Adequate Sight Distance – Intersection Sight Distance (ISD)
•
Allow drivers to have an unobstructedview of intersection
• Definition: Required ISD is the length of cross road that must be visible such that the driver of a turning/crossing vehicle can decide to and complete the
maneuver without conflict with vehicles approaching the intersection on the
cross road.
Adequate Sight Distance – ISD
Intersection Sight Triangle – area free of
obstructions necessary to completemaneuver and avoid collision – needed
for approach and departure (from stop
sign for example) – Exhibit 9-50•
Allows driver to anticipate and avoid collisions•
Allows drivers of stopped vehicles enough view of the intersection to decide when toIntersection Sight Triangle
•
Consider horizontal as well as vertical, object below driver eye height may not be anobstruction
•
AASHTO assumes 3.5’ above roadwayLEFT RIGHT
Sight Distance Obstruction
Hidden Vehicle
ISD Cases
• No control: vehicles adjust speed
• Stop control: where traffic on minor roadway
must stop prior to entering major roadway• Yield control: vehicles on minor roadway must
yield to major roadway traffic• Signal control
: where vehicles on allapproaches are required to stop by either a stop sign or traffic signal
• All way stop
• Stopped major roadway left-turn vehicles –
Case A – No Control
• Rare? – Not really, Iowa
• Minimum sight triangle sides = distance
traveled in 3 seconds (design or actual?) = 2 seconds for P/R and 1 second to actuate
brake/accel.
• Assumes vehicles slow ~ 50% of midblock
running speed
Case A – No Control
• Prefer appropriate SSD on both
approaches (minimum really)• Provided on lightly traveled roadways
• Provide control if sight triangle not
available
• Assumes vehicle on the left yields to
vehicle on the right if they arrive at same time
d
bda
a
b
Large Tree
47’
72’
Example
45 mph
25 mph
Large Tree
a = 47’
b = 72’
Example
45 mph
25 mph
da = 220 feet
db = a *da = 47’ (220’) = 69.9’
db
da
Large Tree
a = 47’
b = 72’
Example
45 mph
25 mph
25 mph > 15 mph, stopping sight distance is not
db
da
Case B – Stop Control
Three Sub Cases – Maneuvers
• Turn left on to major roadway (clear
traffic left, enter traffic right)• Turn right on to major roadway (enter
traffic from left)• Crossing (clear traffic left/right)
Case B – Stop Control
• Need ISD for departure and completion even if vehicle comes into view at point of
departure = 1.47 V
major * t g where t g =7.5-11.5s; add more for grade or multilane;
decrease by 1sec. for right turns
Left
turn
right turn
and crossing
Case C - Yield Control
• Minor Roadway Yields – must be able to see left/right – adjust speed – possibly
stop
• Sight distance exceeds that on stop
control
• Similar to no-control
Case C - Yield Control
• Must use minimum stopping sight distances for d
aand d
b• SSD calculation should include effect of grade
• Required distance = P/R + stop
Case C - Yield Control
• Typically Known – a, b
• Typically Assume V
a
or Vb
• Similar triangle can be used to calculate
safe approach speeds (given oneapproach speed) or allowable a and b.
• d
a
/db
= (da
– b)/a• d
b
= (da
*a)/ (da
– b)da
db
Yield Control
• Case C-1: Crossing maneuver from minor road
• Assumes that minor road vehicles that do not stop decelerate to 60% of minor road speed
• Vehicle should be able to:
• Travel from decision point to intersection decelerating to 60% of design speed
• Cross and clear the intersection at the same speed
t g
Yield Control
• Case C-2: Left and Right turns at yield
control
db = 82 ft to accommodate left and right turns
d : similar to d for stop-controlled but increase time
da
db
d
a: length
of major
approach
• Case D: Signal control
•
First vehicle stopped should be visible to driver of other approachesAlso …
• Case E: All way stop
• Case F: Left turn from major
• t
g
=5.5-7.5*s + multilane adjustment• Effect of Skew
Sighting Rod and Target Rod (AASHTO)
• For vertical sight
distance with vertical curves
• Sighting rod- 3.5 feet tall
• Target rod- 4.25 feet tall (Top portion and bottom 2 feet are
painted orange)
Sighting
Target Rod
Rod
Measuring at an Uncontrolled Intersection
Assistant
Obstruction X
Y
Assistant
Movement of Assistant
Measuring at a Stop-controlled Intersection
Observer Assistant
Stop Bar
10 ft
Sigh t Line
Assistant with Target Rod (4.25 ft) Observer with Sighting Rod (3.5 ft)
평면교차로의 계획 – 교통 관제
• 신호 교차로
- 설계 속도에 따라 교통관제에 주의
• 일시 정지 및 양보표지
- 주 도로와 부도로의 교통량에 따라 영향
- 일시 정지표지 설치하는 도로 임계 교통량을 용량보다 적게 설정
• 회전교통
- 좌/우 회전 자동차가 직진 교통 류를 방해 하지 않도록 함 - 도로규격, 설계기준 자동차 및 교통운영 방법을 고려
설치 위치
1)평면선형을 고려핚 설치 위치;
교차로는 평면선형이 직선인 곳에 설치 하는 것이 원칙
부득이 곡선부 설치 시 곡선부 바깥쪽에 접속하는 것이 바람직함
2)종단선형을 고려한 설치 위치
교차로는 급경사 구갂 , 그리고 종단곡선 구갂에 설치 금지 부득이핚 경우 오목형 종단 곡선부에 설치 하는것이 유리 (시거확보)
평면교차로의 계획 – 평면교차로 갂의 최소 갂격
• 설치 간격 = f(차로 변경에 필요한 거리, 전 차로 길이, 인지성)
• 차로 변경에 필요한 길이(rot)
– L=a×V×N
L=순간격(m)
V=설계속도(km/h)
N=설치 차로수
a= 상수(시가지 1, 지방지역2~3)
전체사고율(y) = -16.32Ln(x) + 30.45 R2 = 0.7393
복합사고율(y) = -11.79Ln(x) + 21.88 R2 = 0.7345
사망사고율(y) = -1.54Ln(x) + 2.32 R2 = 0.6702
0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
이 격 거 리
사 고 율
전체 사고율
사망 사고율
복합 사고율
Y =사고율 X=이격거리
평면교차로 갂의 갂격
-
최소 설치 간격– 교차로의 모서리에서부터 가장 가까운 접근로 출입 지접까지의 거리
국도의 평면교차 갂격
• 국도Ⅰ 과 다른 도로는 입체교차를 원칙; 불가 피핛 경우에는 3개/10km 이하
• 국도Ⅱ와 다른 도로는 입체 교차를 원칙으로 하되, 5개/10km 이하
• 국도Ⅲ과 국도ⅠⅡ를 제외핚 다른 도로와의 연결은 평면교차를 원칙, 5~10개소/10km를 기준