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6
Design Guidance
6.3 Roadway Design Guidance
Access Management
Access management is defined as “…the systematic control of the location, spacing, de-
sign, and operation of driveways, median openings, interchanges, and street connections
to a roadway,” as well as other design factors such as local/state policies, spacing of
traffic signals, and median treatment types.
1
Managing roadway access from driveways
and cross-streets has become increasingly important as funds to widen major roadways
on the secondary transportation system have dwindled. Core reasons to enforce access
management include reduction of accidents, improving bicycling conditions, providing
safer pedestrian pathways, increasing business market area / access, and extending the
serviceable life of roadways and roadway capacity. It is important to manage the access
of roadways before the land around them becomes developed, as retrofitting roadways
is often extremely costly and controversial compared to protecting access management
before a roadway becomes saturated with driveways and street intersections.
Figure 6.7 identifies typical actions that should be taken and the common effects on road-
way performance and business activity (often a concern with retrofitting access controls).
This table complements the one on the following page that describes various roadways
and treatments. Among the important factors to consider are that additional, frequent,
and poorly designed driveways can decrease travel speeds by five to 10 miles per hour,
and accidents (as well as associated travel delays during accidents) can increase by 40%
or more if proper access management techniques are not followed. Business failure rates
along unmanaged corridors are also higher than along well-managed roads.
Roadway Hierarchy
Roadways are typically described by just two functions: the mobility that they provide to
move vehicular (especially motorized vehicles) traffic quickly, and the degree to which the
roadway provides access to adjacent lands. In North Carolina, roadways are frequently
given classifications that describe their place in the hierarchy of streets. The following cat-
egories of street are generally recognized by transportation professionals, along with some
of their major characteristics (Figure 6.8).
Notice that some of the values overlap between categories, indicating that the road-
way’s definition is perhaps determined by plans for it instead of simply a matter of how
wide it is or how fast the posted speed limit. This overlap also suggests that other factors
besides mobility and accessibility can play significant roles in the design and development
of roads, shown below in no particular order.
•
Number of large trucks that use the roadway
•
Degree of peak traffic congestion (compared to how much traffic the roadway
generally carries throughout the day)
•
Topography, water courses, wetlands, ridgelines, floodplains, and other natural
features shape roadways
•
The crossings of other roadways, rail lines, and even bicycle/pedestrian accommo-
dations influence roadway design, width, and speed for short intervals
•
Amount of through traffic compared to traffic that has destinations or origins im-
mediately adjacent to the road
•
History of accidents on the roadway and similar roadways
•
The kinds of land uses that border the roadway – commercial, residential, distribu-
tion, and so forth
•
The users of the road – would more cyclists or pedestrians use the road if there were
better provisions for them;
•
How constrained is the public right-of-way for the road, and how does that influ-
ence design and construction costs
•
Are there utilities that use the same right-of-way as the road – water, sewer, electri-
cal, cable, or fiber optic
•
The access to / from the roadway currently controlled by law, policy, design or
some other means?
1
Transportation Research Board, “Access Management Manual.” Committee on Access Manage-
ment, Washington, DC, 2003.
Figure 6.7 Principles of Access Management
Principle
Action(s)
Researched Effects
Maintain a Strong Roadway
and Intersection Hierarchy
Reduce Signals / Mile: 4.0
6.0
8.0
Increase in Travel Time Compared to 2.0:
16%
29%
39%
Limit Direct Access to Major
Roadways
Access Points / Mile: 10
20
30
40 or more
Reduction in Free-Flow Speed (mph): 2.5
5.0
7.5
10
Favor Through Movements
Prohibit on-street parking
20% - 40% reduction in crashes
Separate and Limit Points of
Conflict
Long signal spacing
42% reduction in crashes
59% reduction in delay
57,500 gallons of fuel reduction per mile
Separate Turning Moves (esp.
left turns)
Add left-turn bay
Raised divider separating lefts from through
traffic
25% - 50% reduction in crashes (four-lane roads)
67% reduction in total crashes
Use Medians
Install median
Install continuous, two-way left-turn lane
Replace TWLTL with a median
35% reduction in accidents
30% reduction in vehicular delay
15%-57% reduction in crashes (four-lane roads)
Support Internal and External
Connectivity
Increase driveway illumination
42% reduction in crashes
The street network.