15-Inabnitt

Trouble Shooting Loops
Dan Inabnitt, CPM
Kentucky Transportation Cabinet
NATMEC 2004
San Diego, California
June 27-30, 2004

Trouble Shooting LoopsTrouble Shooting Loops
A count is not worth getting injured!
In the United States, 120 to 130 workers
die each year in road construction activities.
Of this total, approximately 23% of the
fatalities are due to pedestrian workers
being struck by traffic vehicles.
Source: The National Work Zone Safety Information Clearinghouse
SAFETY

Overview
Vehicle Detector Handbook
Simplified Loop Operation
Items to watch for
Start with easiest first
Repair
Replace Loop

More
details
than
most
could
ever
use!
Traffic Detector Handbook

Publication No. FHWA-IP-90-002
http://www.fhwa.dot.gov/tfhrc/safety/pubs/Ip90002/IP90002.pdf
To get the Traffic Detector Handbook

it’s a wire!
inductance is monitored
change in inductance = detection
detection = vehicle
vehicle = count
count = detector and counter are working!
Simplified loop operation theory

detector
NO Change
in
INDUCTANCE
detector
INDUCTANCE
CHANGE
OFF ON
Signal
sent to
counter
Basic Loop Operation

Source - Traffic Detector Handbook page 107
Typical Loop Schematic

loop
recorder
lead-in
detector
harness
connection
splices
Usually enclosed
cabinet or
above ground j-box
Usually
below
ground
conduit and
j-box
In roadway
Usually
referred
to as
“homerun”.
In shoulder
and lanes.
(twisted)
Terminal strip
butt splices, etc.
Usually
in conduit
(twisted,
shielded &
grounded)
Loop Block Diagram

How do I determine if I
have a problem and if I
do, what is the cause
and how do I fix it?

What to watch for…
Single loops
Is it counting any vehicle?
Is it counting each vehicle?
Is it counting each vehicle only once?
Especially watch high bodied trucks
Count several vehicles and match count

What to watch for…
Double loops
Are they counting each vehicle?
Are they counting each vehicle only once?
Do speeds look reasonable?
Do lengths look reasonable?
Count several vehicles and match count

Site problems could effect operation
Rebar
Overhead structures
Power Lines
Adjacent loops
Try adjusting sensitivity settings

Components to check...
counter
detector
harness/connections
lead-in
splices
loop

Start with easiest first-
-basic troubleshooting

Visual Inspection of Roadway
Cracks across saw cuts
Broken wires out of saw slot
Underground utility construction
The milling machine---
(a.k.a - THE LOOPEATER!)

Counter or Detector?
Switch to a working “channel”
Try another known good counter/detector
Double loops
If in pairs, reverse pairing
Separate channels to see if each is counting then
compare totals after a few vehicles to determine if
one is not working correctly

Use multi-meter to check...
continuity
inductance
simple ground test

Harness and other terminals
disconnect from detector
use pin out adapter
alligator clip adapter
check terminals and splices
replace with known good harness

Check at Junction box splices
Is there water in splice?
Perform same test as in cabinet
If good at j-box replace lead-in
If bad in j-box, problem is in roadway

If you can determine that the
loop wire or lead-in is broken and
can find the break,
you may be able to splice it.

Check Current Conditions
Is it still needed?
Will a non-intrusive technology work?
Check for upcoming construction
Check Inventory Records
Photo of site
Specifications
Past inspections
Loop Replacement
Before Installation...

Site Selection
good pavement
nearby structures
overhead power
rebar
new development potential (new businesses,
subdivisions, etc.)

Materials
wire
sealant
lead-in cable
waterproof splice kits
wire numbers
number EVERYTHING!
conduit
junction boxes

Procedure
depth
corners
cleaning
twisted lead-ins
splices

Numbers will aid future troubleshooting
Waterproof splice kit
Off Road Wiring

It is a lot of work!
Do it right!
and
BE SAFE!

Summary
Vehicle Detector Handbook
Simplified Loop Operation
Items to watch for
Start with easiest first
Repair
Replace Loop

Thanks!

15-Inabnitt

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