How Are Highways Designed?

Summary

This script from Practical Engineering explores the complexities of roadway design, emphasizing the balance of safety, comfort, and efficiency. It delves into the geometrical aspects of highways, including cross-sections, alignment, and profile, and how they are engineered to accommodate speed, drainage, and visibility. The video explains how superelevation, crowning, and sight distance are crucial for driver safety, while also highlighting the artistry involved in creating a continuous, flowing road experience.

Takeaways

• 🛣️ Designing a road involves balancing multiple constraints and making hundreds of decisions to ensure safety and efficiency.
• 🚗 A well-designed road allows drivers to focus on their surroundings without being preoccupied by the road's design flaws.
• 🏎 The maximum speed of a roadway is determined by considering terrain, traffic, existing obstacles, and safety.
• 📏 Roadway geometry includes three main features: cross-section, alignment, and profile, each with its own set of intricate details.
• 🔄 Cross-sections are rarely flat to facilitate water drainage and prevent slippery conditions, with most roads being crowned or having a cross slope away from the center.
• 🌀 Superelevation of the outer edge in curved sections enhances safety by reducing reliance on tire friction for turning.
• 🛍️ Banking roads not only improves safety but also passenger comfort by counteracting centrifugal forces during turns.
• 🔄 The radius of a curve impacts safety; tighter turns require more centripetal force, so engineers design curves to be gentle based on design speed.
• 🌀 Spiral easements are used to transition between straight and curved sections, smoothing out the centrifugal force and allowing gradual steering adjustments.
• 👀 Sight distance, the length of roadway needed to recognize and respond to changes, is crucial for safe driving and varies based on driver reaction time and vehicle speed.
• ↗️ The vertical alignment, or profile, of roads must consider slope, transitions between grades, and sight distance over hills and through valleys to ensure safety.

Q & A

What is the primary goal of roadway design?

The primary goal of roadway design is to create a simple, efficient path between two points while ensuring the safety and comfort of motorists.

Why do we rarely think about roads while driving?

We rarely think about roads while driving because well-designed roads are intuitive and require minimal conscious thought, allowing drivers to focus on their destination.

How does roadway geometry contribute to safety and comfort?

Roadway geometry contributes to safety and comfort by considering factors like cross-section, alignment, and profile, which affect how drivers navigate turns, gradients, and other road features.

What is the significance of a road being 'crowned'?

A road being 'crowned' means it has a cross slope away from the center, which helps in shedding water quickly and keeping the road surface dry, reducing the risk of accidents due to slippery conditions.

Why do engineers superelevate the outside edge of a curved road?

Engineers superelevate the outside edge of a curved road to increase friction and reduce the need for tire friction to navigate the curve, enhancing safety and comfort.

How does the radius of a curve impact safety?

The radius of a curve impacts safety by determining the amount of centripetal force needed to keep a vehicle in its lane. Tighter turns require more force, increasing the risk of accidents.

What is a spiral easement and why is it used?

A spiral easement is a transitional section of road that gradually transitions between straight and curved sections. It is used to smooth out the centrifugal force and allow drivers to steer gradually into and out of curves without sudden adjustments.

Why do lanes often appear wider around curves on highways?

Lanes appear wider around curves to provide additional space for vehicles, compensating for the natural tendency of tires to track imperfectly and the changing forces experienced by vehicles during turns.

What is sight distance and why is it important in roadway design?

Sight distance is the length of roadway required for drivers to recognize and respond to changes or obstacles. It is important in roadway design to ensure drivers have enough time to react safely.

How do vertical curves affect sight distance?

Vertical curves, both crest and sag, affect sight distance by either hiding or revealing parts of the road ahead. Engineers must design these curves to be gentle enough to maintain adequate sight distance for safe driving.

Why is highway design considered both a science and an art?

Highway design is considered both a science and an art because while it involves applying engineering principles and mathematical calculations, it also requires a designer's judgment and creativity to balance various factors and create a safe, comfortable, and visually appealing driving experience.

Outlines

00:00

🛣️ The Complexity of Roadway Design

This paragraph delves into the intricacies of roadway design, emphasizing the numerous decisions that contribute to driver safety and comfort. It highlights the idea that a well-designed road is one that drivers don't consciously think about while driving, suggesting a poorly designed road would require their attention. The script introduces the concept of roadway geometrics, including cross-section, alignment, and profile, and explains how these elements are crucial for determining safe driving speeds. The importance of road crowning for water drainage and superelevation in curves to enhance friction and reduce the reliance on tire grip are also discussed.

05:04

🔍 Roadway Geometry: Curves and Sight Distance

The second paragraph focuses on the specific aspects of roadway geometry that affect vehicle safety, particularly the design of curves and the concept of sight distance. It explains how the radius of a curve impacts the required centripetal force for a vehicle to maintain its lane, with tighter turns necessitating more force and being more prone to accidents. The paragraph also discusses the use of spiral easements to transition smoothly between straight and curved sections, the gradual widening of lanes in curves to provide more room for error, and the importance of sight distance for driver reaction times. Additionally, it touches on the challenges of vertical alignment, including the design of crest and sag curves to maintain visibility and safety.

10:07

🎨 The Art of Highway Design and Sponsor Mention

The final paragraph of the script highlights the artistic aspect of highway design, where the driver's view and experience are carefully considered to create a continuous flow of gradual turns that enhance alertness and enjoyment. It acknowledges the complex interplay of factors and criteria in roadway design, requiring significant judgment to balance for safety and comfort. The paragraph also includes a sponsor mention for Ting Wireless, a service that offers pay-as-you-go mobile plans, which is positioned as a solution for those not using much mobile data due to increased Wi-Fi connectivity. The sponsor segment provides an average cost savings and a promotional offer for the channel's viewers.

Keywords

💡Roadway Design

Roadway Design refers to the process of planning and creating a road system that is safe, efficient, and comfortable for motorists. It involves a multitude of decisions, each aimed at balancing various constraints such as terrain, traffic, and safety. In the video, the theme revolves around how roadway design is intricately linked to the safety and comfort of drivers, with the script discussing the importance of geometrics in highway design.

💡Cross-Section

The cross-section of a roadway is the shape that would be seen if the road were cut across its width. It includes details like the number of lanes, their widths, slopes, and the presence of medians, shoulders, sidewalks, or curbs. The script emphasizes the importance of the cross-section in shedding water quickly to maintain road safety, with the example of roads being 'crowned' to aid in drainage.

💡Alignment

Alignment in roadway design refers to the horizontal positioning of the road, including the curvature of the road and the transitions between straight sections and curves. The script mentions how engineers carefully design the alignment to ensure that vehicles can navigate turns safely at the designed speed, using elements like spiral easements to create smooth transitions.

💡Profile

Profile, or vertical alignment, pertains to the vertical positioning and shape of a road, including how it goes up and down hills and valleys. The script discusses the importance of considering slope and smooth transitions between grades for driver comfort and safety, as well as the impact of vertical curves on sight distance.

💡Superelevation

Superelevation is the practice of elevating the outside edge of a road in curved sections to increase friction and help vehicles navigate turns safely. The script explains how this reduces the reliance on tire friction for centripetal force, enhancing both safety and comfort for drivers, as illustrated by the ball demonstration in the script.

💡Sight Distance

Sight distance is the length of roadway that a driver can see ahead to recognize and respond to changes or obstacles. The script highlights its importance in road design, as it affects the ability to safely navigate turns, pass other vehicles, or avoid hazards, with examples of how sight distance requirements influence curve design and vertical alignment.

💡Crest Curves

Crest curves are convex upward curves in the road profile that can limit sight distance by hiding the road beyond the top of the hill. The script points out the need for gentle crest curves to ensure that drivers have enough sight distance to react to any obstacles that may be out of view at the top of the hill.

💡Sag Curves

Sag curves are concave upward curves that do not limit sight distance during the day as much as crest curves. However, the script notes that at night, the tightness of a sag curve can affect the effectiveness of headlights in illuminating the road ahead, thus impacting sight distance and safety.

💡Speed Limit

A speed limit is the maximum speed at which vehicles are legally allowed to travel on a roadway. The script discusses how engineers must be confident that vehicles can safely traverse the road at the speed limit, which is determined by factors such as roadway geometry, terrain, and safety considerations.

💡Geometric Features

Geometric features in roadway design include the cross-section, alignment, and profile, as well as elements like superelevation and curve radius. The script emphasizes that these features are crucial for determining the manageability of a road at a given speed and ensuring driver safety and comfort.

💡Safety

Safety is a paramount concern in roadway design, influencing decisions about speed limits, geometric features, and sight distances. The script repeatedly underscores the importance of safety in all aspects of road design, from preventing water accumulation to ensuring drivers have adequate time to react to hazards.

Highlights

Designing a road involves numerous small decisions aimed at ensuring motorists' safety and comfort.

Roads are designed to be intuitive, requiring little conscious thought from drivers while driving.

The development of roadways is a remarkable feat in human history and evolution.

Engineers balance terrain, traffic, obstacles, and safety to determine the maximum speed of a roadway.

Roadway geometry is crucial for ensuring vehicles can safely traverse at designated speeds.

The cross-section of a road reveals its shape, including lane numbers, widths, slopes, and medians.

Most roads are crowned to facilitate water drainage and prevent slippery conditions.

Superelevation in curved sections enhances safety by reducing reliance on tire friction for turns.

Banking of roads can make turns more comfortable and reduce the risk of rollovers.

The radius of a curve impacts safety, with tighter turns requiring more centripetal force.

Spirals are used to transition between straight and curved sections, smoothing out centrifugal forces.

Lanes are often widened around curves to provide more space for vehicles to navigate safely.

Sight distance is critical for drivers to recognize and respond to upcoming changes or obstacles.

Crest and sag curves must be designed with sight distance in mind to prevent surprises on the road.

Highway design involves a mix of art and science, considering the driver's view and the flow of the road.

Different regions have their own guidelines for roadway design, impacting the speed limit and safety measures.

Ting Wireless is highlighted as a sponsor offering a pay-as-you-go mobile data plan.

Transcripts

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 right now, which means you’re probably connected

to a wifi network most of the time.

But, I bet you didn’t call your cell phone company to adjust your data plan accordingly.

You’re paying for a set amount of data per month whether you use it or not, and you may

not be using much at all.

Today’s sponsor, Ting Wireless solves this problem entirely.

Ting is the anti-unlimited U.S. wireless provider where you only pay for what you use.

Pretty much everything I do on my phone gets downloaded ahead of time on wifi: podcasts,

audio books, and YouTube videos.

That means I use mobile data for driving directions, emails, and not much else.

If you’re in a similar boat, take your latest phone bill, go to practicalengineering.ting.com,

and use their estimation tool to see how much would pay.

The average bill for a single phone line is just $23 per month.

No contracts, commitments, or strings attached - at the end of the month you just pay for

what you used.

It works on almost any phone, they offer coverage on three nationwide networks, and right now

they’re offering a $25 credit for fans of the channel.

That’s basically a free month of coverage just to give it a shot.

That’s practicalengineering.ting.com or just click the link in the description.

Thank you for watching and let me know what you think.