Category: Transportation

Can We Predict Black Ice – Factors to Prepare Winter Roadway Treatments

Snow covered road

Winter is officially here, which means icy temperatures, snow days and longer nights. This time of year we all pay a little closer attention to the weather and in particular, how it could potentially affect driving conditions. The idea that pavement temperature directly corresponds to air temperature is a common misconception and is in fact only one of several factors that needs to be taken into consideration. Air and pavement temperatures can differ by several degrees. The difference is critical in predicting and preparing for black ice, which poses a serious threat to all motorists each year because of the difficulty it takes to identify.

Forecasting pavement temperatures and conditions is difficult but not impossible. When predicting conditions, four major factors are considered: air, sun, moisture, and the amount of heat beneath the pavement.

Air:

According to the Law of Thermodynamics, every object is in a constant state of temperature change. If you place a cold object in a warm room, the temperature of that object will steadily increase; if you place a warm object in a cold room, the temperature of the object will decrease. If you hold the temperature of the room constant, the object will adjust accordingly until it reaches room temperature. Roadways are no exception to this rule. However, the balance of heat is a gradual process and the speed by which it occurs is heavily dependent on several factors, such as surface area, density and material. For example, in the event of a temperature drop, a bridge (which is exposed to the air on all four sides and made of metal and concrete) will cool faster than the street, with only one surface area exposed to the air.

Below the Pavement:

It’s easy to forget what isn’t out there in the open for us to see. Half of the pavement surface area is affected by the ground beneath it, so subsurface temperatures play an equally important role when considering pavement temperature.

In the fall, the pavement is usually warmer than the air because the subsurface temperatures are still cooling down from the summer months.

In the spring the air is warmer than the pavement because a lot of the ground is still thawing from months of below freezing temperatures.

During a snowstorm the air is below freezing; snow may accumulate but if the ground underneath is warmer than the outside air, the snow will melt.

In addition, rain falling on pavement atop freezing subsurface temperatures may be enough to freeze over the roads.

While this general information is good to know as a guideline, accurate subsurface temperatures can only be measured with a Road Weather Information System (RWIS) installed by the department of transportation. Therefore, it is best to be cautious on the roads when seasons are changing, especially if you’re in a new area.

Sun:

Even in the winter, the sun still has a huge influence on pavement temperatures. It is so powerful, in fact, that a cloudy day can cause a decrease in the pavement temperature by 10 degrees. Despite cold and miserable weather conditions the pavement is constantly being affected by solar radiation. When dealing with the effects of radiation, a meteorologist considers elevation as an additional factor. Higher elevation means closer proximity to the sun and increased exposure to radiation.

Moisture:

Rain, snow and water vapor are the three forms of moisture in the atmosphere. Just like cooler temperatures, rain and snow typically cool down the surface of the pavement. The harder the rain or snow falls the faster the pavement will cool down.

Out of the three forms, water vapor is the most difficult form of moisture in the atmosphere to measure, because you cannot visibly see it. The most commonly accepted form of measuring water vapor in the air is dew point; the temperature below which water condensation occurs. The greater the difference between dew point and the air temperature, the drier the air. We know from the water cycle that once rain falls, it will evaporate back into the air. Evaporation requires heat to occur and there is heat in the pavement. Therefore, the drier the air, the faster evaporation will occur and in conclusion the faster the pavement will cool.

Finally, it is important to understand how air temperature and moisture come together in the formation of black ice. A dangerous misconception is that it needs to be snowing or raining for black ice to occur. Black ice usually occurs when the dew point and air temperatures converge. At this point, the air can no longer hold the moisture, so it condenses onto the pavement. Black ice can also occur when the air temperature is below zero but is warmer than the pavement temperature — requiring only that the pavement temperature is below freezing.

As we have already stated, predicting pavement temperature is complicated. Predicting the air temperature for 5:00 pm tomorrow is already a difficult task. If they predict the air temperature incorrectly, it automatically throws off the accuracy of the pavement temperature prediction. In addition, there are a variety of other factors that change quickly and with less notice (air temperature, precipitation, clouds, thickness of the pavement, etc.). That’s why we rely so heavily on site specific RWIS technology for the most accurate prediction of pavement temperature. State DOTs rely heavily on these pavement forecasts to determine when to pretreat roads, when to schedule crews, and how much material will be required throughout the duration of the event in order to ensure your safety.

Despite predictions and precautions, dangerous winter storm conditions are not 100% preventable. Stay safe this winter and listen to winter weather advisories. Even if there is no snow, black ice is a real possibility.

Why We Don’t Design Roads for Holiday/Summer Traffic Volumes

Arial image of a roadway intersection in Derry NH showing traffic

It never seems to fail, you make careful plans, you wake up early and hit the road, eager to get to your Thanksgiving destination or your holiday weekend getaway, but almost inevitably you round a corner on the highway and see a stretch of nothing but brake lights in front of you and your best laid plans are dashed. We all know what it’s like to travel during holidays – there can be dead stops on highways, long waits at traffic signals, and hazard lights from crashes. When you’re frustrated by all the other cars on the road, you might wonder why it doesn’t go more smoothly and the reason is transportation and traffic engineers do not design roads just for peak holiday travel times.

The reason is more complicated than you might think.

Why Are the Roads Congested during Holiday Travel?

Whether it’s the traditional annual pilgrimage home to see family and relatives or a long holiday weekend to kick off summer, certain holidays (such as Thanksgiving and Independence Day), generate a very sharp increase in road travel compared to most other times of the year. While the congested traffic may feel like a torture trip designed to make your vacation less relaxing, the simple fact is it is not responsible to design every road and intersection to meet the peak traffic demands of a holiday weekend. For the vast majority of roads, the costs for construction and on-going maintenance are paid by towns, cities, or state DOTs, which means taxpayer money. For example, if there are 10 days out of the year where the local interstate would require five lanes each direction to have an acceptable Level of Service (a performance measure used to analyze capacity of and intersections, aka LOS), but the other 355 days of the year, just three lanes each direction provides a good LOS for users, then it would be an irresponsible use of taxpayer money to build that 10-lane interstate.

Now just because all of the roads and intersections are not being designed specifically to service the peak demand of holiday travelers does not mean those traffic volumes will immediately cause backups at intersections or stand stills on the roadway or highway; those headaches generally involve the driver behavior factor. Whether someone was slow to get on the gas at a stoplight because they were looking at their phone or slammed on the brakes because someone cut them off on the highway, all of these actions have much more widespread impacts once the roads are inundated with higher traffic volumes.

These behaviors do occur all throughout the year, regardless of the traffic volumes, but it’s the flexibility or capacity of the road system that gets affected.

Consider a room and a pool of the same size; both have you and a few of your friends spread about. In the room, you flail your arms wildly, maybe your closest one or two friends feel a slight breeze, nothing significant, and anyone beyond that feels nothing. In the pool, you merely take a step and you’ve sent ripples throughout that are reaching even the most distant people. Silly as it may sound, this comparison applies to traffic as well. During normal day-to-day operations the space between cars is much like the space between air molecules where there is enough empty space that not every action causes a chain reaction, while the holiday travel is more like the pool scenario where the roads are so densely packed that any action, no matter how seemingly insignificant, is likely to cause or require a reaction from nearby drivers.

Okay, so How Much Traffic Do you Design for?

The traffic volumes accounted for in design vary depending on the element being talked about.

Let’s talk about traffic signals and road segments first. Say we’re upgrading an existing stop-controlled intersection to be a signal and need to provide turning lanes for every direction. The design of those turning lanes will usually be based on near-peak traffic volumes, the definition of which varies from state-to-state.

For other elements of the road affected by traffic volumes such as roadside barrier needs and thickness of pavement, it is usually standard practice to use the Annual Average Daily Traffic (AADT) for design. This is the overall average volume from the high traffic summer months to the lower traffic winter months.

Many projects require advanced planning to accommodate both vehicle and multi-modal traffic flow. Whether it be for roads, bridges, highways or pathways, proper traffic operations and accommodations for all modes of travel through urban or rural areas is essential. That’s where we come in. From traffic and safety studies to signal design, pedestrian/bicycle infrastructure, and alternative intersections, our engineers have the resources that help you arrive safely no matter when you travel.

Stickney Hill Road: An Unexpected Culvert Replacement

Project Manager Audrey Beaulac recalls one of her most memorable projects from 2019 that stands out from the rest. It’s the story of a deteriorated culvert that wasn’t on the town’s to-do list but quickly became a priority when it failed its routine New Hampshire Department of Transportation (NHDOT) inspection. Her story is below:


Stickney Hill Road in Hopkinton, New Hampshire, provides residents quick access to I-89, is a highly utilized bicycle route for those that want to ride into Concord, and is a school bus route. In June 2019, NHDOT performed a routine bridge inspection of the 10-foot-wide Stickney Hill Road Bridge that conveys Boutwell Mill Brook beneath the roadway. The inspection revealed the corrugated metal pipe had deteriorated significantly since the previous inspection; NHDOT sent a letter to the town advising them that the bridge is at critical deficiency, requiring posting a  BRIDGE CLOSED sign with suitable barricades at each end of the bridge to prevent vehicle use. As a result of this notification, the town closed the road and bridge to  motor vehicles, but fortunately bicycle and pedestrian access and use was maintained for those wanting to connect to the regional trail network close by.

However, closing the road meant potential emergency response delays for residents in the area between the crossing and I-89, Exit 3. Exit 3 is not a full-service exit and only allows access from I-89 northbound and onto I-89 southbound. The detour included using Exit 2 of I-89 to reverse direction to utilize I-89 northbound from Stickney Hill Road or using Exit 2 to reverse direction to access Stickney Hill Road from I-89 southbound. The town line between Concord and Hopkinton is also between the crossing and I-89, which would cause issues for school buses when school starts in August. Additionally, mail and package carriers were impacted by the closure, and some even stopped delivery. The town wanted to reopen the important roadway as soon as possible.

Project Site Map

Thinking outside the box, and knowing lead time to cast a precast concrete culvert could take months, the town asked Hoyle Tanner if the culvert they just had cast for the culvert replacement project they were currently working on along Briar Hill Road would provide the necessary hydraulic capacity required to convey Boutwell Mill Brook beneath Stickney Hill Road – and if it would be structurally strong enough to support the road and earth materials at that location. The town was a week away from installing the Briar Hill Road culvert but quickly switched directions when the Stickney Hill Road bridge replacement became a priority. Since Hoyle Tanner designed the precast box culvert for the Briar Hill Road project, we took a look at the size and design and compared it to what would be required for the Stickney Hill Road location. Upon further investigation, the similarities of the two sites proved advantageous, allowing for the already-cast Briar Hill Road culvert to be used for the Stickney Hill Road location.

With a quick replacement solution in hand, all that was left was the permitting. Since there would be wetland impacts at the crossing needed in order to replace the structure, a wetland permit would be required from the New Hampshire Department of Environmental Services (NHDES)  with final approval from the Army Corps of Engineers. On behalf of the town, Hoyle Tanner coordinated with NHDES and initially sought Emergency Authorization to repair the crossing. The structure was subsequently determined to not be an immediate threat, and the standard process was used for wetland permit application and approval.  

As time was of the essence, Hoyle Tanner quickly finalized the design and coordinated with NHDES. Upon approval of the wetland permit by NHDES and concurrence by the Army Corps of Engineers, the town’s contractor began construction in the fall and opened the roadway to traffic in late fall 2019.

“Hoyle Tanner is wonderful to work for and that is why the Town keeps working with Hoyle Tanner.”

Dan Blanchette, Director of Public Works

The project’s primary goals were to utilize the already-precast structure the town had on hand and open the roadway to through traffic before winter. The town was grateful Hoyle Tanner was able to meet their goals and remain on budget. Dan Blanchette, the Director of Public Works said, “Hoyle Tanner is wonderful to work for and that is why the Town keeps working with Hoyle Tanner.” And don’t worry, Briar Hill wasn’t forgotten, the town got a new culvert cast right away and was able to complete that project, as well.

Traffic Modeling 101: Using Traffic Modeling Software to Improve Mobility

Traffic model snip showing intersection and cars

What it is

Traffic modeling takes raw data (in the form of traffic counts and speed data) and builds a visual representation. This visual representation allows us to see how things interact with each other, which can be as simple as a stop-controlled intersection or as complicated as an entire city grid. The modeling allows us to look at how intersections perform in terms of level of service, traffic delay, and capacity utilized among other metrics.

Traffic modeling doesn’t just show how cars move in a straight line on a road. Instead, the modeling shows how traffic might back up at an intersection based on how much green (light) time each direction of traffic is given, how side roads are affected by long lines of vehicles, and what is happening at turn lanes. We also include pedestrians when there’s significant data for them; at small, rural intersections, there is not enough demand to show them in the model.

The level of service is the key metric for analyzing how well a signal functions. Level of service is categorized by five letter grades (A through F), but it’s really just an incremental delay in seconds. For example, if the average driver is stuck at a traffic light for less than 10 seconds, that’s level of service A. If it’s over 10 but less than 20, that’s level of service B, and so on. So really, the level of service is just a way to say this is the range of delay that the average person gets at this intersection. It’s key that it’s the average driver; so the first person who pulls up to a red light is likely going to be sitting there for more of the full signal cycle, but someone that arrives on green had a zero second delay – that’s why it’s key to measure the average.

Why it’s useful

I’ve been using the modeling software since I started here in 2013. It was pretty basic for the first few years, really just using it to model temporary signals; like if we had to go to a one-lane work zone with alternating directions of traffic, we’d use a temporary signal for that and need to model it to make sure the queues didn’t cause any big problems. In terms of how traffic modeling differs from pure calculations, it really has to do with its scale. You know, you can input some parameters into the software, and it runs all the iterations you need and can simulate random traffic patterns that a calculation wouldn’t be able to do. It also helps give you a visual representation of it. I could do a calculation that says, okay there’s a 300-foot queue here, but then when we put it in the modeling software, we can see that the queue is actually blocking a side road or spilling into the next traffic signal.

The flexibility to play around in the software is also significant. With a calculation, if you want to change something, you more or less have to restart the calc; but in the model, you can toggle a switch and it just completely changed your model – and you can change it back if you need to.

Our standard traffic modeling program is Synchro which is the static model, and then we also have SimTraffic which creates a video simulation of cars moving through the model. The video is the simulation of when the system is populated – that’s what uses the random traffic patterns, which is helpful because there is no calculation for random traffic patterns. You need to have the computer algorithm that best approximates random traffic driving patterns. With that simulation, you get to see how signals interact with each other; so you have one signal, and then you have another one 300 feet away; they might not be coordinated, but they will still influence the traffic patterns at each other, and it’s crucial to see what sort of problems they may cause.

What the challenges are

There are only minor downsides to traffic modeling software. There are so many different parameters in the programs that you might get a totally different result if you overlook one that’s buried deep in the dialogue boxes. In terms of reporting, there are also several different analysis methods you can get from the program. The simulation doesn’t change, but I can have the same traffic volumes and signal timing and still get three slightly different results based on the analysis method. There’s no significant difference, but depending on what the client or agency expects when they review it, it can impact the program’s options.

A good example is New Hampshire Department of Transportation (NHDOT) has published preferences for their report formats, but many clients do not have preferences, and so the lack of standardization can be a challenge.

Where it’s headed in the future

In the future, we will be using traffic modeling software more often. The developers of the traffic modeling software are continuously working on and releasing updates for the programs. We as designers are constantly trying to come up with new ways for traffic signals to be safer or to handle higher capacity. Sometimes, the software doesn’t have the availability to model those correctly because it’s a new innovation that hasn’t made it back into the software yet. So sometimes these updates are just the software catching up to what’s actually being in done in the field.

I expect there will also be some improved bicycle and pedestrian modeling capabilities. Right now, we can say there’s X number of bicycles per hour, but I envision software developers will be adding bicycle signal heads next to traffic lights because that’s an up-and-coming technology. It’s been tested in a couple of states already, and it could become an important part of traffic modeling software updates in the near future.

I’m part of a team that prepares traffic modeling projects for municipalities and state agencies across New England. Reach out to me with traffic questions or to learn more about NHITE.

How the ADA has Changed Transportation Infrastructure After Nearly 30 Years

Nearly one in four adults has a disability in the United States. The most common disability is mobility, followed by hearing and vision. “At some point in their lives, most people will either have a disability or know someone who has one,” according to Coleen Boyle, Ph.D., director of CDC’s National Center on Birth Defects and Developmental Disabilities. After almost 30 years since it was first established, the Americans with Disabilities Act (ADA) has had a significant impact on transportation engineering and design for public use.

Recognizing the challenges of adapting the ADA guidelines to existing transportation infrastructure (sidewalks, street crossings, curb ramps, etc.), the United States Access Board is in the process of developing new guidelines for these facilities called “Accessibility Guidelines for Pedestrian Facilities in the Public Right-of-Way,” popularly known as PROWAG. These guidelines, which are currently a draft but recognized as an industry best practice, establish practical design standards for these elements while recognizing the various constraints posed by space limitations, roadway design practices, slope, and terrain.

Learning about ADA compliant engineering designs can help make streets a safer place for those 61 million Americans living with a disability. Below are two types of engineering designs that you commonly see but might not fully know their purpose:

  1. Accessible Pedestrian Signal and Push Button:  These devices are used at pedestrian crossings to provide non-visual information (audible or vibratory) on WALK and DON’T WALK intervals.  Through tones and vibration, they help a visually-impaired pedestrian locate the push button, know when the walk interval has begun, and know the direction of crossing. It can also provide information on the crossing location through braille or speech messages. The location and design of a pedestrian push button are also critical for those with disabilities. The buttons should be located adjacent to a level all-weather surface and within 3.5’ to 4’ above the sidewalk to provide access from a wheelchair (shown below). To ease operation, they should also be large enough to push with a closed fist and no more than 3.5 pounds of force.ada-document-image_edited
  2. Curb Ramps and Detectable Warnings on Surfaces: Accessible sidewalks are imperative to those living with mobility issues. Safely crossing a street or entering/exiting a parking lot can be difficult for those in wheelchairs if there are no curb ramps. Curb ramps (as shown below) provide a maximum of 1:12 grade to smoothly transition from the sidewalk elevation. As the ramp is flush with the roadway, a detectable warning surface is required to provide tactile cues to those with vision impairments to alert them that they are entering the roadway. A change in surface texture and color, such as concrete, can also be more conspicuous for those with minor vision impairment.

ada-second-image_edited

The ADA was a Civil Rights Movement that prevented discrimination of those with disabilities. The two engineering designs described above were designed to comply with ADA. These designs, and others, will continue to help pedestrians feel more comfortable making their way safely across the street.