Horizontal Alignment
Typically horizontal curves consist of a circular curve with a constant radius. For faster design speeds circular curves are joined to the tangents using transition curves which have varying radii. These curves improve the occupant safety and comfort by providing a gradual increase of the sideways force felt by the vehicle due to the introduction of the curve. A circular curve may also be accompanied by the introduction of superelevation. This is were the road is tilted into the curve to reduce the likelihood of vehicles running off the road at the curve.
Below we can consider some of the key features of a horizontal circular curve joining 2 tangents. Note there is no transition curve in this example.
Typically horizontal curves consist of a circular curve with a constant radius. For faster design speeds circular curves are joined to the tangents using transition curves which have varying radii. These curves improve the occupant safety and comfort by providing a gradual increase of the sideways force felt by the vehicle due to the introduction of the curve. A circular curve may also be accompanied by the introduction of superelevation. This is were the road is tilted into the curve to reduce the likelihood of vehicles running off the road at the curve.
Below we can consider some of the key features of a horizontal circular curve joining 2 tangents. Note there is no transition curve in this example.
PC is the Point of Curvature. This is the point where the straight tangent ends and the curve starts.
IP is the point of intersection between the 2 tangent lines. It can be used to help setting out of the road curve on site.
PT is the Point of Tangency. This is the point where the curve ends and the second straight starts.
R is the radius of the curve determined as detailed below.
Δ is the deflection angle.
IP is the point of intersection between the 2 tangent lines. It can be used to help setting out of the road curve on site.
PT is the Point of Tangency. This is the point where the curve ends and the second straight starts.
R is the radius of the curve determined as detailed below.
Δ is the deflection angle.
Design Standards for Horizontal Curves
The National Roads Authority have prepared design standards for Irish roads. In their document, Volume 6 Section 1 Part 1 NRA TD9/12, the NRA have linked design speed to a number of design parameters in accordance with Table 1/3: Design Speed Related Parameters, see below.
The National Roads Authority have prepared design standards for Irish roads. In their document, Volume 6 Section 1 Part 1 NRA TD9/12, the NRA have linked design speed to a number of design parameters in accordance with Table 1/3: Design Speed Related Parameters, see below.
You may have noticed very large sweeping radius curves along the motorways on which you have driven. The above table tells us that for a design speed of 120km/h we need to provide a radius of 2880m, nearly 3km when there is no super elevation on the road. The addition of superelevation reduces the radius of the curve required. This will be dealt with here.