PE Transportation key terms

A letter grade from A to F describing how well a roadway facility serves its users, based mainly on speed, travel time, freedom to maneuver, and delay compared with free-flow conditions.

The maximum number of vehicles that can discharge from a lane or approach during one hour of green time, assuming continuous queued traffic; a core input to signal capacity and level-of-service analysis.

The ratio of total hourly volume to four times the busiest 15-minute volume within that hour, used to describe how unevenly traffic arrives during the peak hour.

The yellow plus all-red time at a signal that lets vehicles clear the intersection before conflicting traffic is released, computed from approach speed, distance, and vehicle length.

The range of distances from a signalized intersection where a driver can neither stop comfortably on yellow nor clear the intersection before red, creating indecision and crash risk.

Planned control of driveway locations, spacing, and street connections to limit conflict points and preserve the capacity and safety of a major roadway.

Physical or geometric treatments such as speed humps, chicanes, and narrowed lanes intended to reduce vehicle speeds and cut-through volumes on local streets.

An unsignalized circular intersection where entering vehicles yield to circulating traffic, reducing severe crossing conflicts by converting them to lower-angle merging and diverging movements.

The selected speed used to determine geometric features such as curvature, superelevation, and sight distance so that a roadway is consistent and safe for expected drivers.

The total distance traveled during perception-reaction time plus braking distance needed to stop before a stationary object in the travel lane, measured along the roadway.

The rate of roadway cross-slope provided on horizontal curves, banking the pavement so vehicle weight helps counteract centrifugal force and reduces reliance on side friction.

The radius of the circular arc defining a roadway's horizontal alignment; smaller radii require lower design speeds, more superelevation, or greater side friction.

A parabolic transition between two grades that provides gradual change in slope, with length set to satisfy stopping sight distance and comfort criteria.

The horizontal length of a vertical curve per one percent change in grade; multiplying by the algebraic grade difference gives the minimum curve length for a given sight distance.

The unobstructed, traversable roadside area beyond the edge of the travel lane where an errant vehicle can recover safely, with width based on speed, traffic, and slope.

The transverse slope of the pavement, usually expressed as a percentage, that drains water from the surface and affects vehicle handling and hydroplaning risk.

A mathematical representation of how land use, population, and employment generate trips that are distributed, assigned to modes, and loaded onto the network.

The step in travel demand forecasting that links trip productions and attractions between zones, often using a gravity model based on travel time and activity size.

The forecasting step that estimates the share of trips taken by each travel mode, typically using utility-based logit models of cost, time, and convenience.

A table giving the number of trips between each pair of traffic analysis zones; it is the central product of trip distribution and input to traffic assignment.

Additional traffic that appears on a roadway after capacity is expanded, because lower travel times attract trips that were previously suppressed, shifted, or made by other modes.

Vehicle miles traveled, the total distance driven by all vehicles in an area over a period; used to measure exposure, emissions, and demand.

A multiplicative factor expressing the expected change in crash frequency at a site after a treatment, where values below one indicate a reduction.

The percentage reduction in crashes attributed to a treatment, equal to one minus the crash modification factor expressed as a percent.

A regression equation that predicts the expected average crash frequency at similar sites from traffic volume and roadway characteristics, forming the basis of empirical Bayes analysis.

A statistical method that combines a site's observed crash count with the count predicted by a safety performance function, weighting each by its reliability to estimate true safety.

An independent, formal review of a roadway project's design or operation by a qualified team that identifies potential crash risks before problems occur.

A location where the paths of two vehicles, or a vehicle and a pedestrian, cross, merge, or diverge, creating a potential collision; fewer conflict points generally mean safer intersections.

Equivalent single axle load, a damage unit representing one 18,000-pound axle load; pavement design counts accumulate ESALs to reflect mixed truck traffic.

An index of the total structural capacity of a flexible pavement, computed by summing layer thicknesses times layer coefficients and drainage factors.

The elastic modulus of a soil or pavement material under repeated load, used to characterize subgrade support and stiffness in mechanistic-empirical pavement design.

A rating from 0 to 5 of ride quality and user satisfaction, with the present serviceability index declining as distress accumulates over traffic loading.

The number of 18,000-pound ESALs produced by one pass of a given axle load and configuration, increasing sharply with axle weight.

A performance-graded asphalt mix design system that selects binder grade for climate and specifies aggregate and volumetric criteria to resist rutting and cracking.

The densification of soil, aggregate, or asphalt by mechanical effort to increase strength, reduce settlement, and lower permeability; measured as a percentage of maximum dry density.

A thin spray of asphalt emulsion applied to an existing surface before placing a new asphalt layer so the layers bond instead of slipping.

An asphalt or emulsion applied to a granular base course to penetrate and bind loose aggregate, seal the surface, and improve adhesion with the asphalt layer above.

A systematic process of inventorying pavement condition, predicting deterioration, and prioritizing maintenance and rehabilitation treatments to optimize spending across a network.

A temporary area where construction or maintenance affects traffic, requiring a traffic control plan with signs, channelizing devices, and worker protection per MUTCD guidance.

The Manual on Uniform Traffic Control Devices, the national standard setting design, placement, and use of signs, signals, pavement markings, and work zone devices.