Geometric Design (I)

Learning Objectives

• To understand the considerations and quantifiable aspects of geometric design

Locational Design

• Consider– Current land use– Geology– Future land use– Existing infrastructure

Controls and Criteria (1)

• Design Vehicles (p. 168-173)– Passenger cars, buses, trucks, RVs– Physical characteristics: weight,

dimensions– Establish intersection radius, pavement

markings

• Vehicle Performance– Operating characteristics: accel/decel – Impacts air quality, noise, land use

Controls and Criteria (2)

• Driver– Information handling– Reaction time

• Time to perceive + react to a hazard in vehicle’s path

• Expected/unexpected

– Speed– Driver errors

Controls and Criteria (3)

• Traffic– Composition and volume

• Average daily traffic (ADT) is not adequate• Design hourly volume (DHV)• 30th-highest hourly volume (30HV) in one year• K-factor (% of ADT; 8~12% urban, 12~18% rural)

– Speed• Operating Speed (typically the 85th percentile speed)• Free-flow Speed (close to zero density)• Running Speed (actual speed)• Design Speed (as high as practical)

Controls and Criteria (4)

• Capacity– Maximum hourly flow rate (per lane) under

prevailing conditions– Determines adequacy of existing roadways– Helps select roadway type– Helps define needs– Design level of service

(LOS)

Stopping Sight Distance (1)

• Length of roadway that should be visible ahead of you in order to ensure that you will be able to stop if there is an object in your path

• Calculate the SSD for a vehicle traveling on your roadway at the design speed, and then make sure the actual sight distance that you provide is at least as great as the stopping sight distance

Stopping Sight Distance (2)

• Assume – Driver eye height of 3.5 feet– Height of object between 2.0 and 3.5 feet

• Reaction distance + braking distance

• Design standard: tr=2.5, a=11.2

Ga

VtVSSD

sft

mphsrmphft

2.3230

47.1)/(

2)(

)()()(2

Other Sight Distances

• Decision sight distance– Allow longer tr for information processing for

different maneuver conditions (table 6-5)

• Passing sight distance– Ensure safe passing maneuver (figure 6-5)– 4 distance components (figure 6-6)

• At 70 mph– SSD = 730 ft– DSD = 1445 ft (maneuver E)– PSD = 2480 ft

• Basic controlling expression

e = rate of superelevationu = side friction factor (dep. on pavement, speed, …)V = vehicle speedR = radius of curve

Horizontal Alignment

)(

2

15)(

ftR

Vue mph

Horizontal Alignment

• Overall design procedure– Determine a reasonable maximum

superelevation rate.– Decide upon a maximum side-friction factor.– Calculate the minimum radius.– Iterate and test several different radii until

you are satisfied with your design.– Make sure that the stopping sight distance is

provided. Adjust your design if necessary.– Design the transition segments.

Superelevation

• Tilting the roadway to help offset centripetal forces developed as the vehicle goes around a curve

• General Practice– Highways, no ice/snow emax = 0.10– Highways, snow/ice emax = 0.06– Traffic congestion or roadside development,

limit speeds emax = 0.04 ~ 0.06

e1

Side Friction

• Design based on point where centrifugal force creates feeling of discomfort for driver

Speed umax udesign

20 0.50 0.17

30 0.35 0.16

40 0.32 0.15

50 0.30 0.14

60 0.29 0.12

70 0.28 0.10