Tuesday, March 22

Want a traffic signal? Here's how it works....

We get, on occasion, a request from folks to install a traffic signal at an intersection location along one of the state-maintained highways across our district. Truth be told, these requests come in with some level of regularity. The reasons for the requests vary slightly, but boil down to a few main reasons:
  1. Congestion levels of roads connecting a residential neighborhood to a main road
  2. Road rage issues with individuals becoming rude or mean in their treatment of other drivers, i.e. cutting people off or not letting others turn onto a road (does this actually happen around here?)
  3. Safety of drivers, kids on buses and pedestrians crossing the road (though these claims are more ostensible than anything)
Truth be told, we conduct routine studies for traffic signals at a number of intersections all over the district. Whether the request has been submitted or not, chances are we've already conducted - recently - a study at your nearest intersection of interest. When these studies are being conducted we're looking for a few things to stand out. Each requirement, or each stand-out item, is called a warrant. The guidelines we follow in these studies are outlined in our Manual for Uniform Traffic Control Devices (MUTCD) section 4C.01. For those who prefer to speak English rather than Engineer, we'll break it down for you.

Warrant 1: Eight-hour vehicular volume
This looks for a certain volume of traffic on the cross street over any eight hours of a given day. Those don't need to be consecutive hours, but can be (for instance) a three-hour period in the morning and a five-hour period in the evening.
There's a minimum threshold we're looking for here, and that minimum threshold is determined by the type of highway and the population density of the area. We look for raw volume as well as potential for interruption of continuous traffic on the cross street. Page 460 of the MUTCD shows a chart and the details of what we're looking for, but generally speaking we're looking for a minimum of 500 vehicles per hour on the main road with at least 120 cars per hour on the side street. That, or we're looking for more than 700 vehicles per hour on the main road with at least 60 cars per hour on the side street.
Though that looks pretty easy to reach, truth is those numbers - sustained over eight hours of the day - can be tough to reach.

Warrant 2: Four-hour vehicular volume
This looks for a certain volume of traffic over any four hours of a given day. This is similar to Warrant 1, but a plot graph is used. On the graph (figure 4C-1 in the MUTCD) you'll have 24 data points plotted - one point for each hour of the day. We're looking for just four points above the appropriate line.
You'll note (if you look at the graph on page 462) three lines here. The bottom line is for intersections where both the major street and side street are one lane each direction. The middle line is for roads with two (or more) lanes on the major street and one lane on the side street (again, in each direction). The top line is for situations where both the major and minor streets have two or more lanes in each direction.
You'll note the volumes we're looking for over this four-hour warrant are significant. If the cross-street holds 80-100 trips per hour, the major street would need to have more than 1,000 trips per hour on a major road with just one lane in each direction. For those keeping score, that's double the volume of traffic on the major road required for an eight-hour warrant.

Warrant 3: Peak-hour vehicular volume
Some intersections are an absolute nightmare (a term we hear often from folks calling in) during a one- or two-hour block during the day, but the rest of the 24-hour period the intersection is nearly a ghost town.

One glaring area that's near and dear to our hearts lately is in the Leon Springs area, particularly between Ralph Fair Road and Boerne Stage Road. That area is absolutely hammered by traffic in the morning for about a two-hour period, and the rest of the day is pretty well free and clear.
At these locations we have the peak-hour warrant, where we're looking for a lot of dominoes to drop all at once. An important thing to remember here is these intersections are given signals only in very unusual circumstances, such as where a major traffic generator is at that intersection (think of the USAA entrances along Fredericksburg Road).
One way to go about this is a plot graph, just as in Warrant 2. The numbers are different again, but this time we're looking for just one or two dots above the appropriate line.
The other thing - and, really, the warrant we're looking at most here, is a combination of three items that must exist. These include:
  1. Total stopped time delay on the minor street equals or exceeds four vehicle-hours
  2. No fewer than 100 vehicles are using the minor street per hour
  3. No fewer than 650 vehicles per hour are turning onto the minor street from the major street
These warrants are pretty darned difficult to reach, and that's kind of intentional. Again, this warrant is one that's to be used sparingly. Why? Because if we started granting signals willy nilly we'd end up with a gridlocked system, bound by a saturation of signals. Our preference is a free-flowing system of responsible drivers with minimal stops along the way.

Warrant 4: Pedestrian volume
This is only used where we've got a high volume of pedestrian traffic and that foot traffic is inhibited by the heavy volumes of cross traffic. This is done by looking at a four-hour period (minimum 100 pedestrians per hour) or a peak period (minimum 75 pedestrians per hour) and follows a similar pattern of thought as Warrant 2 and Warrant 3 using plot maps and curve lines.
If this crossing point is within 300 feet - the distance of a football field - of a signalized crosswalk or an intersection with a stop sign in place, we really won't be looking to install the signal. There are some exceptions to that rule, but those exceptions come with a ton of compelling evidence (including the fact the signal doesn't hinder or restrict progressive movement of traffic).

Warrant 5: School crossing
This is really for areas we have school children crossing a major street and that's the driving reason for the signal. Keep in mind what we're looking for here is a volume of kids crossing the street as pedestrians on their way to or from school. This includes elementary through high school kids.
Before these signals are to be installed, however, we've got to consider other measures. Traffic signals under Warrant 5 are to be, therefore, kind of the last option on the table. We try to address issues like this with warning signs and flashers, school speed zones, use of school crossing guards and so forth - all the normal stuff you'll see in or near a school.
Heck, the MUTCD even tells us to consider installing a pedestrian bridge before installing a traffic signal in a school zone for a crosswalk.
Again, if there's a controlled crossing nearby - within 100 feet - this warrant cannot be met except in very specific circumstances.

Warrant 6: Coordinated signal system
The goal of this warrant is to keep traffic moving in proper platoons to maximize overall traffic flow. This study looks particularly at one-way streets but can also be used on two-way roads. Volume of traffic isn't really the question for Warrant 6, but the spacing between traffic signals. This warrant cannot be used where a signal exists within 1,000 feet, but can be used to construct a signal in a spot where traffic flow can be best regulated.
This isn't a warrant we use very often with the San Antonio District.

Warrant 7: Crash experience
Remember where we said safety was ostensibly cited as a need for traffic signals? This is where we look at the actual data to see if safety is, indeed, an issue. During this portion of the study we look at the frequency of crashes as well as their severity. This is one of those areas where we need an "all of the above" requirement before we can justify a signal. Why? Because a single crash caused by some incredibly selfish and irresponsible individual paying more attention to their phone than to the traffic isn't enough to necessitate a signal. But if there's a pattern ... well, now we have an issue.
First, we've had to have tried alternatives to mitigate the risk of wrecks before immediately going for the signal. This can include stuff like signs and increased law enforcement presence.
We've also got to have more than five reported crashes within a 12-month period that would have been prevented by a signal. These crashes must me more than a simple fender-bender.
Finally, the intersection must meet similar traffic volume requirements found in Warrant 1.
All of this data is compiled and if each of the three boxes are checked, the warrant has been met. To be honest, next to Warrant 1, this one is the big'un. If we see a warrant met here, you can bet the location will be vaulted to the top of our priority list.

Warrant 8: Roadway network
This one is really quite simple: if both roads of an intersection are major roadways - that is, a route that serves as the principle roadway network for through traffic flow, is a rural or suburban highway, appears as a major route on a transportation plan map, connects major hubs or is a surface street with ramps and overpasses - we'll get a signal at that location. That goes for intersections that are projected to become a major intersection.
We're talking about the bigger intersections, too - intersections featuring traffic volumes of about 1,000 cars per hour on both streets. In instances where the intersections are only projected to meet this warrant, we may opt to address the issue with future development projects (say, for instance, we have a plan to expand one of those roadways; we'd include the signal as part of that expansion project).

Warrant 9: Intersection near a grade crossing
This involves proximity to a railroad crossing - that's what engineers mean when they say "grade crossing", is a point where the roadway intersects a railroad.

At issue here is when an intersection is very near - within 140 feet - to a railroad crossing. We see this more than just once along FM 78 through Cibolo and into Marion. We're looking for an intersection where a stop or yield sign would be otherwise used and we have a certain level of traffic volumes. Here we use a plot map similar to what we had in Warrant 2 and Warrant 3, but we're instead comparing traffic volumes on both roadways to the proximity of the railroad crossing. These are found on page 469, for the curious.
The goal with this warrant is to avoid having the traffic stopped at the railroad stack up through the intersection in question, causing potential for unnecessary congestion and safety hazards.

Let's sum this up
One major item of note, verbatim from the MUTCD: "The satisfaction of a traffic signal warrant or warrants shall not in itself require the installation of a traffic control signal."
This is absolutely critical to keep in mind. Why? Because, truthfully, installing a "marginally warranted" signal could pose a safety risk. Per our transportation engineer, "the increase in rear-end collisions on the primary street and the potential for a lot of red-light running on the primary street" make us look long and hard at these intersections before actually putting in a signal. Human behavior is the factor here - drivers just don't like stopping for a small amount of side-street traffic.
These intersections are instead placed on a watch list and, as traffic volumes or other factors change to meet other warrants, they are elevated on the list until they reach a priority level where we'd go ahead and install the signal.
Here's the deal: Traffic signals are expensive. Not just to construct (our price tag is in the neighborhood of $250K, depending on whether or not we already have any conduit infrastructure in place), but also to maintain. Where the signal falls within city limits of one of our major cities, that maintenance (including timing) falls on the shoulders of the municipality. Putting a signal in may mean some strain on the already stretched resources many of these cities are working with.
All that said, safety is our absolute priority. Our top core value is people, and it's incumbent upon us as an agency by the people and for the people of Texas to keep safety measures in place. If there's an absolute need, we'll fill it. If congestion levels are becoming an issue, we'll address that as well.
Most often this is done by adding the signal to an upcoming project - it's actually less expensive this way, as we don't have to mobilize forces for a single intersection, but address the issue with an active construction project. If we have a project programmed in the upcoming few years, we might simply take that route to address the need (if the project budget affords it). If the need is more pressing, we respond accordingly.
Either way, we really are taking a look at our intersections - far more often than many may surmise - and determining actual need against actual numbers available. The more warrants an intersection meets, the higher on the priority list it goes.