Road Design

Topics: Distance, Length, Stopping sight distance Pages: 12 (2693 words) Published: April 1, 2011
Question 1

In the pre-construction stage what information would you source to help you ‘best fit’ a design to the environment?

The information that may be sourced during the pre-construction stage could include: • Terrain details
• Survey data, existing plans & maps (including topographical & geological maps) • Cadastral boundaries
• Location of existing structures
• Ownership information
• Details of existing road alignments, are there any concerns with the current alignment? Is there a history of accidents on the existing road alignment? • Underground and overhead services (electrical, water, sewerage & gas) • Important environmental features

• Traffic volumes & composition
• Class of road
• The possibility of stage construction
• Budget
• Environmental impact assessment
• Hydrological reports

The information gathered must include a corridor of enough width to allow the designer scope to vary the position of the center line of the road if necessary. It may also be necessary for the designer to walk over the proposed route, to gauge the terrain and the environment and to assist in predicting in any possible problems that may arise.

Question 2

What design factors will influence the road location and standard? Nominate the dimensions that you would expect for all of the individual road elements and why. Nominate the horizontal and vertical geometry limits that you would expect to incorporate and explain why.

The principle factors that influence the choice of a design standard for a road are as follows: • Traffic volume
• Traffic composition
• Terrain
• Environment
• Cost
• Safety
• Stage construction
• Class of road
• Appearance

The vertical alignment of the road will include a series of straight sections joined by a series of curves, being parabolic in shape. A convex (crest) curve will be present at point B and a concave (sag) curve will be present at point C. The horizontal alignment of the road will comprise a series of straights and curves, joined by a transition curve.

o Width of shoulders
The width of shoulders required on undivided roads for an AADT of 500 to 1000 vehicles per day is one to two metres (from Austroads table 4.2). Two meter wide shoulders would be preferred to allow greater room for vehicles to pull over clear of the flow of traffic. However if one meter wide shoulders are used, provisions should be made to allow for parking areas.

o Crossfall of the traffic lanes
The recommended pavement crossfall on straights for a bituminous seal coat surface is 3% (from table 1.6 USQ Study Book).

o Crossfall of the gravel shoulders
The recommended shoulder crossfall for a gravel type shoulder is 4% to 5% (from table 1.7 USQ Study Book). o Stopping sight distance
The minimum stopping sight distance (SSD) on level pavement for a design speed of 100 Km/Hr is 170 metres (from table 1.12 USQ Study Book).

o Maximum grades
The recommended general maximum grade for a design speed of 100 Km/Hr in rolling terrain is 4% to 6% (from table 1.8 USQ Study Book). This is satisfactory considering that the steepest section of the new road alignment is 4%, after point C.

o Width of the traffic lane


From current and past AADT figures, a graph may be drawn and an estimate made for the year 2020 AADT figure. From extrapolation on the graph above the AADT for the year 2020 will be in the vicinity of 800 vehicles per day. The suggested traffic lane configuration for this level of traffic is two lanes and the suggested width is on the borderline of 3.0 to 3.5 metres (from table 1.5 USQ Study Book). However if the budget were sufficient, a lane width of 3.5 metres would be desirable to allow greater room for larger vehicles.

Question 3

What field surveys would be required to help you plan your...
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