Ambulance

Focus on Global Traffic Technologies
was a lack of communication between
intersections to implement a central ‘greencorridor’
system (manually providing green
lights along the length of an ambulance’s
route). Even with communications in place
the green-corridor systems performed
poorly and caused severe traffic disruptions.
Strobe systems offered exceptional signalling
range and consistent, reliable performance.
Well suited for North America’s wide and
straight streets, this line-of-sight system
was less effective in Europe where curved
streets and narrow roads reduce range. This
limited the applications and localities suitable
for emergency vehicle priority. In Europe,
sound and green-corridor systems suffered
from the same issues as in North America
and were not widely adopted.
Only with the emergence of radio/GPS
based priority control systems in the early
2000s was a solution developed that truly
began to meet Europe’s emergency vehicle
preemption requirements. Now, a properly
configured EVP system can provide:
• An improvement in response times of up
to 25 percent
• A reduction of up to 70 percent in
intersection crashes involving emergency
responders
• A prevention of crash injuries and related
costs
• A reduction in property damage costs
• Decreased liability for crashes with civilian
motorists
• A quick demonstrable recovering of the
investment from the above benefits
Centralised architectures
EVP systems can be divided into two main
categories: those that use a distributed
architecture and those that implement a
centralised approach to signalling for a green
light.
In a centralised system, the request for
preemption is sent from the emergency
vehicle to the intersection via a central office
that controls if and when a call for a desired
green phase is made.
In this scenario, the equipment on the
vehicle is the priority request generator. The
vehicle sends its location, speed, heading
and identification credentials over a radio
link to the priority request server located
at a central office. This server validates the
identity of the sender and determines which
intersection and what turning movement is
being requested. The priority request server
then communicates the request to the traffic
controller software, which activates the signal
via a wired or wireless link.
Distributed architecture
In a distributed EVP architecture, the priority
request generator is again located on the
vehicle, but each intersection now has a
priority server. When preemption is desired
the message is sent from the priority request
generator either directly over a two-way
radio link or via a cellular or trunked radio
to the priority request server. The priority
request server verifies the vehicle location,
heading, speed and user identification.
If the call is valid and meets triggering
requirements, the priority server signals
the traffic signal controller requesting a
preemption at the intersection for the
desired green phase. The priority request
server may be monitored and configured
by software located at the central office if
communications exist between the central
facility and the intersection.
Advantages and disadvantages
Each EVP architecture has advantages and
prerequisites for proper operation.
Centralised systems can leverage existing
central office to intersection control of
traffic signals, and require no additional
equipment in the local traffic cabinet,
making them an attractive proposition for
European applications where traffic cabinets
are either limited in space or control of
the signals is centrally managed. Priority
request generators on the vehicle can use
existing public safety radio channels, or GPRS
networks to communicate with the central
office priority request server.
But it’s worth remembering that centralised
systems do have critical performance
requirements which must be addressed
if they are to be effective for EVP. Most
important is the ability to accurately and
consistently determine and communicate
to the priority request server when the
emergency vehicle (priority request
generator) has reached the activation point.
Inaccurate position determination, or latency
in communications will cause the trip point
to move forwards and backwards tens of
metres. The result is preemptions that may
take more side street green than is really
necessary and increased traffic disruption.
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Distributed systems, as noted above, may
use dedicated point-to-point or cellular
radio links. Dedicated radio links ensure
low latency and deterministic access to
the priority server from the emergency
vehicle. This is important as typically multiple
vehicles will be approaching the intersection
at the same time and must all be serviced
with no delay. Continuous updating is also
key for optimal performance. By constantly
signalling the intersection priority request
server, a map matching verification of the
vehicles route may be obtained.
Integration with TSP
EVP systems can also be integrated with
TSP systems, meaning users are able to use
the same intersection equipment. Both EVP
and TSP can be monitored and controlled
using Central Management Software (CMS).
This integration can prove effective in the
case of large events, which are often served
by park-and-ride vehicles that efficiently
transport many attendees. If these public
transport vehicles are equipped with
priority control devices, CMS can cause red
lights to turn green when they approach.
However, if a high priority vehicle requires
the other approach, it will override the inprogress
priority control and give it to the
emergency responder.
This capability is also useful for disaster
evacuation. Event and disaster evacuation
plans may be set up in advance, ready to
implement at the touch of a button.
About Tim Hall and GTT:
Tim Hall is the market and
development director at Global
Traffic Technologies (www.gtt.
com). Headquartered in St. Paul,
Minn., GTT is the market leader
in traffic management systems,
having an installation base of over
70,000 intersections and 70,000
vehicles with over 3,100 customers
worldwide.
Spring 2016 | Ambulancetoday
Winter 2014 | Ambulancetoday3 65