- 2066 Bhadra · 1+3+4 marks
Grade separation types; conflict points
Short answer
Grade separation carries roads at different levels to eliminate crossing conflicts; types include overpass, trumpet, diamond, and cloverleaf. A two-way four-leg intersection has 32 conflict points; with one one-way road it has 19.
What is grade separation? Mention their types with sketches. Draw a right angle four legged intersection of two roads and show their various types of conflict points if (i) both roads are with two way movements; (ii) one road is with one-way and another is with two way movements.
Grade separation
Grade separation means carrying crossing roads (or a road and a railway, or pedestrians) at different levels by a flyover, underpass, bridge or tunnel, so that the crossing conflicts are removed. The roads are linked by ramps where turning is needed (an interchange).
Types with sketches
1. Overpass (flyover) and underpass: one road passes over or under the other, with no connection.
===========//========= upper road
______/ \______
------------------------ lower road
2. Trumpet interchange (three legs): one loop ramp for the turn.
main road
========+=========
| \
| ) loop
branch
3. Diamond interchange (four legs): four ramps; the minor road meets the ramps at-grade.
minor road
\ | /
=======\-+-/======= main road (over/under)
/ | \
4. Cloverleaf interchange: four loops carry the turns that would cross; free flow but large area.
() || ()
\ || /
======X====X======
/ || \
() || ()
5. Directional (stack) and rotary interchange for high-volume junctions.
Conflict points at a four-leg intersection
Count for the through, left and right movements from each entering approach (a pair of movements gives a diverging, merging or crossing conflict).
(i) Both roads two-way (4 approaches, 12 movements)
| Type | Number |
|---|---|
| Crossing | 16 |
| Merging | 8 |
| Diverging | 8 |
| Total | 32 |
(ii) One road one-way, the other two-way (3 entering approaches, 9 movements)
| Type | Number |
|---|---|
| Crossing | 8 |
| Merging | 5 |
| Diverging | 6 |
| Total | 19 |
(i) two-way x two-way (ii) one-way x two-way
N N (two-way)
D\ | /M D\ | /M
M-----+-----D ----+-----> (one-way road,
X X X X 16 crossing X X X enters from W only)
D-----+-----M ----+----
M/ | \D M/ | \D
S S
X crossing M merging D diverging
Classification: diverging (one stream splits), merging (two streams join) and crossing (two streams cut across), in increasing order of severity. A one-way system removes the opposing-flow conflicts and cuts the total from 32 to 19, and to 9 if both roads are one-way. Grade separation removes the crossing conflicts altogether.
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