6. Geometrics Flashcards
What is the flow rate?
The number if vehicles (n) passing some designated roadway point in a given time interval.
q, Flow (veh/hr) = number of vehicles/ time interval OR
q, (veh/hr) = Speed (mph) x Density (veh/mi)
Spacing Equation =
Spacing (ft/veh) = 5280 (ft/mi) / Density (veh/mi)
What is headway?
the time (in seconds) between successive vehicles, as their font bumpers pass a given point.
headway, h (sec/veh) = 3600/Flow Rate
Spacing (veh/mi) =
Speed (fps) =
Headway (s/veh) =
5280/density (ft/veh)
spacing (ft/veh)/headway (s/veh)
3600/flow rate (veh/hr)
What is the R (arc definition)?
R = 5,729.6/D
What is the R (chord definition)?
R = 50/sin(D/2)
What is the equation for tangent, T?
T = R*tan(delta/2)
What is the equation for the length of curve, L?
L = (pi/180)Rdelta
What is the equation for the deflection angle, alpha?
a = (Sta. A - Sta. PC)*(delta/2L)
What is AADT?
Annual Average Daily Traffic (total yearly traffic divided by 365)
What is ADT?
Average Daily Traffic (average 24-hr volume by season, month, week, day)
What is the K factor?
the percentage of traffic occurring during the peak hour during average weekday.
What is DHV?
the Design Hourly Volume, represents the 30th highest hourly volume during a year; DHV = AADT(K)
What is the Directional Distribution, D?
the proportion of the peak traffic traveling in the peak direction.
What is Peak Hour Factor?
peak hour volume/(4 x peak 15-min volume)
What is DEF?
Daily Expansion Factor = average daily traffic for a week/volume for particular day
What is MEF?
Monthly Expansion Factor = AADT (est. as avg of 12 months)/ADT for a particular month
What are the two speed percentiles most important to understand?
- 50th - median speed of observed data
- 85th - speed at which 85% of observed vehicles were at or below.
What does MUTCD say about posted speed limits?
the posted speed limit should be within 5 mph of the 85th-percentile speed of free-flowing traffic. Round to the closes 5 mph increment.
What is the definition of pace?
a 10-mph increment in speed that encompasses the highest portion of observed speeds.
What is time mean speed?
the arithmetic mean of speeds.
What is space mean speed?
the harmonic mean of the speeds.
