Laying out a new roadway seems like a simple
endeavor. 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 motorists 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. Either that or you, like me, are just innately
curious about the constructed environment. 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. Hey, I’m Grady, and this is Practical Engineering. On today’s episode, we’re talking about
roadway geometrics and the shape of highways. Designing a road is like designing anything
complicated. There are a multitude of 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 60 mile per hour (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, and there are fascinating details involved in each one. 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. For example, if I roll this ball down a flat
roadway, it’s not going to go around the corner of the road because there’s no traction. Rubber tires provide this traction against
a road surface, but it’s not entirely reliable. Rain, snow, and ice significantly reduce friction. Different weights of vehicles and conditions
of tires also create variability. Rather than design every curve for the worst-case
scenario, it would be nice not to have to count on tire friction for this needed centripetal
force. Superelevating a roadway around a curve reduces
the need for tire friction by utilizing the normal, or perpendicular, force from the pavement
instead. In my demonstration, if I get the bank angle
just right, the ball goes around the corner perfectly even without any lateral friction
with the track. Banking roadways also makes them more comfortable,
because the centrifugal force pushes passengers into their seats rather than out of them. If the superelevation angle is just right,
and you’re traveling at precisely the design speed of the roadway, your cup of coffee won’t
spill at all around the bend. Superelevation also helps reduce rollover
risk by lowering a vehicle’s center of gravity. If you pay attention on a highway, you’ll
notice that the cross slope changes direction on the outside of curves, and you go from
a crown to a superelevation. 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 I want to discuss. 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. Even with all those measures to make curves
safe and easy to navigate, drivers still usually have a little bit of trouble staying centered
in a lane around a bend. This is partly because tires don’t track
perfectly inline with each other when turning (especially for large vehicles like trucks),
but also because the forces are changing, and that takes compensation. Because of this, engineers often widen the
lanes around curves to provide a little more wiggle room for vehicles. This happens gradually, so it’s relatively
imperceptible. But if you pay attention on a highway around
a curve, you may notice your lane feeling a little more spacious. 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. Crest curves - the ones that are convex upwards
- cause the roadway to hide itself beyond the top. If you’re traveling quickly up a hill, a
stalled vehicle or animal on the other side could take you by surprise. If that curve is too tight, you may not have
enough distance to recognize and react to the obstacle. So, crest curves must be gentle so that you
can still see enough of the roadway as you go up and over. Sag curves - the ones that are concave upwards
- don’t have this same issue. You can see all of the roadway on both sides
of the curve. Or at least you can during the day. At night things change. Vehicles rely on headlights to illuminate
the road ahead, and sometimes this can be the limiting factor for sight distance. If a sag curve is too tight, your lights won’t
throw as far. That has the effect of obscuring some of your
sight distance, potentially making it difficult to react to obstacles at night. So, sag curves also need to be gentle enough
to maintain headlight 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. If you pay attention, you’ll notice newer
roadways are less of a series of straight lines connected by short curves and more of
a continuous flow of gradual turns. This is not only more enjoyable, but it also
helps keep drivers more alert. 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. Not many of us are doing a lot of traveling
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just click the link in the description. Thank you for watching and let me know what
you think.
Arbitrarily is the answer around where I live.
These factors are mostly about design speed; the author does mention that other factors may affect the posted speed. The most familiar example is in school zones, where a nearby school means posted speed is lower but only at specific times.
I think that the main cause of pervasive speeding is a mismatch of design with posted speed.
By and large, not the way they're supposed to be set.
Take the analytic approach it looks like is in this video, or the 85th percentile, then knock off 10-20 mph.
germany cant hear you
Heard a whole lot about highway design. Didn't hear a thing about how speed limits are determined.
I feel let down. I love this guy’s videos, I’ve learned a lot from him but this dude just lied to me and did 0 talking about highway speed limits... just a long rambling on what can affect them. I wanna know about why they choose this speed and how they designed the road to accommodate. Like if this is a 55 road there should be 10’ditch between or this interstate has concrete dividers that are 1.5’thick so a simi going 80 cant break through.
Speed limits are set by idiot lawyers in legislatures who are pretending to know what the hell they’re doing, and they screw it up like they always do.
IMO, they are set artificially low on a lot of roads so that the state has more opportunity to issue speeding tickets.
Speed limits are largely based on gut feel of a person who is very suspectable to motion sickness and has poor depth perception.
They were once based on math formula that took smoothness of the road, line of sight, grade, average traffic density, and a few other factors into accountancy but they tossed that shit out the window because many highways would be rated for 100+mph. This is still the case but then they also look at a table that says if speed limit is over 65 then use 65mph.
Most highways in Texas are 75, there are sections that are 85 and if you go way out north west there are some
95signs./Edit -drainage +grade /Edit it appears the 95 mph signs are no longer a thing