Tag: Traffic

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.

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.