20 30 40 50 60 70 10 80 0 90
Speed Scale/design geometrics
Source: New Jersey DOT
15 MPH can mean the difference between crash avoidance and almost certain death
Speed Complete Streets Scale/design geometrics
Source: AASHTO for flat urban/suburban roadways.
Many overly wide roads could use a road diet
What saroaddiet? Classic road diet shrinks 4 lanes to 3 + bike lanes
Another road diet example Classic road diet shrinks 4 lanes to 3 + bike lanes
What else does a road diet do? Creates room for wider sidewalks
What else does a road diet do? An inexpensive tool for retrofitting existing streets
4 2 12 12 12 12 2 4 6 2 12 12 12 12 2 6 10 2 4 11 10 11 4 2 10 6 2 8 11 10 11 8 2 6
X vs.
X vs.
Source; Converting four lane undivided roadways to a three-lane cross-section: Factors to consider. Iowa State University and Iowa DOT
Existing 2 12 12 2 12 2 12 12 2 Proposed 2 4 11 11 12 11 11 4 2 Stripe 4 bicycle lane Reduce travel lane width to 11 Eliminate offset (design speed =< 35 mph)
6 10 6 6 7 6 9
Source: Highway Capacity Manual
Oregon DOT
Four-step travel demand model Microsimulation
1 Design speed is 35 mph or less
Horizontal clearance
Designers and the public should understand that, once a vehicle leaves the road, a crash or potentially serious encounter with the roadside may occur, regardless of the clear-zone width established by the designer.the selected clear-zone width is a compromise, based on engineering judgment, between what can practically be built and the degree of protection afforded the motorist. Limitations in available right-of-way, the location, frequency, and nature of roadside objects, or the presence of valued resources such as wetlands, or the need to provide for pedestrian or other activities may practically limit the clear-zone width.
Pedestrians feel exposed and vulnerable
3 1.5 Source: AASHTO Green Book minimums
4
Pedestrians Bicycles Cars, trucks, buses
Compact, low-speed, human scale Vs. high-speed, high-capacity
40% larger 120% larger
Large radii 1. Increase crossing distance and 2. Make crosswalk & ramp placement more difficult
Effect of large radius on drivers They drive fast, ignoring pedestrians Tigard OR
Keeping it tight: Curb radius Effective radius is larger than built radius if travel lanes are offset from curb w/ parking and/or bike lane Oregon DOT
Keeping it tight: Curb radius On one-way streets, corners with impossible turns can have a very tight radius Impossible turn
Must consider design vehicles, but don t choose larger vehicle than necessary
We shouldn t t design for the exceptional vehicle Moving vans use this street once or twice a year; hundreds of pedestrians cross every day Orlando FL
Medians improve safety at driveways
Safety 30% potential increase in crashes 12% potential increase in crashes Lowest crash rate Safety issue becomes more pronounced at 24,800 to 28,000 AADT threshold Safety issue becomes more pronounced at 24,800 to 28,000 AADT threshold Mobility Highest delay Minimize turn movements (driveway consolidation) Least signal delay (storage) Must provide adequate storage Access Unrestricted access Unrestricted access Most restrictive Pedestrian safety Pedestrian must cross four lanes of traffic Widest crossing distance Center turn lane not a safe refuge Pedestrians cross two lanes at a time Raised median provides refuge
Medians reduce crash risk for random mid-block crossings
Flush median is not much of a refuge
Flush median is not much of a refuge
Basic Principle Break long complex crossing into two simpler crossings
4 10
2007 Alabama ASCE Summer Conference 148