--- title: "Factors controlling geometric design" subject: "Transportation Engineering I" chapter: 3 key: "semester_6/transportation_engineering_i:geometric-design-factors" source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/geometric-design-factors site: BCT Engineering --- # Factors controlling geometric design Transportation Engineering I · Chapter 3: Geometric Design of Highway - Asked in: 2081 Ashwin, 8 marks; 2078 Chaitra, 8 marks; 2076 Bhadra; 2074 Bhadra, 8 marks; 2070 Bhadra; 2069 Bhadra; 2066 Magh (old course), 2 marks (asked 7 times) - Video: [Factors of design control and criteria for geometric design of highways in hindi](https://www.youtube.com/watch?v=NBkjyeutZ_U) — ENGINEER MIND - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/geometric-design-factors - Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-3 ## Question Explain the factors/design controls and criteria controlling the geometric design of highways (importance of geometric design). ## Short answer Geometric design is controlled by design speed (most important), topography, traffic volume/composition/growth, design vehicle dimensions/performance, human factors (reaction time, eye height), environmental/aesthetic considerations, economy, safety/capacity, and road classification. Proper design ensures safe, efficient movement, lower vehicle operating costs and travel time, optimal land/fund use, and accommodation of future improvements. ## Answer Geometric design fixes the visible dimensions of a highway - cross-section, horizontal and vertical alignment, sight distance and intersections - so that traffic is safe, comfortable and economical. The following factors control it. #### 1. Design speed The most important control. It depends on road class, terrain and traffic. All elements (curve radius, sight distance, superelevation, gradient) are linked to it. NRS 2070 gives design speed by road class and terrain. #### 2. Topography (terrain) Plain, rolling, hilly (mountainous) and steep terrain are classified by cross-slope. In steep terrain, speed and standards are lower. #### 3. Traffic factors - **Volume** (AADT, design hour volume such as the 30th hour) decides the number of lanes and widths. - **Composition** (cars, buses, trucks, non-motorised vehicles) and the capacity in PCU. - Future traffic growth over the design life. #### 4. Design vehicle Dimensions and turning radius of the largest vehicle (width, length, wheelbase, height) fix lane width, radius, extra widening and clearance. Vehicle performance (acceleration, braking) decides gradient and sight distance. #### 5. Human factors Driver's reaction time (PIEV), eye height, visual ability, behaviour and pedestrian characteristics. #### 6. Environmental and aesthetic factors Noise, pollution, scenery, land use; harmony of the road with the landscape. #### 7. Economy Cost of construction, land, maintenance; benefit-cost basis. Standards must be balanced against funds. #### 8. Safety and capacity Geometry should minimise accidents, provide sufficient capacity and level of service. #### 9. Road classification National highway, feeder road, district, urban and village road have different standards (NRS 2070). #### Importance of geometric design - Safe, smooth and efficient movement; lower accident rate. - Lower vehicle operating cost and travel time. - Proper use of land and funds, with room for future improvement. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is the most important control in geometric design? A. Design speed B. Colour of road C. Pavement type D. Rainfall Answer: A. Curve radius, sight distance, superelevation and gradient are all linked to design speed. (Key point: Main control) 2. What does design hour volume decide? A. Soil type B. Number of lanes and widths C. Camber shape D. Bench mark spacing Answer: B. Volume determines lanes and width needed for capacity. (Key point: Traffic volume) 3. What do dimensions of the design vehicle fix? A. Rainfall design B. Land price C. Lane width, radius, extra widening and clearance D. Traffic signal timing Answer: C. Width, length, wheelbase and turning radius control these elements. (Key point: Design vehicle) 4. Which is a human factor? A. Terrain B. Soil C. Cost D. Driver reaction time (PIEV) and eye height Answer: D. Reaction time and eye height affect sight distance and design. 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