Laying out a new roadway seems like a simple endeavor.
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
You have two points to connect, and you’re trying to create a simple, efficient path

between them.
between them.

But, there are lots of small decisions that make up a roadway design, nearly every one
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.
of which is made to keep motorists safe and comfortable.

Although many of us are regular drivers, we rarely put much thought into roads.
Although many of us are regular drivers, we rarely put much thought into roads.

That’s on purpose.
That’s on purpose.

If you’re thinking about the roadway itself at all while you’re driving, it’s probably
If you’re thinking about the roadway itself at all while you’re driving, it’s probably

because it was poorly designed.
because it was poorly designed.

Either that or you, like me, are just innately curious about the constructed environment.
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
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
able to put ourselves in metal boxes that hurtle away at incredible speeds from place

to place.
to place.

It’s not entirely safe, but it’s safe enough that most of the world chooses to do
It’s not entirely safe, but it’s safe enough that most of the world chooses to do

it on a regular basis.
it on a regular basis.

And the place that level of safety and comfort starts isn’t immediately evident to the
And the place that level of safety and comfort starts isn’t immediately evident to the

casual observer.
casual observer.

Hey, I’m Grady, and this is Practical Engineering.
Hey, I’m Grady, and this is Practical Engineering.

On today’s episode, we’re talking about roadway geometrics and the shape of highways.
On today’s episode, we’re talking about roadway geometrics and the shape of highways.

Designing a road is like designing anything complicated.
Designing a road is like designing anything complicated.

There are a multitude of conflicting constraints to balance and hundreds of decisions to make.
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,
In an ideal world, every road would be a straight, flat path with no intersections, driveways,

or other vehicles at all.
or other vehicles at all.

We could race along at whatever speed we wanted.
We could race along at whatever speed we wanted.

But reality dictates that engineers choose the maximum speed of a roadway based on a
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.
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
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
choose a speed motorists are allowed to travel, you have to be confident that vehicles can

traverse the road at that speed safely.
traverse the road at that speed safely.

That confidence has everything to do with the roadway’s geometry.
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.
You would never put a 60 mile per hour (100 kph) speed limit on a city street.

Why?
Why?

Because hardly any competent driver could navigate a turn that fast, let alone avoid
Because hardly any competent driver could navigate a turn that fast, let alone avoid

a hazard, maneuver through traffic, or survive a speed bump.
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?
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:
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
the cross-section, the alignment, and the profile, and there are fascinating details

involved in each one.
involved in each one.

The first one, cross-section, is the shape of the road if you were to cut across it.
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
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.
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
One thing you might notice looking at roadway cross-sections is that they’re almost never

flat.
flat.

The reason is that a flat surface doesn’t shed water quickly.
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
This accumulation of water on the road is dangerous to vehicles by making roads slippery

and creating more ice in the winter.
and creating more ice in the winter.

So, nearly all roads are crowned, which means they have a cross slope away from the center.
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.
This accelerates the drainage of precipitation and keeps the surface of the road dry.

But, not all roadways are crowned.
But, not all roadways are crowned.

There’s another type of cross slope that helps make roads safer.
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.
In curved sections, engineers make the outside edge higher or superelevated above the centerline.

This is also to help with friction.
This is also to help with friction.

Any object going around a curve needs a centripetal force toward the center of the turn.
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.
Otherwise, it will just continue in a straight line.

For a vehicle, this centripetal force comes from the friction between the tires and the
For a vehicle, this centripetal force comes from the friction between the tires and the

road.
road.

Without this friction - on a flat surface - there would be no way to make a turn at
Without this friction - on a flat surface - there would be no way to make a turn at

all.
all.

For example, if I roll this ball down a flat roadway, it’s not going to go around the
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.
corner of the road because there’s no traction.

Rubber tires provide this traction against a road surface, but it’s not entirely reliable.
Rubber tires provide this traction against a road surface, but it’s not entirely reliable.

Rain, snow, and ice significantly reduce friction.
Rain, snow, and ice significantly reduce friction.

Different weights of vehicles and conditions of tires also create variability.
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
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.
count on tire friction for this needed centripetal force.

Superelevating a roadway around a curve reduces the need for tire friction by utilizing the
Superelevating a roadway around a curve reduces the need for tire friction by utilizing the

normal, or perpendicular, force from the pavement instead.
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
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.
perfectly even without any lateral friction with the track.

Banking roadways also makes them more comfortable, because the centrifugal force pushes passengers
Banking roadways also makes them more comfortable, because the centrifugal force pushes passengers

into their seats rather than out of them.
into their seats rather than out of them.

If the superelevation angle is just right, and you’re traveling at precisely the design
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.
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.
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
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.
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 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.
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
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.
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
Crashes are most likely when radii are small, so engineers follow guidelines based on the

design speed to make sure curves are sufficiently gentle.
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
It’s not only the curves that need to be gentle but also the transitions between straight

sections.
sections.

At first glance, connecting circular curves to straight sections of roadway looks like
At first glance, connecting circular curves to straight sections of roadway looks like

a perfectly smooth ride.
a perfectly smooth ride.

But forces experienced by vehicles and passengers are a function of the radius of curvature.
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
So if you go directly from a straight section (which has an infinite radius) to a circular

curve, the centrifugal force comes on abruptly.
curve, the centrifugal force comes on abruptly.

Another way to think about this is by using the steering wheel.
Another way to think about this is by using the steering wheel.

Every position of your wheel corresponds to a certain radius of turn.
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
If straight sections of roadway were connected directly to circular curves, you would have

to turn the steering wheel at the transition instantaneously.
to turn the steering wheel at the transition instantaneously.

That’s not really a feasible or safe thing to ask drivers to do.
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
So instead, we use spiral easements that gradually transition between straight and curved sections

of roadway.
of roadway.

Spirals use variable radii to smooth out the centrifugal force that comes from going around
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
a bend, and they allow the driver to steer gradually into and out of each curve without

having to make sudden adjustments.
having to make sudden adjustments.

Even with all those measures to make curves safe and easy to navigate, drivers still usually
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.
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
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,
(especially for large vehicles like trucks), but also because the forces are changing,

and that takes compensation.
and that takes compensation.

Because of this, engineers often widen the lanes around curves to provide a little more
Because of this, engineers often widen the lanes around curves to provide a little more

wiggle room for vehicles.
wiggle room for vehicles.

This happens gradually, so it’s relatively imperceptible.
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
But if you pay attention on a highway around a curve, you may notice your lane feeling

a little more spacious.
a little more spacious.

One other important aspect when designing a curve comes from the simple but crucial
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.
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
Sight distance is the required length of the roadway required to recognize and respond

to changes.
to changes.

It varies by driver reaction time and vehicle speed.
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
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.
turns or obstacles and decide how to manage.

Sight distance also varies by what is required of the driver.
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
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.
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
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.
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.
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.
The final aspect of roadway geometry is the profile - or vertical alignment.

Roads rarely traverse areas that are perfectly flat.
Roads rarely traverse areas that are perfectly flat.

Instead, they go up and over hills and down into valleys.
Instead, they go up and over hills and down into valleys.

Engineers have to be thoughtful about how that happens as well.
Engineers have to be thoughtful about how that happens as well.

The slope, or grade, of a roadway, is obviously essential.
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
You don’t want roads that are too steep, mainly because it would be hard for trucks

to go up and down.
to go up and down.

You also want smooth transitions between grades for the comfort of drivers.
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.
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
Crest curves - the ones that are convex upwards - cause the roadway to hide itself beyond

the top.
the top.

If you’re traveling quickly up a hill, a stalled vehicle or animal on the other side
If you’re traveling quickly up a hill, a stalled vehicle or animal on the other side

could take you by surprise.
could take you by surprise.

If that curve is too tight, you may not have enough distance to recognize and react to
If that curve is too tight, you may not have enough distance to recognize and react to

the obstacle.
the obstacle.

So, crest curves must be gentle so that you can still see enough of the roadway as you
So, crest curves must be gentle so that you can still see enough of the roadway as you

go up and over.
go up and over.

Sag curves - the ones that are concave upwards - don’t have this same issue.
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.
You can see all of the roadway on both sides of the curve.

Or at least you can during the day.
Or at least you can during the day.

At night things change.
At night things change.

Vehicles rely on headlights to illuminate the road ahead, and sometimes this can be
Vehicles rely on headlights to illuminate the road ahead, and sometimes this can be

the limiting factor for sight distance.
the limiting factor for sight distance.

If a sag curve is too tight, your lights won’t throw as far.
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
That has the effect of obscuring some of your sight distance, potentially making it difficult

to react to obstacles at night.
to react to obstacles at night.

So, sag curves also need to be gentle enough to maintain headlight sight distance.
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
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,
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.
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
Different countries and even different states, counties, and cities often have their own

guidelines for how roadway design is done.
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
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.
that gets posted as the speed limit.

There are just so many factors that go into highway safety, many of which are more philosophical
There are just so many factors that go into highway safety, many of which are more philosophical

or psychological than pure physics and engineering.
or psychological than pure physics and engineering.

It may seem like you can just plug in your criteria to some software that could spit
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.
out a roadway project in a nice neat bow.

But to a certain extent, highway design is an art form.
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.
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
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.
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.
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
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
significant judgment to keep them in balance and make sure the final product is as safe

and comfortable for drivers as possible.
and comfortable for drivers as possible.

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You’re paying for a set amount of data per month whether you use it or not, and you may
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Thank you for watching and let me know what you think.
Thank you for watching and let me know what you think.