Dimensions and Weights of Road Design Vehicles
1983 Edition

This section introduces essential terminology related to vehicle configurations and axle arrangements vital for road design. It specifies that a tandem axle consists of two or more consecutive axles spaced 1.2 to 2.5 meters apart that share the load equally. Definitions include truck-tractor (a power unit designed to tow without bearing trailer load), truck-trailer combinations, and how overall vehicle width is measured excluding safety apparatus and tyre bulge. It also explains the notation system for vehicle types, such as '2-S2' indicating a two-axle truck-tractor with a tandem-axle semi-trailer. These foundational definitions underpin the detailed specifications of dimensions and weights in later sections.

This section describes the extent of IRC 3's application in designing road elements to accommodate the maximum prescribed dimensions and weights of design vehicles. All road components, whether newly constructed or upgraded, should be planned to support vehicles meeting this standard, ensuring current sufficiency and future adaptability. Critical vehicle parameters influencing design include width (affecting lane and shoulder sizing), height (for under-bridge and overhead clearance), overall length (impacting curve design and traffic safety), axle loads (dictating pavement thickness), and total weight (influencing gradient limitations).

Key terms and vehicle types defined here include:

• Tandem Axle Weight: The combined weight on two or more consecutive axles spaced between 1.2 m and 2.5 m.
• Trailer: A non-powered vehicle towed by a motor vehicle without bearing any of its own weight on the towing unit.
• Truck: A motor vehicle primarily purposed for goods transportation.
• Truck-Tractor: A vehicle designed to pull trailers, carrying only a portion of the trailer’s load.
• Truck-Trailer Combination: A truck or tractor unit coupled with a trailer.
• Overall Width: The total transverse dimension across the vehicle including load but excluding safety devices and tyre bulge. The section also explains the notation for vehicle types where digits represent axle counts and letters denote trailer types, facilitating clear categorization for design considerations.

This part elaborates on the vehicle type notations used in IRC 3, where:

• The initial digit indicates the number of axles on the truck or truck-tractor.
• The letter 'S' signifies a semi-trailer.
• The digit following 'S' specifies the number of semi-trailer axles.
• Additional digits indicate trailer axles. Examples include '2-S2' for a two-axle truck-tractor with a tandem-axle semi-trailer and '2-2' for a two-axle truck with a two-axle trailer. Definitions of tandem axle weights and truck-tractor roles are also clarified, which are crucial for interpreting vehicle configurations in road design.

This section identifies critical dimensional parameters for road design vehicles, including width (which influences lane and shoulder widths), height (governing clearance requirements), overall length (affecting curve geometry and traffic safety), axle loads (which determine pavement thickness), and total vehicle weight (impacting gradient limits). These parameters serve as the foundation for designing road infrastructure to safely accommodate the largest permitted vehicles.

The standard specifies maximum allowable weights for transport vehicles based on axle and gross weight restrictions. Tandem axles spaced 1.2 m to 2.5 m apart must not exceed 18 tonnes. The gross vehicle weight is the aggregate of single and tandem axle limits. A comprehensive table details maximum gross and axle weights for various vehicle types, including distinctions for front and rear axle weights and tandem axle configurations, ensuring road infrastructure is designed to withstand these maximum loads.

Axle weight limits defined in IRC 3 include:

• Single axle with dual wheels: up to 10.2 tonnes.
• Single axle with single wheels: up to 6 tonnes.
• Tandem axles (two or more consecutive axles spaced 1.2 to 2.5 m): combined maximum weight of 18 tonnes. The maximum gross permissible weight for vehicles sums the individual axle loads, and detailed tables list typical vehicle types alongside their axle restrictions, ensuring design compliance and vehicle safety.

This section clarifies that tandem axles, defined as two or more consecutive axles spaced between 1.2 m and 2.5 m apart, have a maximum combined weight limit of 18 tonnes. Tandem axle weight is the total load transmitted to the road surface by these axles. The maximum gross vehicle weight is calculated by summing single axle and tandem axle weights. Examples include tandem axles fitted with eight tyres carrying up to 18 tonnes, while single axles with dual wheels are limited to 10.2 tonnes and single wheel axles to 6 tonnes.

The code prescribes maximum overall vehicle lengths for various configurations:

• Single unit trucks (2 or more axles): up to 11 meters (excluding bumpers).
• Single unit buses (2 or more axles): up to 12 meters.
• Truck-tractor with semi-trailer: up to 16 meters.
• Truck-trailer combinations: up to 18 meters. No vehicle combination may include more than two units. Height limits are generally 3.8 meters, except for ISO series 1 freight containers (up to 4.2 meters) and double-decker buses (up to 4.75 meters). These dimensions are critical for designing lane widths, clearances, and curve geometries.

According to IRC 3, the maximum vehicle height is generally capped at 3.8 meters, with exceptions for double-decker buses allowed up to 4.75 meters and vehicles carrying ISO series 1 containers permitted up to 4.2 meters. The overall width encompasses the vehicle’s full transverse dimension including loads but excludes safety features and tyre bulges. Length limits vary by vehicle type with single unit trucks allowed 11 meters, buses 12 meters, truck-tractor semi-trailer combos 16 meters, and truck-trailer combos 18 meters. No combination may exceed two vehicles. These size restrictions guide design parameters such as lane widths and clearances.

Vehicle selection for design purposes should consider terrain, economic viability, and road hierarchy. For steep or mountainous regions, single unit vehicles suffice, while semi-trailers are considered where economically justified. Vehicle dimension and weight parameters affect the design of lanes, clearances, curves, pavement thickness, and gradients, guiding the choice of appropriate vehicle types to ensure functional and safe roadway design.

Vehicle size and weight directly influence key road design elements: width affects lane and shoulder sizing; height dictates clearances for bridges and overhead structures; length impacts horizontal and vertical curve design and passing safety; axle load determines pavement thickness; and total weight limits gradient steepness. Design must accommodate the maximum anticipated vehicle sizes and weights to ensure safety and future upgradability.

This section addresses how vehicle axle loads and configurations affect pavement design. Axle loads govern pavement thickness requirements, while total vehicle weight influences allowable road gradients. Tandem axles, spaced 1.2 to 2.5 meters apart, transmit combined loads considered in structural design. Vehicle classifications based on axle numbers and trailer configurations inform load distribution analyses essential for durable pavement construction.

Safety considerations for passing and overtaking rely heavily on vehicle dimensions and weights, which affect lane width, shoulder dimensions, and curve geometry. Overall vehicle length is critical for designing curves that allow safe overtaking distances, while width impacts clearance during such maneuvers. Axle loads and total vehicle weights influence pavement strength and gradient limits, affecting vehicle handling and safety on roads.

This concluding section summarizes essential definitions and clauses from IRC 3, including tandem axle weight definitions, vehicle type notations, overall width measurements, and truck-tractor and trailer classifications. These form the technical backbone for understanding and applying the standard’s requirements in road design.