CIVE3065 Traffic Engineering - Key Definitions and Concepts

Comprehensive vocabulary flashcards covering key definitions, traffic signal parameters, transportation planning terms, and core analytical relationships for CIVE3065 Traffic Engineering.

Aspect

The signal indication displayed to traffic at a signalized junction.

Stream

Traffic that is treated as a single queue.

Phase

Traffic or a collection of streams receiving the same aspect simultaneously.

Stage

A time period within a signal cycle when specific phases display green signals.

Cycle

A complete sequence of all signal phases at a junction.

Cycle Time ($C$)

The total time taken to complete one full cycle of signal phases.

Combined Green and Amber ($G$)

The total duration of the green and amber display for a phase, given by $G = k + a$, where $k$ is actual green time and $a$ is the duration of the amber phase.

Saturation Flow ($s$)

The maximum continuous rate of flow possible to discharge from a traffic lane or approach during the green indication.

Phase Lost Time ($l_p$)

The time lost due to starting delays when a signal turns green and flow fall-off during the amber aspect (standard assumption is $l_p = 2\,\text{s}$).

Effective Green Time ($g$)

The sum of the green and amber periods less the phase lost time, expressed as $g = G - l_p = k + a - l_p$.

Intergreen Period ($I$)

The time period from one phase losing green to the next conflicting phase receiving green (standard minimum assumption is $I = 5\,\text{s}$ plus any all-red time). Minimum of 4s with red/amber overlap

Actual Green ($k$)

The displayed duration of the green signal aspect, related to effective green by $k = g - 1$ when $a = 3\,\text{s}$ and $l_p = 2\,\text{s}$.

Intergreen Lost Time ($l_i$)

The lost time occurring during a phase transition, calculated as $l_i = I - a$, which consists of all-red time plus red/amber time.

Total Lost Time Per Cycle ($L$)

The sum of intergreen lost times across stage changes plus phase lost times across relevant phases: $L = \sum_{\text{stage changes}} (I - a) + \sum_{\text{phases}} l_p$.

Approach Capacity

The maximum traffic volume an approach can discharge per cycle, given by $\frac{gs}{C}$, where $g$ is effective green time, $s$ is saturation flow, and $C$ is cycle time.

Degree of Saturation ($x$)

The ratio of actual arrival flow ($q$) to maximum capacity ($\frac{gs}{C}$) on an approach: $x = \frac{qC}{gs}$.

Trip

A one-way movement from one place to another for a particular purpose.

Origin

The starting point of a trip.

Destination

The end point of a trip.

Study Area

The geographic area through, and within, which traffic movements are to be studied.

External Cordon

A continuous (imaginary) boundary line drawn around or across the perimeter of the study area.

Cordon Crossing Point (Cordon Station)

A location where the external cordon crosses highways having significant levels of traffic flow.

Vehicle Desire Movement

That part of an origin-destination trip recorded within the defined study area.

Zone

An individual origin (destination) or a grouping of geographic origins (destinations), categorized as internal if inside the study area or external if outside.

Zone Centroid

A single representative point within a zone assumed (for transport modelling purposes) to be the origin or destination of all trips starting or ending in that zone.

Trip Matrix (Origin-Destination Matrix)

A table or matrix in which each entry specifies the traffic flow (number of trips) occurring from a particular origin zone to a destination zone.

Screenline

A line dividing the study area into two portions such that traffic flows crossing it during a survey can be measured easily.

Fundamental Relationship of Traffic Flow

The macroscopic relation linking flow ($q$), concentration/density ($K$), and space mean speed ($v_s$): $q = K v_s$.

Time Headway ($\bar{h}_t$)

The average time elapsed between successive vehicles passing a point, defined as $\bar{h}_t = \frac{3600}{q}\,\text{s}$, where $q$ is flow in $\text{veh/h}$.

Spacing ($\bar{s}$)

The average distance between front bumpers of successive vehicles, inversely related to concentration/density by $K = \frac{1}{\bar{s}}$.

Flow Ratio ($y$)

The ratio of arrival flow ($q$) to saturation flow ($s$) for an individual lane or approach: $y = \frac{q}{s}$.

Total Flow Ratio ($Y$)

The summation of the critical or governing flow ratios for each stage of a signal cycle: $Y = \sum_i y_i$.

Reserve Capacity ($RC$)

The percentage margin by which practical junction capacity exceeds existing critical traffic demand: $RC = \frac{Y_{pract} - Y}{Y} \times 100\%$.

User Equilibrium (Wardrop's 1st Principle)

A condition where journey times on all used routes connecting an origin and destination are equal and less than those that would be experienced on unused routes ($c_1(f_1) = c_2(f_2)$).

System Equilibrium (Wardrop's 2nd Principle)

A condition where total network travel cost is minimized by equating marginal link travel costs: $\hat{c}_1(f_1) = \hat{c}_2(f_2)$, where $\hat{c}(f) = c(f) + f \frac{dc}{df}$.

Multinomial Logit Model

A discrete choice modal split model calculating the probability of choosing mode $m$ via utility $U_m$: $P_m = \frac{e^{U_m}}{\sum_j e^{U_j}}$.

Furness Method

An iterative bi-proportional matrix balancing procedure in trip distribution that successively multiplies rows by origin growth factors ($G_i^O$) and columns by destination growth factors ($G_j^D$) until factors converge near 1.