Transportation Defined: Recessed Pavement Markers

Recessed pavement markers
are used in colder climates.

Most drivers are probably familiar with pavement markers…

They’re the reflective objects typically placed on the road in between pavement stripes – you’ll often see them along the solid pavement lines, too, or on exit/entrance ramps. Basically, they’re used to give drivers a better guide at night.

Nothing new there, but did you know that the markers can vary depending on their location?

That’s right … if you travel up north you may notice that the pavement markers are recessed, rather than raised as they are in most other parts of the state.

Any ideas why that is?

We asked ADOT Project Supervisor Rick Schilke for an explanation and he says it has to do with snowplows.

A diagram of recessed
pavement markers.

“Up here, we sink the pavement markers down for the plows, otherwise they’d just pop right off,” he said.

Makes sense to us!

Installing recessed pavement markers requires a little more work than the raised markers. Schilke says crews must grind a groove into the road that tapers down at an angle allowing for reflection. The marker is then put into place with an epoxy.

One more thing to note: In areas where snow is expected in the colder seasons, ADOT crews actually recess stripes, too. (We’re not talking about painted stripes, ADOT uses striping “tape” that’s more durable and reflective than paint.) The striping tape is recessed so that it won’t be displaced by snowplows.

Transportation Defined is a series of explanatory blog posts designed to define the things you see on your everyday commute. Let us know if there's something you'd like to see explained ... leave a comment here on the blog or over on our Facebook page!

Transportation Defined: Climbing Lanes

A climbing lane on SR 87.

Next time you’re traveling uphill only to get stuck behind a slow-moving vehicle or truck, you’ll appreciate the concept of a climbing lane.

A climbing lane is an “extra” lane that’s used for short distances in certain areas to improve safety, ease congestion and prevent delays. These lanes help facilitate the passing of trucks and slow moving vehicles whose speed drops because of the sustained steep grades.

A few more facts:

• A climbing lane looks the same as any other lane and is the same width. 
• Climbing lanes usually are used on uphill segments of highway that have a steep grade (usually 5-6 percent grade). 
• Climbing lanes typically are marked with signage advising slower traffic to keep right. 

“Climbing lanes help reduce collisions and backups by providing slower moving trucks and vehicles an additional, safe lane to travel in. This reduces conflicts between slower moving trucks and passing vehicles,” said Prescott District Engineer Alvin Stump in a recent news release focused on a new ADOT project involving a climbing lane (we’ll tell you more about that tomorrow!).

Transportation Defined is a series of explanatory blog posts designed to define the things you see on your everyday commute. Let us know if there's something you'd like to see explained ... leave a comment here on the blog or over on our Facebook page!

Transportation Defined: Dynamic Message Signs

Dynamic Message Sign

We’ve got a blog post coming up real soon that’s going to focus on Dynamic Message Signs (Don’t know what those are? Keep reading!).

That blog post isn’t ready quite yet, so in the meantime we wanted to at least get started by defining a couple of terms for you …

First up, Dynamic Message Signs: These are the large signs over/near roadways used to display messages to the public. You’ll often see travel times posted on these signs during rush hours or safety messages and other travel information.

The larger DMS have three rows of 18 characters. Each of those characters measures 18 inches tall.

Now that you know what DMS is, take a look at a Node Building. These “buildings” are about the size of a typical storage shed and are located near the roadway. There are several node buildings in the Phoenix-metro area. 

Node Building

These nodes are filled with equipment capable of collecting regional information (e.g., from various controller cabinets) and sending it to the TOC. Node buildings are equipped with fault tolerant air conditioning systems to maintain moderate summer temperatures.

OK … now you’ll have to stay tuned for a more in-depth blog post on Dynamic Message Signs.

Transportation Defined is a series of explanatory blog posts designed to define the things you see on your everyday commute. Let us know if there's something you'd like to see explained ... leave a comment here on the blog or over on our Facebook page!

Transportation Defined: Guardrails

ADOT crews repair a guardrail.

Guardrails. You see them all the time, right? But, how often do you really think about the important job they’re performing?

If your answer is, “never,” that’s perfectly fine and, quite frankly, it’s what we expected, because who sits around contemplating guardrails?

But, guardrails really do perform a vital task, which is to separate vehicles from a non-traversable slope past the edge of the roadway or any type of fixed object (think power pole or building). They’re also the topic of this blog post, so hopefully your interest in guardrails is now sufficiently piqued!

How they work
First off, there are different types of barrier out on the state’s roads, but what you see on many highways (and what we’re talking about here), is the w-beam guardrail that is secured to wooden posts (see photo above). Fun fact: It’s called a w-beam because if turned on its side, it looks like a W.

When the guardrail is hit by a vehicle, the posts move back and rotate with the force of the vehicle. The wooden posts, the w-beam, the dirt and the vehicle all absorb some of the impact, but the idea is that the w-beam will stay intact and will prevent the vehicle from traveling off the side of the road into whatever obstacle the guardrail is there to protect you from.

The energy of the impact is distributed to many of the posts, which are able to hold the w-beam against the vehicle, allowing the vehicle to come to a stop without crossing the barrier or flipping over it.

We don’t use them unless we have to
Guardrails are only used when necessary. If a road can be built to avoid the need for guardrail, that’s typically the way to go, but it’s not always possible.

We won’t get too technical, but engineers do have calculations for determining whether or not to use guardrail that take into account speed, traffic volume and the amount of clear area that there needs to be.

If it is determined that guardrail is needed, engineers also have to figure out how much is necessary. The term, “length of need” refers to the length of guardrail needed to shield the motorist from the potential hazard. Again, we’re not going to get technical here, but there’s a formula to calculate the length of need, too.

But wait, there’s more!
If a guardrail is hit, there’s some maintenance involved. According ADOT Maintenance Superintendent Craig Cornwell, his crews are out there repairing guardrail hits just about every day.

He says the w-beam railing comes in sections (from 12.5 feet to 25 feet).

“If a section is damaged, we replace that whole section,” said Cornwell, adding that the beams actually overlap and are connected to each other with eight bolts (there’s also a big bolt (about 18 inches) that connects the beam to the wooden post.

On top of repairs, Cornwell says his crews perform routine maintenance on the guardrails, which includes regular inspections.

Transportation Defined is a series of explanatory blog posts designed to define the things you see on your everyday commute. Let us know if there's something you'd like to see explained ... leave a comment here on the blog or over on our Facebook page!

Transportation Defined: Inclinometers

An inclinometer measures slope
inclination at a specific location.

If you've been following our US 89 updates on Facebook, YouTube or the US 89 web page, you know that we have been talking a lot about geotechnical engineers lately. Today, we want to take a look at one of the tools they work with: inclinometers.

Stay tuned to the blog to find out how geotechnical engineers are using inclinometers to assess the site at US 89.

***

When engineers suspect a slope or embankment might be moving, they can use an inclinometer to see if their hunch is correct…

But, what’s an inclinometer?
An inclinometer is a device that measures the slope inclination (movement) at a specific location.

ADOT Geotechnical Operations Manager J. J. Liu explains that an inclinometer consists of a precision-tooled plastic casing (it looks basically like PVC pipe) that has internal grooves carved out of it.

That casing is installed into the ground through a vertically drilled hole.

Next, the casing is weakly grouted into place, allowing it to shift with the ground when (or if) it moves. A probe (not just any probe – this one measures tilt and can calculate the magnitude, direction and scope of any ground movement) is then inserted into the casing in order to measure the inclination at various points along the length of the casing.

“The inclination data is compiled and compared with a baseline reading, which we obtain right after the inclinometer is installed,” Liu says. “The difference at each elevation is the magnitude and direction of the movement at that elevation.”

We mentioned the internal grooves above and we don’t want to forget about those…

Inclinometers out in the field.

The grooves are on the inside of the casing and serve as a kind of track for the inclinometer probe (the probe has guiding wheels – did we mention that? You can see them in the photo above).

So, the wheels sit in the grooves and measurements of the inclinations at various points are taken by the unit as the probe is pulled up from the bottom of the casing.

Hopefully this is making sense, but if not, try to picture a big milkshake (it can be any flavor!).

We’ll say it’s a very thick milkshake … thick enough that you can drill a hole through the ice cream, all the way down to the bottom of your glass. Now imagine sticking a big straw down that hole that you’ve just drilled out.

The straw is kind of like the casing that we described above. If the milkshake moves or shifts a little bit, the straw is going to move along with it.

There’s no real food analogy to go with this next part, so we’ll just say that you’ll place a probe (that same sophisticated probe we described earlier – the one with the wheels that can calculate the scope and direction of any movement) down the hole of the straw…

You’ll use the probe to take periodic readings and, over a period of time, you’ll know how much your shake is shifting.

Transportation Defined is a series of explanatory blog posts designed to define the things you see on your everyday commute. Let us know if there's something you'd like to see explained ... leave a comment here on the blog or over on our Facebook page!

Transportation Defined: Truck Escape Ramps

Unless you drive a big rig, truck escape ramps probably aren’t something you think of too often...

But, you are a reader of the ADOT Blog, which means you're the sort of inquisitive person whose curiosity demands to know what things are and how they work. So, about those truck escape ramps…

There are two primary types – gravity ramps and gravel arrester beds.

A gravity ramp works essentially like this: a truck takes the ramp and experiences an uphill grade that naturally will stop the truck.

A gravel arrester bed works in a completely different way, but the goal is the same. This ramp is on a flat-level grade. Trucks will take the ramp and run into a bed that’s full of small, round gravel. The friction from the wheels going through the gravel is what slows the truck down to a stop.

An ADOT roadway design manager explains that the type of ramp used depends on the spot in which it is being built.

Ramp or no ramp 
Roadway design guidelines help engineers decide whether or not a truck escape ramp is necessary at a certain location. But other factors are looked at, too, including the grade of the highway and whether or not there are curves or a stop at the end of the grade. Observation also plays a part – if ADOT district employees, DPS officers or truck drivers notice a lot of the big trucks are experiencing hot brakes at a certain location, an escape ramp may be considered.

Getting out 
Trucks typically can maneuver off the gravity ramp fairly easily, but because the gravel arrester beds are pretty deep, a towing vehicle has to pull trucks out of those.

There are service roads next to the ramps, so a tow truck can drive alongside and get into position. There also are concrete anchors spaced along the service road that assist with towing the vehicle out of the gravel arrester bed.

One more thing
We thought maybe you’d like an explanation of why these ramps are necessary in the first place. Here’s the answer, according to ADOT’s Roadway Design Guidelines:

The combination of heavy trucks and steep highway downgrades presents a potential safety hazard... Defective or incorrectly adjusted braking systems on trucks or trailer, among other things, can contribute to brake overheating and failure resulting in the driver's inability to control vehicle speeds on downgrades. 

Truck escape ramps offer an opportunity for out-of-control trucks to exit the highway and come to a controlled stop.

But we can’t forget…
Brake check areas are just as important as the truck escape ramps. You’ll find these areas at the top of a summit or just ahead of a long downhill grade – they give truck drivers the chance to check their brakes before heading down.

Transportation Defined is a series of explanatory blog posts designed to define the things you see on your everyday commute. Let us know if there's something you'd like to see explained ... leave a comment here on the blog or over on our Facebook page!

Transportation Defined: Pavement Design Life

After 300-plus blog posts, you’d think that by now we’d have covered everything there possibly is to know about pavement...

But you’d be wrong.

Certainly, it’s not from a lack of trying – we’ve blogged about quiet pavement, pavement at airports, pavement materials testing, pavement markers and even the impact weather and climate can have on pavement. Oh, and remember when we explained how our crews are able to make sure pavement is as smooth as it can be (bonus points if you can recall how a profilograph works)?

It’s just that there’s so much to know about pavement (we’re not kidding – there really is) that we haven’t even scratched the surface.

Which brings us to today’s topic: pavement design life.

Pavement design life is a term that engineers use when they’re planning to build a new road or maintain an existing roadway. They’ll also use a number of years to go along with it, for example: 10-year pavement design life, 20-year pavement design life, etc.

The phrase should not be taken to imply that a road is only being built to survive for a set number of years. What it does represent is the road’s age at which some preventative maintenance or reconstruction will be considered so the road can continue to be durable and useful for the traffic it’s serving.

“For a typical highway, we generally will design an asphalt pavement for 20 years,” said ADOT Pavement Design Group Manager Paul Burch. “It does not mean that the road’s going to be falling apart and rubble in 20 years.”

A little more on pavement design…
Burch says that when engineers want to build a road, they take a lot into consideration, including soil condition, location, expected traffic levels and the area’s climate. Once those factors are determined – and the engineers know the pavement design life they want to build for – design begins.

All those conditions play a role in how the pavement is designed. Say, for example, the road’s being built in an area that gets very cold weather. If that’s the case, engineers will adjust the asphalt pavement mix to account for the temperature extremes.

Now, what if there’s a road that was constructed decades ago that’s not serving the traffic levels very well … can it be built up to a 20-year pavement design life?

Of course it can!

“What we would do if we were to inherit a road like that is we’d start by getting soil samples to test so we can determine what the strength of the soil is. Or, we could do Falling Weight Deflectometer testing so we could determine the strength of the existing roadway and the underlying soils through a method called 'Back Calculation.' From there, we would estimate what our traffic level would be and what design life we would want to design for,” said Burch, adding that the next step would be to create a design that takes all those important factors into account.

For more on pavement, check out our previous blog posts. And, stay tuned – we promise there are more pavement posts to come!

Transportation Defined is a series of explanatory blog posts designed to define the things you see on your everyday commute. Let us know if there's something you'd like to see explained ... leave a comment here on the blog or over on our Facebook page!