All About: Intersections - Part 1 of 2

I believe that in transportation, some of the really interesting things happen when things come together. Literally! Grand Central for example is a crazy happening place simply because of all the transportation modes that feed to one location. In an Auto network we see this occur a lot more frequently at Intersections. Two streets crossing requires a safe and effective means for all vehicles to travel safely in their intended direction. I see the intersection as a market place where people come to share time together before they continue on their journey. For large intersections this requires money (~$200,000), advance planning, engineering, and important technology to keep drivers happy. In this first post of a two part series, I want to layout three aspects of basic traffic elements of an intersection that keep cars, pedestrians and Bicyclists moving.

Pedestrian Crossings/ Bike Buttons

Facilitating the movement of pedestrians is also a key component of a safely operating intersection. Typically pedestrian buttons and signs give walkers, joggers and dog walker’s informative information about the traffic signal and green directions. More commonly today, these facilities are being upgraded to have audio chirps, tactile padding, and large “mushroom” buttons. All of these features are synergized together to make a pleasant walking environment for pedestrians.

In addition to these pedestrian improvements, bicycle buttons have been added on the outward facing pole to give “in-gutter” bikers the ability to request a crossing. One major difference between his button and a pedestrian button is the amount of time allocated for each button to cross the street. A pedestrian button can give 20 to 30 seconds, while a bike button would give the same amount of time as a single vehicle to cross the intersection. This helps the efficiency of the intersection and prevents too much time given to pedestrians who won’t use it.

More recently we have seen bicycle detectors in the bike lane on the ground. Similar to a vehicle inductance loop to detect a car, these bicycle loops are sensitive enough to detect a bike moving over the loop. While these are yet to be perfect, depending on the mass and material of the bike, they have helped bicyclist automatically interact with the intersection, the only issue still seams to be detecting carbon fiber bikes. Solving this could lead to a huge jackpot for entrepreneurs. With bike detection becoming more common, I can only hope we begin to see bike signal coordination to allow bicyclist to have a pleasant green light stroll the same way cars do!

Vehicle Detection

Traditionally, a fixed time is allocated for each traffic direction no matter how many vehicles line up at each approach. The downside of this, is that it can waste time by giving a green light to a direction without any vehicles waiting. To solve this, wires have been put in the ground in the shape of a circular loop to detect vehicles. These are typically placed closest to the crosswalks to capture the first few row of vehicles.

Installation of these wires requires lane closure to make cuts in the pavement, allowing for the wire to be dropped inside, and then a sealant is placed over the cut to securely embed the wire in the ground. All the wires from the different lanes are pulled to the side of the road into what is known as a pull box, and fed into a traffic cabinet on the side of the road. Electric current  is run through the wires and when a large metal object (like a car) hovers over the wire, the change in current is measured and a vehicle is detected.

Cycle Length

Besides some of the physical geometries of an intersection signal, we can also consider the underlying operation of how a light changes between red, yellow and green for the different movements. No matter how the intersection is set up, a signalized intersection typically operates on a cycle length. A cycle length is simply the amount of time given to have a green period for all the different movements of traffic. Typically a cycle length varies from 60 seconds to 2 minutes depending on the size and amount of vehicles using the intersection.

The cycle length is determined by the traffic engineers either manually or via computer programs for optimization purposes. This and all the other relevant information of a traffic signal is stored on signal timing charts with the city. Through a public records request, anyone can request this information and use it for their own purposes (usually Traffic Engineering Consulting firms will request this from cities when performing a project dealing with the city signals). In addition to cycle length these charts provide NEMA phase information, green, yellow and red times, as well as time of day charts to show when a cycle length may change during the day based on historical demand traffic volumes.

I hope this has outlined some of the basic characteristics of an intersection and provided relevant information for your next encounter with one (hopefully on a bike!). In my next segment focused about intersections, I plan to explore information about signal coordination and optimization and give some ideas for future intersection technology.

Continue Reading Part 2 of 2.