ITE Journal - April 2020 - 46

In the United States, it is common practice for the design
speed of a roadway to be greater than the posted speed limit.
Doing so provides a buffer for driver reaction or correction
if drivers exceed the posted speed limit but do not exceed the
roadway's design speed. However, designing roadways in this
way can be problematic to the degree it encourages faster speeds
despite discouragement by the posted speed limit.
An important safety factor transportation engineers consider
when designing roadways is stopping sight distance. This is often
defined as the minimum sight distance required for a vehicle
to come to a stop when traveling at the roadway's design speed.
One of the main factors that affects stopping sight distance is
brake reaction time and distance, the distance a vehicle travels
from the moment the driver sees an object in the roadway
requiring a stop to the moment the brakes are applied.11 To
calculate brake reaction distance, a brake reaction time of 2.5
seconds is typically assumed, but recent evidence suggests this
assumption may be inadequate and average reaction times may
be longer due to increased instances of "distracted driving."12
With current infrastructure designed for a 2.5 second reaction
time, drivers may be unable to see a VRU or object, react and
brake, and reach a complete stop in time to avoid a collision.
As a result, it is increasingly important to supplement strategies
to reduce distracted driving with strategies to reduce vehicle
speeds, which help give drivers more time to react to situations
on the road.
Since speed and reaction time are such important factors
in determining collision outcomes-and in light of the rise of
traffic-related fatalities among VRUs, the number of speed-related collisions, and increasing driver reaction times related to
distracted driving-standardizing HAV speeds and reaction
times will be important for safety in the future. The pending
commercialization of HAVs provides us with a window to
meaningfully impact future roadway safety outcomes, particularly for VRUs.
46

Apri l 2020

i te j o urn al

San Francisco MTA Vision Zero Action Plan, February 2015

Figure 2. Relationship between vehicle speed and likelihood of
VRU fatality.

How Might HAVs Influence Safety?
While HAVs have the potential to improve collision rates and safety
in the future, they could also introduce new safety risks, and many
questions remain as to how HAV will be used and regulated in the
future. Advocates often point to HAVs' potential to reduce human
errors by removing the need for a human driver, given that human
error is a factor in more than 90 percent of collisions.13 With automated
driving systems (ADS), collision types related to distracted driving and
driving under the influence of alcohol could be reduced.14, 15 Further,
the ability to manage vehicle speeds through automated technologies
based on roadway classification, surrounding land use, or other
factors (i.e., areas with VRUs) could reduce speeding, which is a key
contributor to collision frequency and severity.
HAVs could also have better awareness of their surroundings
than human operators by using sensors with wide spectrum
visibility to "see" 360 degrees around the vehicle. However,
reaction time for ADS varies from humans based on differences
in recognition and processing time. In particular, HAVs' ability
to detect and predict the movement of pedestrians and bicyclists
across contexts is currently limited and represents a key challenge
area for HAV safety.16 For example, in the fatal AV-pedestrian crash
in Tempe, AZ, USA in 2018, the pedestrian was detected as an
object-not a pedestrian-5.6 seconds before impact. It took the
ADS 4.4 seconds to calculate that a collision would occur, making
it too late in this case to avoid a collision without human driver
intervention.17 This illustrates that HAV reaction time can be longer
than that of a human operator, depending on the circumstance.
There are also larger questions about how general HAV use
should be regulated in the future to maximize potential societal
benefits while decreasing potential downsides, like negative effects on
traffic congestion, emissions, and the design and function of cities.
So far, cities and states have taken different approaches to policies
around AV testing, and no national legislation has been passed.
Many factors will influence AV safety impacts such as:
ƒ	 How vehicles are designed (similar to today, larger and heavier
vehicles-and those with higher bumpers-would result in more
severe injuries).18
ƒ	 How vehicles are programmed (i.e., to maximize mobility and
take more risk in unpredictable situations, or to maximize
safety and drive slower and more cautiously).
ƒ	 Whether HAVs increase or reduce total vehicle travel, which
will depend on public policy and level of vehicle sharing and
ride-pooling. To the degree vehicle miles traveled (VMT) increases,
vehicle exposure and collision risk for VRUs would increase.16
ƒ	 How human drivers and VRUs will respond to and interact with
HAVs as they are phased in.
ƒ	 How the right of way is adapted (i.e., lane widths could be
reduced to accommodate additional lanes, which would increase
throughput, but also reduce the margin for error).



ITE Journal - April 2020

Table of Contents for the Digital Edition of ITE Journal - April 2020

ITE Journal - April 2020 - Cover1
ITE Journal - April 2020 - Cover2
ITE Journal - April 2020 - 3
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ITE Journal - April 2020 - 5
ITE Journal - April 2020 - 6
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ITE Journal - April 2020 - Cover3
ITE Journal - April 2020 - Cover4
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https://www.nxtbook.com/ygsreprints/ITE/ITE_May2020
https://www.nxtbook.com/ygsreprints/ITE/ITE_April2020
https://www.nxtbook.com/ygsreprints/ITE/ITE_March2020
https://www.nxtbook.com/ygsreprints/ITE/ITE_February2020
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https://www.nxtbook.com/ygsreprints/ITE/G110939_ITE_November2019
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https://www.nxtbook.com/ygsreprints/ITE/G103582_ITE_February2019
https://www.nxtbook.com/ygsreprints/ITE/G102868_ITE_January2019
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https://www.nxtbook.com/ygsreprints/ITE/G100154_ITE_November2018
https://www.nxtbook.com/ygsreprints/ITE/G99495_ITE_October2018
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