6. Geometrics

Question 1

What is the flow rate?

Answer 1

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)

Question 2

Spacing Equation =

Answer 2

Spacing (ft/veh) = 5280 (ft/mi) / Density (veh/mi)

Question 3

What is headway?

Answer 3

the time (in seconds) between successive vehicles, as their font bumpers pass a given point.

headway, h (sec/veh) = 3600/Flow Rate

Question 4

Spacing (veh/mi) =
Speed (fps) =
Headway (s/veh) =

Answer 4

5280/density (ft/veh)
spacing (ft/veh)/headway (s/veh)
3600/flow rate (veh/hr)

Question 5

What is the R (arc definition)?

Answer 5

R = 5,729.6/D

Question 6

What is the R (chord definition)?

Answer 6

R = 50/sin(D/2)

Question 7

What is the equation for tangent, T?

Answer 7

T = R*tan(delta/2)

Question 8

What is the equation for the length of curve, L?

Answer 8

L = (pi/180)Rdelta

Question 9

What is the equation for the deflection angle, alpha?

Answer 9

a = (Sta. A - Sta. PC)*(delta/2L)

Question 10

What is AADT?

Answer 10

Annual Average Daily Traffic (total yearly traffic divided by 365)

Question 11

What is ADT?

Answer 11

Average Daily Traffic (average 24-hr volume by season, month, week, day)

Question 12

What is the K factor?

Answer 12

the percentage of traffic occurring during the peak hour during average weekday.

Question 13

What is DHV?

Answer 13

the Design Hourly Volume, represents the 30th highest hourly volume during a year; DHV = AADT(K)

Question 14

What is the Directional Distribution, D?

Answer 14

the proportion of the peak traffic traveling in the peak direction.

Question 15

What is Peak Hour Factor?

Answer 15

peak hour volume/(4 x peak 15-min volume)

Question 16

What is DEF?

Answer 16

Daily Expansion Factor = average daily traffic for a week/volume for particular day

Question 17

What is MEF?

Answer 17

Monthly Expansion Factor = AADT (est. as avg of 12 months)/ADT for a particular month

Question 18

What are the two speed percentiles most important to understand?

Answer 18

• 50th - median speed of observed data

- 85th - speed at which 85% of observed vehicles were at or below.

Question 19

What does MUTCD say about posted speed limits?

Answer 19

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.

Question 20

What is the definition of pace?

Answer 20

a 10-mph increment in speed that encompasses the highest portion of observed speeds.

Question 21

What is time mean speed?

Answer 21

the arithmetic mean of speeds.

Question 22

What is space mean speed?

Answer 22

the harmonic mean of the speeds.