Lecture 4 - Geometric Design of Roads

Question 1

Example of Geometric Design of Roads

Answer 1

• Design speed
• Sight distance
• horizontal alignment
• vertical alignment
• road safety
• cross sections

Question 2

What should you refer to for the National highways design standards?

Answer 2

• DMRB
• Manual for streets

Question 3

How to select a suitable design speed

Answer 3

• usually the 85th percentile speed
• the speed is usually influenced by motorists expectations

Question 4

Factors which influence speed of the road

Answer 4

• weather
• daylight conditions
• traffic on the road

Question 5

Main factors which affect the design speed

Answer 5

• visibility
• curvature
• road width
• junctions/accesses
• surface conditions

Question 6

What are factors which affect the design speed in rural areas?

Answer 6

• Bus stops
• cyclists
• pedestrians
• crossings

Question 7

What are the Road safety audit stages?

Answer 7

Stage 1 - Completion of preliminary design
Stage 2 - Completion of detailed design
Stage 3 - Completion of construction
Stage 4 - Monitoring which include the analysis and reporting of 12 months road traffic collision data from when scheme began

Question 8

What are road safety audits?

Answer 8

• they identify road safety problems
• carried out independently of the design team
• Collision monitoring to identify any road safety issues that may occur after opening.

Question 9

What are the main sight distance of interest to road designers?

Answer 9

• Sight stopping distance (SSD)
• Overtaking sight distance or full overtaking sight distance (OSD or FOSD)

Question 10

What is sight distance?

Answer 10

• length of carriageway which is visible to the driver in both the horizontal and vertical planes
• the most important factor in the safe and efficient operation of a highway
• standards established for the minimum sight distances have a major significance on the cost of a road.

Question 11

What are the main components of a road geometric design?

Answer 11

• Horizontal alignment
• vertical alignment

Question 12

What are four elements of the minimum distance required for safe passing on two lane single carriageway roads (FOSTD)

Answer 12

d1 – the perception – reaction time taken or distance travelled by a vehicle while the driver determines if it is safe to pass the driver in front
d2 – overtaking time taken or distance travelled by the overtaking vehicle in undertaking the real passing action
d3 – safety time of distance between the overtaking vehicle and the opposing vehicle at the moment the overtaking vehicle returns to its correct lane
d4 – closing time taken or distance travelled by the opposing vehicle during the time d1 plus d2

Question 13

In the UK the FOSD used on two-lane roads assumes that?

Answer 13

• safety distance d3 = 0.2d4
• 85% of overtaking actions last less than 10 seconds

Question 14

What is the Stopping sight distance?

Answer 14

• Minimum distance required to stop a vehicle before it hits a stationary or slow moving object.

Question 15

What parameters should be taken into account when computing stopping sight distance (ssd)?

Answer 15

• Speed
• reaction time
• braking distance
• eye and objects height