Traffic Engineering and Pavement Design

Traffic engineering

Traffic engineering deals with the planning, geometric design and traffic operations of roads, streets and highways — their networks, terminals and abutting lands — for safe, convenient and economic movement of people and goods.

Traffic safety is traditionally addressed by the three Es — Engineering, Enforcement and Education (often extended with Environment and Emergency care).

Road user characteristics

• Physical: vision, hearing, strength, reaction time.
• Mental: knowledge, skill, experience, intelligence.
• Psychological: attentiveness, fear, anger, maturity, impatience.
• Environmental: traffic stream, atmosphere, facilities.

PIEV theory

The total perception–reaction time of a driver is made up of:

1. Perception — time to see or perceive an object.
2. Intellection — time to understand the situation.
3. Emotion — time for emotional reactions (fear, anger, superstition, etc.).
4. Volition — time to decide and initiate action.

It varies from about 0.5 s to 4 s or more with conditions; IRC uses a total reaction time of 2.5 s for stopping sight distance.

Vision: the cone of clear vision is narrow (a few degrees), fairly clear vision extends to about 10–12°, and peripheral vision is wider but less clear — signs must be placed within the driver's clear field of view.

Vehicular characteristics

• Static: dimensions (width, length, height), weight, axle configuration, turning radius — affect lane width, clearances, pavement design and curve widening.
• Dynamic: speed, acceleration and deceleration, braking performance, power — affect sight distances, gradients, intersection design.
• Resistances: rolling, air, grade, curve and inertia resistances determine power requirements.

Traffic volume studies

Traffic volume — number of vehicles crossing a section per unit time.

Methods: manual counts (tally sheets), mechanical/automatic counters (pneumatic tubes, inductive loops), video and camera-based counting, radar and infrared sensors, moving observer method.

Uses: planning, geometric and pavement design, signal design, capacity analysis, traffic trends, economic evaluation.

PCU values (IRC)

Typical PCU equivalency factors for rural roads (IRC:64) include: passenger car/jeep/van 1.0; motorcycle/scooter 0.5; light commercial vehicle 1.5; truck or bus 3.0; truck–trailer/tractor–trailer 4.5; bicycle 0.5; cycle rickshaw 2.0; bullock cart 8.0. For urban roads (IRC:106), PCU values vary with the percentage of each vehicle type in the traffic stream (ranges are given; e.g. two-wheelers about 0.5–0.75, buses and trucks about 2.2–3.7). Indo-HCM (2017) gives dynamic PCU values from Indian field studies.

Speed studies

Spot speed study

Speed of vehicles at a particular location — measured by radar guns, enoscope (mirror box over a known base length), pressure contact tubes, video.

Cumulative speed distribution is used to find percentile speeds:

Modal speed (most frequent) and pace (the 10 km/h range containing the most vehicles) are also reported.

Time mean speed and space mean speed

Space mean speed is used in traffic flow relations ($q=k{v}_s$).

Speed and delay studies

Give running speed, journey (overall) speed and the location, cause and duration of delays along a route.

• Journey speed = distance ÷ total journey time (including stops).
• Running speed = distance ÷ running time (excluding stopped delays).

Methods: floating car (moving observer) method, license plate method, interview technique, elevated observations, GPS probe vehicles.

Origin–destination (O–D) studies

Determine where trips start and end, their purpose, mode and route — used for planning new roads, bypasses, public transport and parking.

Methods: roadside interview, license plate method, return postcard method, tag-on-car method, home interview surveys, workplace surveys, mobile phone and GPS data.

Presentation: O–D matrix (trip table) and desire lines (straight lines between origins and destinations with thickness proportional to trips).

Parking studies

• In–out survey, fixed period sampling, license plate method (records duration).
• Parameters: parking accumulation (vehicles parked at a time), parking volume (total vehicles parked in a period), parking load (area under accumulation curve, vehicle-hours), parking duration, parking turnover = parking volume ÷ number of bays, parking index (occupancy) = accumulation ÷ capacity × 100.

Accident studies

• Objectives: identify black spots, causes, and remedial measures; evaluate safety measures.
• Records: first information reports, accident report forms; collision diagrams (show type and path of vehicles) and condition diagrams (show roadside features).
• Causes: road user (speeding, drunk driving, fatigue, distraction), vehicle (brake or tyre failure, lighting), road (poor geometry, surface, signs, drainage), environment (fog, rain).
• Accident rates:

($A$ = accidents in $T$ years on a section of length $L$ km.)

• Road safety audit (IRC:SP:88) at design, construction and operation stages.

Types of pavements

Other types: composite pavements (bituminous over concrete or cement-treated base), semi-rigid pavements (cement-treated layers), interlocking concrete block pavements.

Layers of a flexible pavement

Also: prime coat (between granular base and bituminous layer) and tack coat (between bituminous layers).

Stresses in flexible pavements

• A wheel load is applied over a contact area (often assumed circular) with contact pressure roughly equal to tyre pressure.
• Boussinesq's theory (homogeneous elastic half-space): vertical stress below the centre of a uniformly loaded circular area of radius $a$:

• Burmister's layered theory — two or three elastic layers of different moduli; basis of modern analysis (a stiffer upper layer reduces stresses below).

Equivalent single wheel load (ESWL)

The single wheel load that produces the same effect (stress or deflection) at a given depth as a group of wheels (dual or tandem).

Boyd–Foster equal stress method (graphical): on a log–log plot of load against depth, ESWL = $P$ (one wheel of the dual) at depth $d/2$ and $2P$ at depth $2S$ ($d$ = clear gap between tyres, $S$ = centre-to-centre spacing), with a straight line between:

Design approaches

CBR method (empirical)

$t$ = total pavement thickness (cm) above the layer of given CBR; $P$ = wheel load (kg); $p$ = tyre pressure (kg/cm²). Thickness above each layer is found using that layer's CBR.

Design traffic

Standard axle and equivalency

The standard axle in IRC:37-2018 is a single axle with dual wheels carrying 80 kN. Damage by other axle loads is expressed by the fourth power law:

• Vehicle damage factor (VDF) — number of standard axle repetitions caused by one passage of a commercial vehicle; found from axle load surveys (or indicative values).
• Lane distribution factor (LDF) — proportion of commercial vehicles in the design lane: single-lane road 1.0; two-lane single carriageway 0.50; four-lane single carriageway 0.40; dual carriageway — two lanes in each direction 0.75, three lanes 0.60, four lanes 0.45.

Only commercial vehicles (laden weight above 3 tonnes) are considered. Design periods of about 20 years are commonly used for National and State Highways (longer for expressways and high-density corridors, shorter for lower categories). IRC:37-2018 is applicable for design traffic of 2 msa and above; low-volume roads are designed as per IRC:SP:72.

IRC:37-2018 mechanistic–empirical design

Design criteria

1. Bottom-up fatigue cracking of the bituminous layer — controlled by the horizontal tensile strain ${\epsilon }_t$ at the bottom of the bituminous layer.
2. Rutting (permanent deformation) — controlled by the vertical compressive strain ${\epsilon }_v$ on top of the subgrade.
3. Additional checks for cement-treated bases (fatigue of CTB) and top-down cracking considerations.

Material inputs

• Subgrade — effective CBR (considering embankment and subgrade layers) → $M_R$ from CBR relations.
• Granular layers — modulus depends on thickness and support: $M_{R,gran}=0.2h^{0.45}{M}_{R,support}$ ($h$ in mm).
• Bituminous layers — resilient modulus from IRC tables depending on binder grade (e.g. VG-30, VG-40) and pavement temperature (commonly 35 °C in design).
• Cement-treated layers — modulus and flexural strength as given in the code.
• Poisson's ratio — typically 0.35 for bituminous and granular layers and subgrade (0.25 for cemented layers).

Procedure

1. Estimate design traffic (msa) and subgrade effective CBR.
2. Select pavement composition (e.g. bituminous surface + DBM + WMM + GSB, or with CTB/CTSB layers) with trial thicknesses.
3. Analyse the pavement with the IITPAVE multilayer elastic program under the standard axle (dual wheels, 20 kN each, tyre pressure 0.56 MPa, 310 mm spacing).
4. Compute allowable ${\epsilon }_t$ and ${\epsilon }_v$ from the transfer functions for the design traffic.
5. Adjust thicknesses until computed strains do not exceed allowable strains; check minimum layer thicknesses and drainage.
6. IRC:37-2018 also provides design catalogues of layer thicknesses for different CBR and traffic combinations.

Components of a rigid pavement

Unlike flexible pavements, the thickness of a rigid pavement depends mainly on the flexural strength of concrete; subgrade support has a relatively smaller effect.

Design factors

• Wheel load and axle configuration — axle load spectrum, tyre pressure, contact area.
• Traffic — number of repetitions of different axle loads over the design period (commonly 30 years for rigid pavements).
• Modulus of subgrade reaction $k$ — from plate load tests (or from CBR correlations), increased for the effect of DLC/granular sub-base (effective k).
• Concrete properties — flexural strength (modulus of rupture), modulus of elasticity (about 30 000 MPa), Poisson's ratio (0.15), coefficient of thermal expansion (about $10\times 10^{-6}$ per °C).
• Temperature differential between top and bottom of the slab — causes warping.

Radius of relative stiffness

It measures the stiffness of the slab relative to the subgrade — the distance over which the slab spreads the load.

Equivalent radius of resisting section

For a load of contact radius $a$ on a slab of thickness $h$ (thin-plate theory correction):

Load stresses (Westergaard)

Westergaard considered three critical positions of the wheel load:

• Interior — load well away from edges and corners.
• Edge — load at the edge, away from corners.
• Corner — load at the corner of the slab.

• Interior and edge stresses are tensile at the bottom of the slab; corner stress is tensile at the top.
• Edge load stress is generally the most critical of the load stresses (without load transfer).

Temperature stresses

Warping stresses

A temperature differential between the top and bottom of the slab makes it curl (warp); self-weight and subgrade restraint resist this, inducing stresses.

• Day (summer, mid-day): top hotter → slab tends to curl down at edges → tension at the bottom (adds to wheel load tension at the edge).
• Night (winter): top cooler → edges curl up → tension at the top (adds to corner load stress).

Frictional stresses

Uniform temperature change makes the slab expand or contract; friction with the sub-base restrains it.

($S_f$ in kg/cm²; $W$ = unit weight of concrete, kg/m³; $L$ = slab length, m; $f$ = coefficient of friction, about 1.5.) This governs the spacing of contraction joints.

Critical combinations