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Road Widening on Curves

Curve Widening on Open Highway Curves (Source: AASHTO – Exhibit 3-50)

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 widthdesign 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.

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.

Table referred in AASHTO – A Policy on Geometric Design of Highways and Streets [page 215]

Table referred in ATJ 8-86 Pindaan 2015

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.

Pavement Width (m)
Design Speed (kph)
Design Radius (m)
Design Vehicles
No of lane roadways
The parameters required in ATJ and AASHTO

Benchmarking on Proposed Curve Widenings by Software #

Referring to Exhibit 3-51 in AASHTO  #

Pavement width : 6.6m

Design speed : 50kph

Design Radius : 90m

Type of vehicle: SU

No. of lane roadways: 2

Based on the table;

The curve widening is 2.1m.

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.

∴ Required Widening

 = Widening (From Exhibit 3-51) + Adjustment (From Exhibit 3-52)

 = 2.1 + (-1.3)

 = 0.8 m 

Referring to Table 4.9 in ATJ #

Pavement width : 6.6m

Design speed : 50kph

Design radius : 90m

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

Based on the table;

∴  The Required Widening is 1.25m 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.

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