Noticeably, when driving through curvy roads, we experience difficulties in manoeuvring our vehicles to be at the centre of the lane, especially for the larger design vehicles. Hence, this is where the pavement (or curve)  widening plays its role.

So, What is Curve Widening? #

It is the enlargement of the travelled route on horizontal curves with the intention of making road operations similar to those on tangents. Applying curve widening will ensure the vehicles entering and leaving fit the designed road, smoothly and safely.

In obtaining the required curve widening, road designers should be equipped with three parameters – the pavement width, design speed and design radius of the horizontal curves. With this information in hand, next will be to refer to the guidelines for road designing by the authorities.

Curve widening at a circular curve starts from the end of the normal crown of a straight road. Meanwhile, the curve widening starting point for a spiral curve is the start of the level crown of the transition curve.

In MiTS, the end of the normal crown and the start of the level crown is shown in the horizontal detailing and superelevation report.

Guidelines Referred and Their Differences #

There guidelines referred by our software in proposing a suitable curve widening are (i) AASHTO – A Policy on Geometric Design of Highways and Streets 2001 (based on Exhibit 3-51 and Exhibit 3-52) and (ii) Arahan Teknik Jalan (ATJ) 8-86 Pindaan 2015 (based on Table 4.9 – Pavement Widening on Open Road Curves), with the latter only implemented starting from MiTS version 2.9.

The concept in determining required widening for both guidelines is the same, only that (i) require adjustment factors (Exhibit 3-52) depending on the type of design vehicles and number of lane roadways. To help you understand better, you can refer to the table below that compares the parameters required in obtaining the curve widening.

PARAMETERS	ATJ (PINDAAN 8-86 2015)	AASHTO (EXHIBIT 3-51)
Pavement Width (m)	✅	✅
Design Speed (kph)	✅	✅
Design Radius (m)	✅	✅
Design Vehicles		✅
No of lane roadways		✅

Benchmarking on Proposed Curve Widenings by Software #

You may refer to the GIF attached where the road widening values apply exactly as the one used in the Spread Input

Referring to Exhibit 3-51 in AASHTO #

Example made based on Exhibit 3-51 in AASHTO

Pavement width : 6.6m

Design speed : 50kph

Design Radius : 70m, 90m, 110m

Type of vehicle : SU

No. of lane roadways : 2

Design radius = 70m

Proposed curve widening = 1.00m

Design radius = 90m

Proposed curve widening = 0.80m

Design radius = 110m

Proposed curve widening = 0.70m

Based on the table;

The curve widening when the radius is 70m, 90m and 110m respectively is 2.6m, 2.1m and 1.8m.

However, this value from the table above is designed for WB-15 design vehicles. Since in the project, the vehicle design chosen is SU, we will need to refer to Exhibit 3-52 for adjustments.

Radius of curve (m)	70	90	110
Required widening (m) = Widening (From Exhibit 3-51) + Adjustment (From Exhibit 3-52)	= 2.6 + (-1.6) = 1.0	= 2.1 + (-1.3) = 0.8	= 1.8 + (-1.1) = 0.7

Referring to Table 4.9 in ATJ #

Example made based on Table 4.9 in ATJ

Pavement width : 6.6m

Design speed : 30kph

Design radius : 30m, 50m, 120m

Interpolation : Set to Nearest (pavement width 6.6m is nearer to 6.5m compared to 7m)

Design radius = 30m

Proposed curve widening = 2.00m

Design radius = 50m

Proposed curve widening = 1.25m

Design radius = 120m

Proposed curve widening = 0.75m

Based on the table;

∴  The Required Widening is 2.00m, 1.25m and 0.75m respectively for road with curve radius of 30m, 50m and 120m as per suggested in software above

From the example above, it is much easier and straightforward to use the ATJ guidelines in obtaining the required curve widening compared to AASHTO. Nevertheless, the choice is yours depending on the authority and the road design.

To know more on how to apply your preferred guidelines in acquiring the required widening of the curve step by step, you may refer here.

Why does the Proposed Curve Widening remain constant despite changes to the Road Width? #

After adjusting the road width to a specific, required width in Road Module, you’ll find that the Proposed Curve Widening remains unchanged under the Spread Input.

For example, despite changing the road width to 3m and 3.2m respectively, while maintaining the Design Speed and Design Curve at 30km/h and 30m, the Proposed Curved Widening remains 1.5m.

Design RoadWidth = 3m, Total RoadWidth= 6m

Design RoadWidth = 3.2m, Total RoadWidth = 6.4m

The values obtained, referring to Table 4.9 in ATJ 8/86:

The difference in these values resulted from the software’s behaviour in determining the required curve widening- the software actually refers to the Generalized (Default) width of 3.6m, rather than any manual overrides in the Spread Input.

This is to ensure consistency with the suggested curve widening tables in both guidelines, AASHTO (Exhibit 3-51) and ATJ 8/86 (Table 4.9 based on REAM GL 2/2002), in which the values are established based on the standard lane width.

Furthermore, by referring to the standard or default width, it can avoid design conflicts, such that the extra width for parking bays or bus laybys is taken as part of the carriageway width in determining the required curve widening; This may actually lead to a non-compliant transition throughout the horizontal curves.

Can the widening be applied with values other than the Proposed Curve Widening? #

The answer is yes- Once the ‘Apply Curve Widening‘ is checked under the Superelevation Spread Input, users can override the widening values, as shown in the image below.

It is always suggested that users apply their own engineering judgement in determining the values applied to the road design, while ensuring the design still complies with the authorities; It is good to remember that MiTS is there only as a tool, easing complex computation in the design process, and should not be the one to ‘control‘ your design.