Month: January 2012

[B’ Spokes: When I read this I could not help but think of conversations from the state around wanting to put rumble strips everywhere. We are designing streets that encourage overrun by motor vehicles and then try to do something else that has little to no effect on the subject or worse discourages biking and walking.
I will also note that while accommodating fast travel by motor vehicle has it’s place, but that place is NOT on every single street.]

by Eric Dumbaugh

…
In this study, I
examine the subject of livable streetscape
treatments and find compelling evidence
that suggests they may actually enhance
the safety of urban roadways. Concerns
about their safety effects do not appear
to be founded on empirical observations
of crash performance, but instead on a
design philosophy that discounts the
important relationship between driver
behavior and safety. This study traces the
origin and evolution of this philosophy,
and proposes an alternative that may better account for the dynamic relationships
between road design, driver behavior, and
transportation safety.

…
Beyond simply acting as thoroughfares for motor vehicles, urban streets
often double as public spaces. Urban streets are places where people walk,
shop, meet, and generally engage in the diverse array of social and recreational activities that, for many, are what makes urban living enjoyable. And
beyond even these quality-of-life benefits, pedestrian-friendly urban streets have
been increasingly linked to a host of highly desirable social outcomes, including
economic growth and innovation, improvements in air quality, and increased physical fitness and health,
to name only a few. For these reasons, many groups and individuals encourage
the design of “livable” streets, or streets that seek to better integrate the needs of
pedestrians and local developmental objectives into a roadway’s design.

…
Interestingly, clear zones are not the only design feature for which such safety anomalies appear. Hauer
reexamined the literature on lane widths and found that
there was little evidence to support the assertion that
widening lanes beyond 11 feet enhances safety. Instead, the
literature has almost uniformly reported that the safety
benefit of widening lanes stops once lanes reach a width of
roughly 11 feet, with crash frequencies increasing as lanes
approach and exceed the more common 12-foot standard.

…
Thus, a key question emerges: why does contemporary
design guidance recommend practices that the best available evidence suggests may have an ambiguous or even neggative impact on safety, and paradoxically, to do so under
the auspices that they constitute a safety enhancement?

…
One of the key problems identified by the AASHO
committee was the large number of fatalities associated
with single-vehicle, run-off-roadway crashes. To address
this issue, they heard testimony from Stonex, a General
Motors employee responsible for designing the “Proving
Ground,” an experimental “crashproof” highway that had
100-foot clearances on either side of the travelway. Based on the test performance of
the Proving Ground, Stonex was of the opinion that “What
we must do is to operate the 90% or more of our surface
streets just as we do our freeways . . . [converting] the surface
highway and street network to freeway and Proving Ground
road and roadside conditions”

…
As shown in Table 3, the livable section is safer in all
respects. By any meaningful safety benchmark—total midblock crashes, injuries, or fatalities—there can be little
doubt that the livable section is the safer roadway.

…
Pedestrian and bicyclist injuries were likewise higher
on the comparison section (see Table 4), which may be
partly attributable to the fact that the livable section provides parked cars and fixed objects to buffer pedestrians
from oncoming traffic. But do the benefits in pedestrian
safety outweigh the hazards these features may pose to
errant motorists?

…
While these results seem to contradict conventional
design practice, they confirm a trend that many researchers
and practicing engineers have observed for some time, but
which has received little substantive elaboration: specifically,
that clear zones and other forgiving design practices often
have an ambiguous relationship to safety in urban environments, and may be associated with declines in safety performance. The best possible explanation for the enhanced
safety performance of the livable sections considered in this
study is that drivers are “reading” the potential hazards of
the road environment and adjusting their behavior in
response.

…
The reason why this subject has not received greater
attention in design literature and guidance appears to be
that it contradicts the prevailing paradigm of what constitutes safe roadway design. Nevertheless, a behavior-based
understanding of safety performance is supported by
research and literature in the field of psychology, which has
focused on the subject of traffic safety as a means for understanding how individuals adapt their behavior to perceived
risks and hazards.

…
The presence of features such
as wider lanes and clear zones would appear to reduce the
driver’s perception of risk, giving them an increased but
false sense of security, and thereby encouraging them to
engage in behaviors that increase their likelihood of being
involved in a crash event. If so, this explains why the livable
streetscape treatments examined in this study resulted in
not only fewer fixed-object crashes, but fewer multiple
vehicle and pedestrian crashes as well. … From the perspective of risk homeostasis theory,
the use of high design values is not “forgiving,” but is
instead “permissive.”

…
The passive approach promotes designs intended to
support high-speed operating behavior, and then attempts
to mitigate a roadway’s hazards through the use of signs
and pavement markings. The problem that emerges, however, is that signs and roadways are often communicating
contradictory information. The result is that the majority
of drivers in urban areas disregard posted speed limits, and
seem to learn to disregard road signs altogether, even when
they display information that is essential to their safety. … This latter finding
suggests that even conscientious drivers may be unable to
comply with posted speed limits when roadways are designed
for higher-speed operation.

…
I have argued that many of the safety concerns that
emerge on urban streets result from design practices that
fail to link a roadway’s design to its environmental context,
thereby providing motorists in urban environments with a
false sense of security and increasing their potential exposure to crashes and injuries. I have further provided a theoretical framework that better accounts for the safety anomalies one observes when examining the literature and data
on the crash performance of urban roadways. Yet theory is
only the first step. There is a clear and demonstrated need
to better develop our professional understanding of the
relationship between driver behavior and transportation
safety, as well as to enhance our overall approach to the
design of urban roadways. This study thus concludes with
the hope that by better understanding the relationship
between design, driver behavior, and safety, we can design
roadways that are not only safe, but also livable.
Continue reading “Safe Streets, Livable Streets”