardly any competent driver could navigate a turn that fast, let alone avoid a hazard, maneuver through traffic, or survive a speed bump. 

So how do we know what kinds of road features are manageable for a given speed?

There are three main features of roadway geometry that are decided as a part of the design: 

the cross-section, 

the alignment, and 

the profile, 

The first one, cross-section, is the shape of the road if you were to cut across it. The roadway cross-section shows so much information like the number of lanes, their widths and slopes, and whether there’s a median, shoulders, sidewalks, or curbs. One thing you might notice looking at roadway cross-sections is that they’re almost never flat. The reason is that a flat surface doesn’t shed water quickly. This accumulation of water on the road is dangerous to vehicles by making roads slippery and creating more ice in the winter. So, nearly all roads are crowned, which means they have a cross slope away from the center. This accelerates the drainage of precipitation and keeps the surface of the road dry.

But, not all roadways are crowned. There’s another type of cross slope that helps make roads safer. In curved sections, engineers make the outside edge higher or superelevated above the centerline. This is also to help with friction. Any object going around a curve needs a centripetal force toward the center of the turn. Otherwise, it will just continue in a straight line. For a vehicle, this centripetal force comes from the friction between the tires and the road. Without this friction - on a flat surface - there would be no way to make a turn at all. 

Superelevating a roadway around a curve reduces the need for tire friction by utilizing the normal, or perpendicular, force from the pavement instead. 

The faster the design speed of the road, the higher the bank around the bend.

The shape of curves themselves is the second aspect of roadway geometry. Just like superelevation, the radius of a curve has a significant impact on safety—the tighter the turn, the more centripetal force needed to keep a vehicle in its lane. Crashes are most likely when radii are small, so engineers follow guidelines based on the design speed to make sure curves are sufficiently gentle. It’s not only the curves that need to be gentle but also the transitions between straight sections. At first glance, connecting circular curves to straight sections of roadway looks like a perfectly smooth ride. But forces experienced by vehicles and passengers are a function of the radius of curvature. So if you go directly from a straight section (which has an infinite radius) to a circular curve, the centrifugal force comes on abruptly. Another way to think about this is by using the steering wheel. Every position of your wheel corresponds to a certain radius of turn. If straight sections of roadway were connected directly to circular curves, you would have to turn the steering wheel at the transition instantaneously. That’s not really a feasible or safe thing to ask drivers to do. So instead, we use spiral easements that gradually transition between straight and curved sections of roadway. Spirals use variable radii to smooth out the centrifugal force that comes from going around a bend, and they allow the driver to steer gradually into and out of each curve without having to make sudden adjustments. 

One other important aspect when designing a curve comes from the simple but crucial fact that drivers need to see what’s coming up to be able to react accordingly. Sight distance is the required length of the roadway required to recognize and respond to changes. It varies by driver reaction time and vehicle speed. The slower you react and the faster you’re going, the more distance you need to observe turns or obstacles and decide how to manage. Sight distance also varies by what is required of the driver. The amount of roadway necessary to bring the vehicle to a stop is different than the amount needed to safely pass another vehicle or avoid a hazard in the lane. Even if a curve is gentle enough for a car to traverse, it may not have enough sight distance for safety due to an obstacle like a wooded area. In this case, sight distance will require the engineer to make the curve even gentler.

The final aspect of roadway geometry is the profile - or vertical alignment. Roads rarely traverse areas that are perfectly flat. Instead, they go up and over hills and down into valleys. Engineers have to be thoughtful about how that happens as well. The slope, or grade, of a roadway, is obviously essential. You don’t want roads that are too steep, mainly because it would be hard for trucks to go up and down. You also want smooth transitions between grades for the comfort of drivers. But, on top of all that, vertical curves also have the same issue with sight distance.

Of course, there are equations for all of these different parts of roadway geometry that can tell you, based on the design speed and other factors, how much crown is required, or how high to superelevate, or the allowable radius of a curve, etcetera. Different countries and even different states, counties, and cities often have their own guidelines for how roadway design is done. And even then, the speed used by the engineers to design the roadway isn’t always the one that gets posted as the speed limit. There are just so many factors that go into highway safety, many of which are more philosophical or psychological than pure physics and engineering. It may seem like you can just plug in your criteria to some software that could spit out a roadway project in a nice neat bow. But to a certain extent, highway design is an art form. Designers even consider how the driver’s view will unfold as they travel along.

There are so many factors and criteria that go into the design of a roadway, and it takes significant judgment to keep them in balance and make sure the final product is as safe and comfortable for drivers as possible.

Thank you for reading...