Transportation Engineering: Geometric Design, Traffic Engineering, and Pavement Design

Comprehensive practice flashcards defining foundational vocabulary, empirical test methods, design criteria, and distress mechanisms in geometric design, traffic analysis, and pavement engineering.

Geometric Design

The branch of highway engineering that deals with the visible elements and dimensional features of the road.

Design Vehicle

The vehicle whose length, height, and width parameters are used as the governing design criteria for road geometric features as per IRC:003.

IRC 73

The Indian Roads Congress standard code that deals specifically with the geometric design of rural highways.

Unevenness Index

A cumulative measure of vertical undulations per unit length of the road, measured using a Bump Integrator.

Carriage Way

The structural portion of the pavement specifically designed and provided to carry vehicular traffic.

Shoulder

A roadside strip provided to accommodate stopped vehicles and to provide lateral confinement to pavement layers, with a desirable width of $4.6\,\text{m}$ and a minimum of $2.5\,\text{m}$ for a 2-lane rural highway.

Kerb

A boundary element that indicates the boundary between the pavement and the shoulder, median, or footpath.

Camber (Cross-Slope)

The transverse slope provided to the road surface to drain off rainwater towards the outer edges.

Median

A traffic separator installed between opposing traffic lanes to prevent head-on collisions of vehicles.

Right of Way (ROW)

The total area of land acquired and reserved along the road alignment for highway development and roadway construction.

Building Line

A line set at a designated distance from the road center representing the boundary up to which no building activity is permitted.

Control Line

A line set beyond the building line that defines the distance up to which the nature and type of building construction is controlled.

Stopping Sight Distance (SSD)

The minimum sight distance that must be available from all points along a highway so that vehicles travelling at design speed can safely stop before reaching a stationary obstacle.

Intermediate Sight Distance (ISD)

A sight distance provided to allow limited overtaking opportunities when overtaking sight distance cannot be provided, calculated as $\text{ISD} = 2 \times \text{SSD}$.

Headlight Sight Distance (HSD)

The distance visible to a driver at night under vehicle headlight illumination, the minimum value of which must equal stopping sight distance.

Overtaking Sight Distance (OSD)

The sight distance required on undivided highways to allow an overtaking vehicle to pass a slower vehicle safely without colliding with oncoming traffic.

Sight Triangle

The clear visibility triangle established at priority intersections by providing a minimum visibility of $15\,\text{m}$ along the minor road and an $8\,\text{s}$ travel distance along the major road.

Superelevation

The transverse slope provided along horizontal curves by raising the outer edge of the pavement relative to the inner edge to counteract centrifugal force.

Equilibrium Superelevation

The superelevation that completely counteracts centrifugal force without mobilizing lateral friction ($\mu = 0$), equalizing wheel pressure across inner and outer wheels.

Mechanical Widening

Extra pavement width provided on horizontal curves to accommodate off-tracking resulting from the rigidity of a vehicle's wheelbase, calculated as $\frac{n l^2}{2R}$.

Psychological Widening

Extra pavement width provided on horizontal curves to compensate for the driver's psychological tendency to steer away from the outer edge, calculated as $\frac{V}{2.64\sqrt{R}}$.

Transition Curve

A curve of continually varying radius inserted between a tangent and a circular curve to introduce centrifugal acceleration and superelevation gradually without abrupt jerks.

Setback Distance

The clearance distance required from the centerline of a horizontal curve to an inner obstacle to ensure adequate sight distance is available across the curve.

Curve Resistance

The loss of effective forward tractive thrust ($T - T\cos(\alpha)$) experienced by a rear-wheel-drive vehicle when turning its front steering wheels along a horizontal curve.

Grade Compensation

The reduction in gradient provided when a sharp horizontal curve occurs on an upgradient to compensate for the reduction in a vehicle's pulling power.

Ruling Gradient

The maximum design gradient up to which an engineer designs the vertical profile so that a vehicle with maximum pulling power can maintain a constant speed over a long stretch.

Limiting Gradient

A gradient steeper than the ruling gradient adopted over short stretches when topographical conditions make the ruling gradient prohibitively expensive.

Exceptional Gradient

An unusually steep gradient permitted in extraordinary situations, restricted to a maximum length of $100\,\text{m}$ and flanked by milder gradients for at least $100\,\text{m}$.

Critical Length of Grade

The maximum continuous length of an ascending grade over which a loaded truck can operate without its speed dropping by more than $25\,\text{km/h}$.

Summit Curve

A vertical curve designed with upward convexity formed at the intersection of two gradients, designed primarily to satisfy sight distance criteria.

Valley Curve (Sag Curve)

A vertical curve designed with upward concavity, governed in design by headlight sight distance and rate of change of centrifugal acceleration.

PIEV Theory

A psychological theory analyzing total driver reaction time through four sequential stages: Perception, Intellection, Emotion, and Volition, establishing a design reaction time of $2.5\,\text{s}$.

Traffic Volume

The number of vehicles passing a designated point or transverse road section during a specified unit of time, expressed in $\text{veh/h}$ or $\text{PCU/h}$.

30th Highest Hourly Volume

The hourly traffic volume that is exceeded exactly 29 times in a year, adopted as the design hourly volume and typically equal to $8-10\%$ of AADT in India.

Spot Speed

The instantaneous speed of an individual vehicle measured at a specific point or location along a roadway.

Time Mean Speed (TMS)

The arithmetic mean of the spot speeds of all vehicles crossing a specific roadway location during a given observation interval ($V_t$).

Space Mean Speed (SMS)

The harmonic mean of vehicle spot speeds over a road stretch, computed as the total distance divided by total travel time ($V_s$).

Running Speed

The speed obtained by dividing the journey length by the running time, completely excluding stopped delays.

Journey Speed

The overall effective travel speed obtained by dividing the journey length by the total journey time, including all stopped delays.

98th Percentile Speed

The speed at or below which $98\%$ of vehicles are moving, theoretically adopted as the design speed for highway facilities.

85th Percentile Speed

The speed at or below which $85\%$ of vehicles travel, adopted in engineering practice as the safe speed or upper regulatory speed limit.

15th Percentile Speed

The speed at or below which $15\%$ of vehicles travel, used to define the lower regulatory speed limit to segregate excessively slow traffic.

Traffic Density

The number of vehicles occupying a unit length of a roadway lane at a given instant in time, expressed in $\text{veh/km}$.

Time Headway

The time interval between the passage of the rear bumpers of successive vehicles past a given reference point.

Space Headway

The distance measured between the rear bumpers of consecutive successive vehicles travelling in the same direction.

Basic Capacity

The maximum number of vehicles that can pass a given point in unit time under ideal roadway, traffic, and environmental conditions.

Level of Service (LOS)

A qualitative rating scale (A through F) describing operational traffic flow conditions based on speed, travel time, density, and delay.

Rotary Intersection

An at-grade intersection layout where entering traffic is channeled to move unidirectionally clockwise around a central island, converting crossing conflicts into weaving maneuvers.

Weaving

The combined traffic maneuver of merging followed immediately by diverging across a shared segment of roadway in the same general direction.

Signal Cycle

One complete rotation through all signal display indications (Green, Amber, and Red) provided at an intersection.

Change Interval (Amber Time)

The yellow phase that warns approaching drivers of an impending red display and allows vehicles inside the intersection to clear safely.

Clearance Interval (All-Red Interval)

An optional interval following the change phase during which all intersection approaches display red to allow vehicles to clear the intersection completely.

Effective Green Time

The actual usable time available during a phase for vehicular flow to clear an intersection, given by $g = G + A - t_l$.

Critical Lane Volume

The maximum traffic volume carried by the most heavily loaded lane among all lanes permitted to move simultaneously during a particular green phase.

Peak Hour Factor (PHF)

The ratio of the total peak-hour volume to four times the maximum volume during the peak 15-minute interval within that hour, quantifying traffic flow variations.

Flexible Pavement

A layered pavement structure that distributes wheel loads to the underlying soil subgrade through grain-to-grain contact across successive layers.

Rigid Pavement

A pavement structure consisting of a Portland cement concrete slab that spreads wheel loads over a broad area primarily through slab flexural action.

Prime Coat

A low-viscosity bituminous treatment applied to an untreated granular base course to seal capillary voids and prepare the surface for subsequent applications.

Tack Coat

A light application of bituminous material applied between two existing pavement courses to ensure a strong cohesive bond between them.

Seal Coat

A thin bituminous surface treatment applied over a wearing course to provide impermeability against water intrusion and enhance skid resistance.

Rigidity Factor

The dimensionless ratio of contact pressure to tyre pressure ($\frac{\text{Contact Pressure}}{\text{Tyre Pressure}}$), expressing the influence of tyre wall tension or compression.

Vehicle Damage Factor (VDF)

A multiplier that converts the damaging effect of commercial vehicles of diverse axle configurations into equivalent repetitions of an $80\,\text{kN}$ standard single axle.

Equivalent Single Wheel Load (ESWL)

The magnitude of a single isolated wheel load that induces equal stress, strain, or deflection at a specified pavement depth as that caused by a multi-wheel assembly.

Group Index (GI)

An arbitrary empirical rating ranging from $0$ to $20$ based on soil gradation, liquid limit, and plasticity index, where higher values denote poorer subgrade quality.

California Bearing Ratio (CBR)

An empirical penetration test measuring the force required to penetrate a subgrade soil sample at $2.5\,\text{mm}$ and $5.0\,\text{mm}$ relative to standard crushed stone values.

Modulus of Subgrade Reaction ($k$)

The reaction pressure per unit elastic deflection of the subgrade foundation, evaluated using a standard $75\,\text{cm}$ diameter plate at a deflection of $0.125\,\text{cm}$.

Radius of Relative Stiffness ($l$)

A parameter defined by Westergaard that quantifies the relative structural rigidity of a concrete slab relative to the resistance of the supporting subgrade.

Equivalent Radius of Resisting Section ($b$)

A modified radius introduced by Westergaard to account for non-uniform flexural stress distribution directly beneath the concentrated wheel load contact area.

Warping Stress

Temperature-induced stress developed in a concrete pavement slab resulting from daily temperature differences between the slab's top and bottom surfaces.

Frictional Stress

Thermal stress developed in a concrete pavement slab when overall seasonal temperature changes produce expansion or contraction that is resisted by subgrade friction.

Dowel Bar

A smooth mild steel bar installed across transverse joints to transfer wheel loads between adjacent slabs through shear and bending while permitting longitudinal thermal expansion.

Tie Bar

A deformed steel bar placed across longitudinal joints to prevent adjacent concrete pavement slabs from separating, designed solely to resist tensile forces.

Flakiness Index

The percentage by weight of aggregate particles whose least thickness dimension is less than $0.6$ times their mean sieve dimension.

Elongation Index

The percentage by weight of aggregate particles whose greatest length dimension exceeds $1.8$ times their mean sieve dimension.

Angularity Number

A measure of particle roundness deficiency in aggregate, calculated as $67 - \%\text{ solid volume}$ to quantify void spaces above the standard $33\%$ base value.

Marshall Stability

The maximum compressive load in $\text{kg}$ sustained by a compacted cylindrical bituminous mix specimen before failure when tested at $60\,^\circ\text{C}$.

Marshall Flow

The total diametrical deformation of a Marshall specimen in units of $0.25\,\text{mm}$ recorded between zero load and maximum load during the stability test.

Voids in Mineral Aggregate (VMA)

The total volume percentage of void space between the compacted aggregate particles in a bituminous paving mix, consisting of the sum of air voids and effective bitumen volume.

Voids Filled with Bitumen (VFB)

The percentage of void space in the mineral aggregate framework that is occupied by effective bitumen volume ($\frac{V_b}{\text{VMA}} \times 100$).

Bleeding

A bituminous pavement distress mechanism characterized by the migration of excess binder to the road surface, creating a shiny, slippery film.

Ravelling

The progressive surface disintegration of an asphalt pavement resulting from the dislodgement and loss of aggregate particles due to poor cohesion or binder deficiency.

Stripping

The separation and loss of the adhesive bond between aggregate particle surfaces and the bituminous binder film, commonly initiated by moisture.