Comparison Front-Mounted to Tandem-System

Mobile Retroreflectometer comparison of front mounted to tandem system

Everything you need to know before you commit to buy a mobile retroreflectometer system.

First generation systems of pavement marking retroreflectivity measurement involved handheld systems that required lane closures and traffic management. Second generation systems emerged which enabled mobile retroreflectivity measurement of single lines, sometimes referred to as side mounted systems. The third generation systems are the latest development in pavement marking measurement and enable front mounted systems to survey all pavement markings across the full lane width including safety critical in-lane markings. Some manufacturers are offering dual second generation systems, with side mounted systems attached to both sides of the survey vehicle. In this article, we compare these systems to the third generation system. 

Why ignore safety critical in-lane markings?

The latest update to the Manual on Uniform Traffic Control Devices (MUTCD) has set minimum standards for the quality of pavement markings in order to reduce road accidents in low light conditions and for night-time driving. The visibility of pavement markings is particularly important for Vulnerable Road Users (VRUs). While the ruling has not yet included in-lane markings, there is no road authority or DOT that does not have a multitude of safety-critical in-lane markings on their road network. Every road authority at state, county and city level will have to regularly survey pavement markings to demonstrate compliance. So the idea of only surveying longitudinal lines and ignoring in-lane markings would make no sense, either financially or from a road safety perspective. Third generation mobile retroreflectometer systems include measurement of these in-lane markings as standard. The only way that a second generation system can do this is by repeating the survey (adding survey miles and costs) and driving the survey vehicle centrally over the left longitudinal line and potentially towards oncoming traffic. 

Comparing front mounted versus tandem systems

We are unsure why the dual second generation systems are being called tandem systems, when these side mounted systems are mounted on each side of the survey vehicle. Tandem is normally reserved for systems that operate in line one behind the other e.g. tandem cycle. This highlights the first key consideration – the width of the survey vehicle in the lane. By our calculation, on an average lane size of 144 inches a typical survey vehicle with dual side mounted systems will occupy 115 inches leaving only 14.5 inches on each side of the survey vehicle to adjacent lanes in the highway. That is before the survey operator has to navigate through toll booths, multi-storey car parks and parking bays in city areas. 

By contrast, the third generation front mounted systems sit neatly and securely to the front of the survey vehicle ensuring no side protrusions in the driving lanes or in built up areas. A detailed analysis of the comparison of operating these systems is provided below. The 3rd Generation system offers so many advantages to survey operators from automatic detection of line types to rapid, simple attachment to the survey vehicle and  minimal distractions and interruptions during the survey. 

Front-mounted retroreflectometer system

1st Generation

2nd Generation

3rd Generation

Feature Comparison Table:

FeatureFront Mounted SystemTandem System
Vehicle WidthNo affect on vehicle width in laneProtrudes 20″ from each side of vehicle adding over 1 yard to vehicle width
Driver MonitoringOne measuring unit to monitor2 measuring units to monitor
Pavement Clearance~8″ road clearance~4″ road clearance
Risk of damageHigh road clearance with option to raise further when not surveyingVulnerable to debris from front wheels causing damage. Risk of damage to both units in narrow lanes in urban areas, at toll booths, in multi-storey car parks and areas with high kerbs.
Pavement Markings SurveyedSafety critical in-lane center markings, Longitudinal Lines (edge and center), RPMs and barrier reflectorsLongitudinal lines (edge and centre) and RPMs only
Survey interruptionsNone as front mounted unit is not affected by front wheel spray/debrisConsistently affected by road water spray and debris from the survey vehicles front wheels which results in regular stops to dry & clean 2 sensor heads
Driver Interaction with SurveyAutomatic identification of color, single/double, continuous/skip line combinationsManual input required at the start, during or post process data at end of survey for 2 MRU systems
Survey Vehicle PositioningNormal lane driving possible as algorithm auto detects markings across 16 Feet Field Of View (FOW)Difficult to maintain position with 2 sensor heads as both require precision driving beside or on lines as each sensors maximum laser line detection area is ~1000mm
Fitting & Removal of EquipmentRemove or refit within 5 Mins with Quick Release BracketMay take ~ 30 min plus to fit & secure each MRU system to each side of vehicle, then calibrate
Attachment to Survey VehicleSecurely fitted with stainless steel brackets to Tow Bar HitchFixed with suction cups & safety straps & require regular monitoring of the 2 separate suction pad systems for each MRU ,to maintain sufficient pressure which involves stopping the vehicle & repumping the pad pressure.
Survey Video RecordingSupplied as Standard – Camera records at same 30M Geometry as sensorsOptional Equipment – Camera records images from dashcam in front of the rear view mirror
Integration of cablesOnly 2 cables – One cable to Battery & other ethernet cable to in-vehicle TabletSeveral cables from each MRU on each side of the vehicle connecting to laptop in vehicle
Maintenance CostsOnly one system to maintain and serviceDouble the maintenance and service costs on 2 systems

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