15.2. Guidelines on superelevation

While designing the various elements of the road like superelevation, we design it for a particular vehicle called design vehicle which has some standard weight and dimensions. But in the actual case, the road has to cater for mixed traffic.

When engineers design roads, they often base their designs on a 'design vehicle'. This vehicle is chosen for its standard size and weight, which represents a typical heavy vehicle that might use the road. However, in reality, many different vehicles of various sizes and weights share the road at the same time. Therefore, while designing superelevation—how much the road is tilted at curves—it's crucial to account for this mix of traffic.

Imagine a grocery store that mainly stocks items for a typical family, like those in a standard recipe book. If a family of seven comes in with a long shopping list, the layout might confuse them, making it hard to find everything. Similarly, designing roads just for one type of vehicle doesn't serve the needs of all the different vehicles that will actually use the road.

For fast moving vehicles, providing higher superelevation without considering coefficient of friction is safe, i.e. centrifugal force is fully counteracted by the weight of the vehicle or superelevation. For slow moving vehicles, providing lower superelevation considering coefficient of friction is safe, i.e. centrifugal force is counteracted by superelevation and coefficient of friction.

Fast vehicles can handle more tilt in the road (higher superelevation) because their speed helps keep them on track, preventing sliding off the curve due to centrifugal force. In contrast, slow-moving vehicles need a gentler slope (lower superelevation), as they rely more on the friction between the tires and the road to stay stable. Understanding this is essential for safe road design.

Think about running on a track. If you're sprinting around a bend, leaning into the curve helps keep you upright. But if you're walking slowly on the edge, you need to be more careful to avoid slipping. The principles are similar in vehicle motion on roads.

IRC suggests the following design procedure:

1. Find e for 75 percent of design speed, neglecting f, i.e e = (0.75v)² / gR.
2. If e ≤ 0.07, then e = e, else if e > 0.07 go to step 3.
3. Find f for the design speed and max e, i.e f = v² / gR - e.
4. Find the allowable speed v for the maximum e=0.07 and f=0.15.

The design of superelevation involves several mathematical steps for accuracy:

1. Calculate the required superelevation (e) based on 75% of the vehicle's design speed.
2. If this calculated value is less than or equal to 0.07, that is sufficient. Otherwise, further calculations must be done to account for friction.
3. This involves checking the frictional forces experienced at the design speed and determining if the maximum acceptable superelevation can still ensure safety.
4. Finally, if the calculated superelevation exceeds safety limits, engineers must lower the expected speed to ensure stability on the curve.

Think of it like adjusting a set of weights on a scale. If the weights equalize nicely, you're done. But if they don't balance, you may need to adjust one side or the speed of the process to get the scale to zero out and ensure everything operates safely and correctly.

IRC specifies a maximum super-elevation of 7 percent for plain and rolling terrain, while that of hilly terrain is 10 percent and urban road is 4 percent. The minimum super elevation is 2-4 percent for drainage purposes, especially for large radius of the horizontal curve.

Different terrains have specific limits for superelevation to ensure safety:

• Maximum superelevation for flat areas is set at 7%, which means the road can tilt at an angle that is safe for vehicles to maneuver at higher speeds.
• Hilly areas can have a tilt of 10% due to the different gravitational and centrifugal effects at play.
• Urban roads must be more restrained with a maximum tilt of only 4%, primarily focusing on low speeds and denser traffic.
• Minimum tilt values ensure proper drainage, especially on curves to prevent water accumulation and maintain surface integrity.

Consider a slide at a playground. If it's too steep, kids may fall off; if it's too flat, they might not slide at all. Just like adjusting the slope of the slide for different ages or abilities, road designers adjust superelevation based on the type of road and expected traffic.

1. Elimination of the crown of the cambered section by: rotating the outer edge about the crown, or shifting the position of the crown.
2. Rotation of the pavement cross-section to attain full superelevation: rotating about the centerline or the inner edge.

There are several methods to achieve proper superelevation on roads:

1. By eliminating the high center point (crown) traditionally present in road design. This can be done by either rotating the road surface from the outer edge around this point or shifting the crown itself outwards to adjust the road angle.
2. The pavement itself can be physically adjusted by either raising or lowering sections of the road. This adjustment helps to create a smooth and effective tilt that supports safe vehicle navigation around curves.

Think about adjusting a tablecloth on a round table; if you want to keep drinks from sliding off the edge, you can tilt one side slightly or adjust the whole cloth to create a more stable surface. Similarly, road designers adjust pavement angles for safety.

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