Know Your Manuals
Published by the FHWA, this manual sets the national standards for all traffic control devices on public streets and highways.
What is the MUTCD (Manual on Uniform Traffic Control Devices)?
In the United States, this eight-sided sign shape is reserved exclusively for the regulatory STOP sign.
What is an octagon?
The sum of two distances: the distance traversed by the vehicle from the instant the driver sights an object to the instant the brakes are applied, plus the distance needed to stop the vehicle.
What is Stopping Sight Distance (SSD)?
Ranging from A (free flow) to F (forced or breakdown flow), this qualitative letter grade describes the operational conditions within a traffic stream.
What is Level of Service (LOS)?
It is the year the Institute of Transportation Engineers (originally the Institute of Traffic Engineers) was officially founded.
What is 1930?
Published by the Transportation Research Board (TRB), this reference is the go-to guide for computing Level of Service and capacity.
What is the Highway Capacity Manual (HCM)?
Proven to reduce left-turn crashes compared to circular green balls, this four-section signal display uses this specific indication to show a permissive left turn.
What is a Flashing Yellow Arrow (FYA)?
Often denoted by the variable e, this is the banking or tilting of a roadway cross-section to help counterbalance centrifugal force on a horizontal curve.
What is superelevation?
This circular intersection features yield control for all entering traffic, channelized approaches, and counter-clockwise circulation around a central island.
What is a modern roundabout?
Nineteen traffic engineers gathered in this Pennsylvania "Steel City" on October 2, 1930, to establish ITE.
What is Pittsburgh?
Published by AASHTO, A Policy on Geometric Design of Highways and Streets is almost universally known by this colorful nickname.
What is the Green Book?
Before installing a new traffic signal, engineers must evaluate the intersection against 9 specific threshold conditions outlined in the MUTCD, known as these.
What are signal warrants?
This unobstructed, traversable roadside area beyond the edge of the traveled way allows an errant driver to stop safely or regain control of a vehicle.
What is the clear zone?
In macroscopic traffic flow theory, this phenomenon occurs when a sudden transition in density and speed propagates upstream or downstream through a line of traffic.
What is a shockwave?
After witnessing a severe carriage crash at an intersection, this African American inventor patented a three-position mechanical traffic signal in 1923.
Who is Garrett Morgan?
Published by ITE, this multi-volume publication provides data on the number of vehicle and person trips produced by various land uses.
What is the Trip Generation Manual?
Consisting of the yellow change interval and the all-red phase, this time period allows vehicles to safely clear the intersection before conflicting traffic is released.
What is the clearance interval (or change and clearance interval)?
In vertical curve design, this single parameter represents the horizontal distance required to achieve a 1% change in the algebraic difference of grades.
What is the K-value?
While a standard four-leg intersection has 32 of these (diverging, merging, and crossing), a single-lane roundabout reduces the total number to just 8.
What are conflict points?
Inspired by the German Autobahn and a 1919 cross-country military convoy, this U.S. President signed the Federal-Aid Highway Act of 1956, creating the Interstate Highway System.
Who is Dwight D. Eisenhower?
Introduced by AASHTO in 2010, this manual provides analytical tools for quantifying and predicting crash frequency and severity.
What is the Highway Safety Manual (HSM)?
This area approaching a signalized intersection is where a driver at the onset of a yellow light can neither safely stop before the stop bar nor clear the intersection before the light turns red.
What is the dilemma zone?
Used to gradually transition a roadway from a tangent section to a circular horizontal curve, this curve has a radius that decreases uniformly from infinity to the radius of the circular curve.
What is a spiral curve (or Euler spiral / clothoid)?
Formulated in 1935, this classic traffic flow model assumes a strict linear relationship between traffic speed (u) and traffic density (k).
What is Greenshields' Model?
Known as the "Father of Traffic Safety," this American pioneer never learned to drive a car himself, yet invented the rules of the road, one-way streets, pedestrian crosswalks, and the rotary.
Who is William Phelps Eno?