Roundabout Design Experience

Roundabouts are a common form of intersection control used throughout the world and increasingly in the United States.  According to the National Cooperative Highway Research Program‘s Report 672, roundabouts have been demonstrated to be safer for motor vehicles and pedestrians than other forms of at-grade intersections. 

TA has analyzed and designed roundabouts incorporating features accommodating a particular set of circumstances.  Such designs have included:

  • Channelized lanes
  • Expandable design to accommodate a second circulatory lane
  • Bike and pedestrian accommodations
  • Diverging ramps to a shared bike/ped path
  • Traversable central island to accommodate larger vehicles

TA helped design a roundabout near West Virginia University’s football stadium that can be converted during game days with an additional entry/exit accessing a parking lot.  The roundabout is somewhat unique in that it lies on a 7 percent grade.

   wvu-roundabout dsc_0098   plan-jpg

Blazing Ahead with Connected Vehicles

The MASITE/ITSPA Annual Conference in State College, PA, showcased connected vehicle technology with a truck safety demonstration.  Dr. Eric Donnell, the Director of the Larson Transportation Institute, and Dr. Sean Brennan, Associate Professor of the Mechanical Engineering Department at Penn State, led the afternoon events. The demonstration featured a Volvo truck cab’s autonomous response to an incident scene while approaching in cruise control. 

The ambulance at the test scene site emitted a radio signal indicating its presence on the roadway.  The signal was received by the truck and the truck came out of cruise control.  Engine brakes were autonomously applied at a calculated distance according to the truck’s speed.  This application of the brakes automatically slowed the truck down to a safe speed which allowed the driver to steer around the scene or come to a complete stop. 

Time is not Negotiable

Trans Associates is currently at the forefront of test track design for autonomous vehicles working with Uber Advanced Technologies Center (UATC) at several locations in and around Pittsburgh. The relationship with UATC has given Trans Associates an opportunity to set aside convention and apply engineering principles in a highly collaborative and schedule driven environment. 

UATC required a design that would accommodate current and future autonomous vehicle testing conditions.    Flexibility was designed in from the start to accommodate ever evolving test variables at the site.  Trans Associates’ open layout accommodates unconventional intersections, roadway twists and turns, grading changes and pedestrian amenities.  The signal systems were also designed to be changeable in order to mimic various signal conditions found in the U.S. and internationally.  

There are a number of considerations involved with test track design and there are many variables.  One variable is not time.  The project schedules are fierce and aggressive in the autonomous vehicle race and Trans Associates is nimble in its response.  

Signal Technology: An Ever-Evolving Practice

The traditional three-segment traffic signal head has been around since 1920. Since its inception, the design of the traffic signal, and its associated equipment has gotten remarkably more advanced, and the technology trend shows promise of continuing. Several technologies have been developed for the purpose of improving traffic signal operation. Some of these technologies include advancements in pedestrian and vehicle detection, transit and emergency vehicle preemption, and logic controllers which continuously optimize traffic flow.  Each of these elements must be designed for each intersection or signalized corridor individually, and the best design will vary with field conditions. There is no cookie-cutter solution to traffic engineering!

Vehicle Detection

When considering detection, it is important to consider that there are many detection methods, but none of them are perfect. Each method has strengths and weaknesses. One way to overcome potential shortfalls is to implement two different detection technologies at the same signal, so that they can work in tandem.

The most common detector type which is currently in use is a loop detector, which is simply a loop of wire in the ground that generates an electric pulse from the movement of a vehicle nearby. Loops work well for how simple they are, and the electrical signature picked up can be used to indicate if a vehicle is a car, bus, or truck.

These in-pavement detectors have strengths and weaknesses. For example:

·    Susceptible to breaking from pavement shifting.

·    Difficult, expensive, and time consuming to repair.

·    Accuracy can be depleted by environmental factors such as ground capacitance, nearby electric fields or small vehicles.

·    Cannot detect bicycles, certain motorcycles, or pedestrians.

 

Detection technologies have seen considerable advancements in recent years.  It is recommended to consider using radar, video cameras, or infrared imaging as the primary detection method.

Radar, infrared, and video detection can all pick up and classify virtually all vehicle types, including bikes, motorcycles, and pedestrians. As with loops, each of these detectors have strengths and weaknesses. For example:

·    Cameras have high fidelity, but can be occluded by lens deposits and bad weather.

·    Infrared works in all weather, but can be spoofed by hot spots left by idling vehicles.

·    Radar works well in all weather, isn’t temperature dependent, but can be spoofed by erratic movements such as branches and debris in the wind. Also, separate radars must be used for moving and for stopping vehicles and pedestrians.

By combining two of these detector technologies, the strengths of one detector can overcome the weaknesses of the other detector. As with signal timing, the ideal detector types will vary with intersection location and field conditions; however, radar has become the preferred choice by most municipalities and DOT’s. 

Vehicle Coordination

When considering signalization at more than one intersection, coordination is required in order to make the signals work in a way that promotes efficient traffic flow. The simplest and most widely used solution is to employ time clocks at each signal with appropriately designed schedules. Due to clock drift and possible hardware faults, a GPS clock should be used so that the signals remain in sync. The signals can also be made to communicate with each other, by means of radio, twisted pair, or fiber optic cables.

Vehicle Coordination technologies have seen considerable advancements in recent years.  A more advanced form of signal coordination is beginning to take hold, and it is called Adaptive Signal Control (ASC). ASC is a set of software algorithms, communications protocols, and controller hardware which works together to allow the signals to “learn” traffic patterns in real time, all of the time. The traffic data is transmitted to and collected at a central server, recorded and processed, then turned into instructions which are sent back to the signals, telling them how to handle traffic flow. ASC is a great solution for busy corridors, or roads with highly variable traffic patterns. Several measures of effectiveness (MOEs) are available, which can be used to quantify the benefits of implementing ASC. Such MOEs include reduction in delay, reduction in fuel emissions, reduction in queue length, and smoother progression of traffic. When considering ASC, it is important to also consider the engineering and cost requirements to implement and operate a system. Often times, the long-term benefits exceed the up-front cost.

At Trans Associates, our staff can and will guide you towards the most appropriate solution for your needs. We will also design the signals, and coordinate with DOTs, municipalities, stakeholders, and vendors to ensure a seamless experience from inception to construction, whether you need design of a new isolated signal, or enhancements to an entire signalized corridor.