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Chapter 12 Intersections

 

This chapter shows you how to create intersections on a main road. The design details for an intersection are handled automatically by HighRoad. However you can control these details manually if you wish.

Note: This option is available only if you have the Intersection module. This module is included in HighRoad Plus and HighRoad Pro and is optional in other models.

 

Preparing for an intersection

In order to create an intersection you must first design the main or through road of the intersection. This should be done completely, even if just in a trial form. It should have a horizontal alignment, a profile, and a typical section. The intersection will be automatically attached on the side of this road at the position that you nominate. This road should be active when you are ready to create the intersection. Choose the road from the Active menu.

Creating an intersection

Choose Place intersection on <Road name>... from the Plan menu. The cursor will change to an intersection shape (). Click beside the through road at the location that you want to place the intersection. HighRoad will, using default dimensions, create the side road and kerb returns to produce an intersection at this location, as shown in Figure 12-1. When the cursor is over the intersection it becomes a hand shape and you can drag the intersection. You can also double-click to show the intersection details. A dialog box as shown in Figure 12-2 will appear. This shows in a diagrammatic form a typical T-intersection together with critical chainages along both control lines. You can alter the details of the intersection that HighRoad has created. As you change these values, and click Calculate, or move to another field, all the related details of the intersection will be recalculated.

Figure 12-1

 

Location

The location at which the side road will be attached to the road is shown as Intersection Chainage. This location is initially set where you clicked along the main road. You can change this as appropriate for your design.

Note: Be sure to leave sufficient room for an intersectuin where you click. It is advisable to click well away from the start or end of the road when placing an intersection. The intersection can be dragged closer to the end later if necessary.

Kerb returns

Kerb returns (or pavement returns as the case may be) allow for a smooth connection between the main road and the side road. HighRoad initially sets the radius of each kerb return to 7.5 metres. You can change this radius to one that is appropriate for your design. The left and right kerb returns are viewed as if you are standing at the start of the side road and looking along the side road. An arrow and the labels left and right are shown in the Intersection calculator in Figure 12-2.

Note: The diagram in Figure 12-2 does not change to show a side road going to the left. You need to be aware of this when specifying the kerb return radii for roads going to the left.

Figure 12-2

 

Changes in the radius of a kerb return will effect the tangent point chainage at which it matches the main road and the tangent point chainage where it matches the side road.

 

Pavement widths

The initial values for width are taken from the half section of the main road where the intersection is attached. The width of the first link of the main road typical section is applied to the first left and right links of the side road typical section. The box marked Keep widths synchronised with typical sections is checked. This is the default setting and is appropriate when the first link left and right of the control line is the pavement width. This may not be the case for your design. For example the pavement may be made up of a number of links. The appropriate value for the width is the sum of these. If you want to control the widths yourself uncheck the box marked Keep widths synchronised with typical sections. The pavement width at the start and end of each kerb return is used to calculate the location of the kerb return arc.

Note: If the box is not checked HighRoad will not automatically adjust the widths if you change any of the typical section details that affect the intersection.

Deflection angle

The default value of the deflection angle of the start of the side road control line is ±90° 00' 00". This places the intersection on the same side of the through road that you clicked and at an angle of 90°. You can change this as needed. This deflection angle is positive for side roads to the right of the through road (as shown in the Intersection calculator in Figure 12-2) and negative for side roads to the left.

Note: The diagram in Figure 12-2 does not change to show a side road going to the left. You need to be aware of this when specifying the deflection angle for roads going to the left.

The elements of the intersection include three new control lines (representing the side road and two kerb returns) with a horizontal alignment, typical section and a profile for each control line. The length of the side road is initially 50 metres.

 

Adjusting the intersection

Either the side road control line or one of the kerb returns should be active when you want to adjust the intersection. When the cursor is over the intersection it changes to a hand shape. Double-click the intersection to adjust its dimensions. A dialog box as shown in Figure 12-2 will appear. You can now edit the details of the intersection. You can also drag the intersection along the main control line. The deflection angle can be adjusted graphically by dragging the second IP of the side road.

 

Side road

Horizontal alignment

The horizontal alignment of the side road is initially created with two IPs. The first is at the start of the side road on the centre line of the through road. The second horizontal IP is located 50 metres away at an angle of 90°. If you drag this second IP of a road, the deflection angle and the distance to this IP will change. HighRoad will use the new chainage and the deflection angle to the first IP of the side road whenever the intersection is subsequently moved along the through road. You can add more IPs to the side road just as you would for any road by choosing New IP from the Plan menu. Be sure that the side road is the active control line when you do this.

Profile

When an intersection is created, a profile is provided for the side road. Three vertical IPs are created. The first two of these are synchronised as outlined below. A circle appears around the IP to show that it is synchronised. The first IP is synchronised to the Intersection Chainage. The second is synchronised by a grade that matches the crossfall of the through road. Figure 12-3 shows a profile for a side road. The elevation for the start of the side road is taken from the levels of the through road. The start of the side road is connected to the centre line of the through road.

 

Figure 12-3

 

 

The side road is initially given a length of 50 metres. The second vertical IP is placed at 25 metres, and the third vertical IP at 50 metres. The second vertical IP is given an elevation based on the grade from the start of the road. The start grade of the side road is matched to the crossfall of the through road. The through road typical section at the chainage of the side road is scanned to find the crossfall of the pavement link to which the side road attaches. This grade is used to calculate the grade to the second vertical IP in the side road. The vertical curve length is calculated to be long enough to just touch the edge of the pavement of the through road. The third VIP is placed at ground level.

Note: For side roads not at 90 degrees, this grade will not be correct. You will need to do some calculations to establish an appropriate grade and then adjust the elevation of the second vertical IP until you get the required grade.

You can choose to disconnect HighRoad's automatic synchronisation of the vertical IPs. Double click the second vertical IP. A dialog box as shown in Figure 12-4 will appear. Uncheck the box marked Synchronise by grade with through road. Similarly you can disconnect the synchronisation of the first IP. It will show a similar check box but it will be labelled Synchronise with through road.

 

Figure 12-4

 

Updating the side road profile

When an intersection is moved along the through road, or the grading of the through road is changed, the profile of the side road will be adjusted if you have not switched the synchronisation off. The start elevation of the side road will be changed to match the new level of the through road. The grade at the start of the side road will be changed to match any change in the cross fall of the through road. The elevation of the second vertical IP will be adjusted so that the grade is correct. Its chainage or vertical curve length will not be adjusted. HighRoad assumes that while you want the side road to still connect correctly to the through road, you may have changed both the chainage of the second VIP and the vertical curve length to suit your design requirements. The third vertical IP will not be changed in any way. Its incoming grade may change as a result of the change in elevation of the second IP.

 

Typical section

The typical section of the side road is created by adapting the typical section of the through road. Consider the example of the through road typical section shown in Figure 12-5.

 

Figure 12-5

The typical section of this road is purposely asymmetrical to make it easy to describe this process. The left side has a kerb and a footpath, such as in an urban road. The right side has a shoulder and a drainage ditch, as in a rural road. The intersection is attached to the right hand side of the through road.

The right side (the rural half of the through road typical section) is used to create the typical section for the side road. This half section is used directly to create the typical section for the right half of the side road, and reflected to create the left half of the side road. The resulting typical section is shown in Figure 12-6.

Figure 12-6

 

Kerb return

Horizontal alignment

When an intersection is created or adjusted the kerb return geometry is calculated to fit the kerb return arc between the tangent points. Figure 12-7 shows the through road and side road before the addition of kerb returns.

 

Figure 12-7

Superimposed on this is a diagram of the right kerb return. (The chainages of the through road are increasing down the page. The side road is attached to the right side of the road.) The shaded area in Figure 12-7 is the area of pavement that is part of the right kerb return. The control line for a kerb return follows the line of the arc shown in Figure 12-7. It starts at the through road tangent and finishes at the side road tangent. In many respects a kerb return control line is just like any other control line.

 

Typical Sections

Like the side road, the typical section for a kerb return is adapted from the typical section of the through road. Consider the shaded pavement area shown in Figure 12-7. At the start of the kerb return (the through road tangent point) the pavement width and slope matches the pavement link on the right side of the through road typical section.

 

Figure 12-8

 

Figure 12-8 shows the pavement link for the through road. The control line for the kerb return follows the arc shown in Figure 12-7. The matching pavement link which is attached to the left of this control line is shown in Figure 12-9.

Figure 12-9

 

The signs (positive and negative) of the offset and crossfall of this link are opposite to the typical section of the through road as shown in Figure 12-8. This is because they are on the opposite side of the control line.

There are some special requirements for a kerb return typical section. The offset and crossfall will vary at different locations around the kerb return. Consider the cross section shown in Figure 12-7. This is a cross section on the kerb return control line. The pavement at this section is required to join to the centreline of the through road. To attach to the control line of the through road an anchor feature is created as part of the intersection. This anchor feature is invisible and is created and moved automatically as necessary when the intersection is moved or adjusted. See Attaching a pavement to a feature, in Chapter 7, Typical sections for details.

Note: Be very careful if you edit anchored links that are created automatically. These links and the matching feature are changed automatically when the cul-de-sac is moved or some aspect of the design is varied. If you alter such a link or its matching feature, HighRoad may not be able to correctly make adjustments later, and you may get unexpected results.

To complete the typical section of the right kerb return, the shoulder, drainage ditch and batter slope must be added to the right side of the kerb return control line as shown in Figure 12-10.

 

Figure 12-10

Profile

When a kerb return is first created by HighRoad it has four vertical IPs. Each of these are synchronised as outlined below. A circle appears around the IP to show that it is synchronised. The chainage, elevation and grade at the start of the kerb return are calculated to match the through road at the edge of the pavement. The chainage, elevation and grading at the end of the kerb return are calculated to match the side road at the edge of the pavement.

 

Figure 12-11

 

The middle vertical IPs are located at third points. The elevations of these vertical IPs are set to match the grade required at the start and the end of the kerb return. See Figure 12-11. The second Vertical IP is controlled by grade to the first IP. If you drag its position, it will remain at the chainage that you specify, but its elevation will be adjusted so that the slope is always correct. Similarly the third vertical IP is controlled by the grade to the fourth IP.

There will be occasions when you want to take control of the grading. You can choose to disconnect HighRoad's automatic synchronisation of the vertical IPs. Double click the second vertical IP. A dialog box as shown in Figure 12-12 will appear. Uncheck the box marked Synchronise with start of kerb return. Similarly you can disconnect the synchronisation of the three other IPs. The second IP will show a similar check box but it will be labelled Synchronised by grade with start of kerb return. The last two IPs have similar check boxes.

 

Figure 12-12

Updating the kerb return profile

Whenever the grading of the through road or the side road is adjusted, the grading of the kerb returns are also adjusted to suit. The grading of the kerb returns in response to an update is similar to the original creation of the kerb return. The vertical IPS that are updated are those that are marked as synchronised. (Initially all the VIPS of the kerb return are synchronised as this is the default setting.) You can insert extra vertical IPs in between the first two and last two in cases where extra IPs are needed to correctly grade the kerb return. These IPs will not be synchronised and will not be adjusted automatically when other changes are made.

Interaction of variables

Figure 12-13 shows a simple case of an intersection on a straight road and with a straight side road. The location of the tangent points of the kerb return in relation to the main road and the side road are calculated to suit the following variables:

* width at tangent point on main road

* width at tangent point on side road

* radius of kerb return.

 

A change in any of these variables will trigger a recalculation of the intersection details. For example, with the kerb return shown, a change in the width on the main road would change the tangent chainage on the side road by an equal amount. This in turn will effect the vertical grading of the side road and the kerb return. The elevation and grade of the kerb return tangent point on the side road will now be different, and will result in a new kerb return grading being calculated.

Figure 12-13

 

If the side road was changed to an angle other than 90°, the tangent point chainage on the main road as well as the side road would change. This would result in a recalculation of the side road grading and the kerb return grading. Note: HighRoad will not make any adjustments to the vertical grading if you have switched off the automatic synchronisation features.

Intersecting batters

The batter slopes of the side road and main road can interact in the vicinity of the intersection as shown in Figure 12-1. The interaction between the batter slopes can be complex. The calculation of the batter interaction line is done as an iterative process. Ultimately an anchor feature is created to which the batter slopes can attach. For further information on anchored links see Attaching a pavement to a feature, in Chapter 7, Typical sections. The anchor feature is invisible and is created and moved automatically as necessary when the intersecting roads are moved or adjusted.

Note: The anchor feature can be complicated. Take the case of a main road with multiple side roads to the left. The left batter of the main road would attach to multiple anchor features. The pop-up menu of the link editor allows attachment to only one anchor feature. This problem is overcome by having a sequence of anchor features. The first in the sequence is referenced by the pop-up menu in the link editor. All features in the sequence are checked, even though only the first appears to be referenced by the link editor. This is done automatically by HighRoad.

To draw the plan of the kerb return, cross sections are calculated close together (at the interval you have specified). These cross sections would normally overlap at the centre point of the kerb return radius. Part of Figure 12-1 relating to the right kerb return is shown in Figure 12-14.

 

Figure 12-14

It highlights the batter slopes which are part of the right kerb return. In this example the batter slopes are in fill and the slope for the through road and the side road in this area is 52%. At this slope the batters extend further than the centre point of the kerb return radius. For the arc of the kerb return, all batter slopes extend to the same point which is the kerb return centrepoint. The elevation of this point, calculated from the through road is quite likely different to the elevation calculated from the side road. For fills, HighRoad uses the highest of these as the fixed level to which all the batter slopes of the kerb return extend to. For cuts, the lowest of these is taken as the fixed level to which all kerb batters are attached.

 

Interrelated intersections

In the small road network shown in Figure 12-15, there is a series of related roads, intersections and cul-de-sacs. The interrelationship between these roads makes for a complex updating procedure when any road is moved. A change in the grading of the through road may effect the grading of the side road, which may in turn effect the grading of its side road. Such inherited effects are automatically handled if you have not switched off the automatic synchronisation features that HighRoad provides.

Figure 12-15

 

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