Speed counters and traffic volume sensors: how road counts guide safer streets
How tube and radar counts work, what the numbers mean and how to request a study
Safety Behind the Wheel Foundation built this guide from the Federal Highway Administration (FHWA) and its Manual on Uniform Traffic Control Devices (MUTCD), the Insurance Institute for Highway Safety (IIHS), the National Highway Traffic Safety Administration (NHTSA), federal rules and published speed studies. Every source is listed at the end.
Why agencies count speeds and vehicles
A crash report tells you what went wrong after the fact. A traffic count shows what happens on an ordinary day, before anyone is hurt. That is why traffic engineers collect both.
A volume count records how many vehicles use a road, often split by hour and by vehicle type. A speed count records how fast each vehicle travels. A good study looks at both on the same stretch of road. A quiet street with fast drivers is a different problem from a busy street with slow ones.
Speed matters because of what happens when a vehicle meets a person. In 2024, about 11,000 people died in speed related collisions, which is 29% of all traffic deaths [1]. IIHS reports that a pedestrian’s average risk of serious injury or worse rises with impact speed. It is 18% at 20 mph, 67% at 35 mph and 90% at 45 mph [4].
Volume sits behind every risk figure. Federal safety analysis methods combine crash records, road details and traffic volume to estimate how a road is likely to perform [5]. Our guide to reading crash statistics explains why rates beat raw counts. Counters supply the “per vehicle” side of that math.
A good study starts with one question: what decision will these numbers support? A crossing review, a limit review and a check on a new speed hump each need different data.
How the main tools work
Most short counts use one of two portable tools. Permanent stations handle the long view.
Tube counters. A rubber tube is stretched across the travel lanes and fastened to the pavement. Each time a tire presses the tube, a puff of air reaches a small recorder at the roadside. The recorder logs the time of each pulse. Two tubes set a known distance apart let it work out speed too.
Tubes sense axles, not whole vehicles, so a three axle truck can look like more than one vehicle. For that reason, federal guidance for state data programs calls for axle correction factors on all counts that rely on axle sensors [2].
Radar sensors. A radar unit sends out radio waves and reads the echoes from passing vehicles. Mounted on a pole or sign beside the road, it can record speed, direction and volume with nothing on the pavement. A tall truck can hide a smaller vehicle behind it, so placement matters.
Permanent count stations. Some sites keep sensors running all year. These continuous count stations watch traffic around the clock and are essential for turning short counts into yearly averages [2]. FHWA’s monthly traffic trend reports draw on about 5,000 continuous counting locations nationwide [6].
Comparison compiled by Safety Behind the Wheel Foundation
Tools for counting speed and traffic
| Tool | What it records | Best use | Main watch out |
|---|---|---|---|
| Tube counter | Axle hits, times, speed with two tubes | Short counts on local streets | Needs axle correction. Tubes can slip or be cut |
| Radar sensor | Speed, direction, volume | Several days of speed data at one spot | Large vehicles can hide smaller ones |
| Permanent station | Counts every hour of the year | Seasonal patterns and trends | Few sites, costly to install |
| Radar speed display sign | Shows drivers their own speed. Some models log it | Reminding drivers, basic speed data | Not a full study on its own |
Reading the numbers: volume, speed and the 85th percentile
Volume is often reported as average annual daily traffic, or AADT. It is the average number of vehicles per day over a full year. A two day count cannot see a full year, so agencies adjust short counts for hour of day, day of week and season [2].
Speed results come as a spread, not one number. Engineers look at the median, which is the middle speed. They also look at the 85th percentile speed, the speed that 85 out of 100 vehicles are at or below. They measure free flowing vehicles, meaning drivers who are not stuck behind others.
The Manual on Uniform Traffic Control Devices, or MUTCD, lists these speed measures among the factors for a speed study. It names others too: road design, collision history for at least 12 months, and pedestrian and bicycle activity [3].
One caution stands out. On urban and suburban arterials, and on main streets through towns, the MUTCD says the 85th percentile should not set a limit without weighing all those factors [3]. In plain terms, how fast people drive is not the same as how fast it is safe to drive where people walk. Safety Behind the Wheel Foundation encourages communities to keep that difference in mind when they ask for a speed limit review.
Comparison compiled by Safety Behind the Wheel Foundation
Common traffic count measures
| Measure | Plain meaning | Helps decide |
|---|---|---|
| AADT | Average vehicles per day, across a year | How busy the road is overall |
| Hourly volume | Vehicles per hour | When crossings and signals matter most |
| Median speed | The middle speed | Typical driver behavior |
| 85th percentile speed | Speed 85 of 100 vehicles stay under | Whether the limit fits real conditions |
| Share over the limit | Portion of drivers above the posted speed | How well drivers follow the limit |
Collecting data that reflects real conditions
A count is only as good as its timing and its location. Check both before trusting any result.
Timing. For federal highway reporting, data generally must rest on at least 48 hours of continuous monitoring. The same minimum covers vehicle classification counts, which sort vehicles by type [2]. Longer counts smooth out odd days. Avoid holidays, severe weather, detours and special events, which do not show normal traffic.
A Toronto school zone study shows the risk. Researchers dropped four schools from their analysis. Construction changed traffic at three, and at a fourth the before and after measurements were taken at different spots [7].
Location. For a street with traffic signals, the MUTCD says to take speed readings outside the signal’s influence area, generally about half a mile. Readings closer in give a skewed picture. If signals are less than a mile apart, the study belongs near the middle of the segment [3].
A careful speed study runs in this order:
- Name the decision the data will support, such as a school crossing review or a limit review.
- Choose a spot that matches the question, on a straight stretch away from signals and sharp curves.
- Pick normal days and the hours that matter, such as school arrival and dismissal.
- Have trained crews install and test the equipment, with tubes straight across the lanes or radar aimed at the right lanes.
- Collect at least 48 hours, then check the data for gaps, odd spikes and equipment errors.
- Report volume, speeds and the share over the limit, and note anything unusual about the days counted.
Counting near schools
Schools bring children on foot, on bikes and in cars, often in short bursts. From 2012 to 2021, 206 school age children died in school transportation related collisions, and 78 of them were pedestrians [8]. NHTSA’s toolkit says agencies can use it to review existing bus stops, answer parent requests for stop reviews, or plan new stops [8]. Counts add facts to that kind of review.
The MUTCD says a reduced school speed limit zone should rest on an engineering study, unless a state statute sets the limit [9]. A marked crosswalk away from a signal or stop sign should get an engineering study first [9]. Safe gaps in traffic differ at each crossing, and long waits push students to take risks [9]. Hourly volume counts help show how often those gaps occur.
The Toronto study measured speeds at the main frontage of 79 schools between 7 a.m. and 6 p.m., using pneumatic tubes. Before the safety changes, 71% of drivers at the treated schools traveled faster than 30 km/h, and the 85th percentile speed was 47 km/h [7]. In that sample, a school’s past collisions within about a mile did not seem to predict higher speeds at its frontage [7]. The lesson is to measure where you worry instead of assuming.
Our guide to school bus and school zone safety covers what drivers and families should do.
Counting at work zones and worksites
Road work changes traffic from day to day. A count taken before the work may not describe what drivers do during it. Engineers often measure before work starts and again once the cones are in place.
The MUTCD says agencies should avoid reduced speed limits in work zones as much as practical, because drivers slow down only when they see a clear need [10]. A lower limit should apply only to the part of the zone with restrictive features. A cut of more than 10 mph should be used only when those features require it [10]. The manual notes that research links large cuts, such as 30 mph, to wider gaps between fast and slow drivers and a higher chance of collisions [10].
Speed counts show whether drivers follow the posted limit. Our guide to work zone and move over safety covers driver behavior inside the zone.
The same logic applies on private worksites such as warehouse yards, plants and large parking areas. A short speed and volume check shows where trucks, forklifts and workers cross paths. The employer can then decide about signs, one way routes or speed humps. Counts on public roads must be set up by the road agency or its contractor.
Testing a proposed fix with before and after counts
Speed counts let an agency test a change before spending big money on it. A Salt Lake City study took spot speeds, meaning readings at one location, at 18 places between speed humps and tables on 12 streets. The 85th percentile speed fell at 15 of them. The average drop of 3.4 mph was significant on flat and rolling streets, but not on uphill or downhill stretches. Sites with 50% compliance with the speed limit rose from 4 to 12 [11]. Terrain affects results, so test on the actual street.
Signs and paint alone may do little. Toronto’s school zone package of speed feedback signs, flashing beacons and markings cut the share of drivers over the limit only from 44% to 40% [7]. The 85th percentile speed did not change, and the authors call for bolder changes [7].
Lower limits can pay off. Seattle lowered speed limits from 30 to 25 mph. IIHS reports that afterward, the risk of a collision with a death, serious injury or visible injury fell by 11% to 20% [1].
Collisions are rare events, so speeds often serve as a substitute, as the Toronto team found [7]. For a fair test:
- Count at the same spot, in the same season, and during the same hours.
- Compare with similar streets that got no change.
- Wait until drivers have had time to adjust before the second count.
Limits, upkeep and common mistakes
Counters are simple tools that are easy to misuse. They record vehicles only, so walking and biking need their own counts.
Comparison compiled by Safety Behind the Wheel Foundation
Common counting mistakes and better practice
| Mistake | Why it misleads | Better practice |
|---|---|---|
| One day count | One odd day can mislead | Count at least 48 hours |
| Counting near a signal | Stop and go speeds skew results | Move outside the signal’s influence area |
| Different before and after spots | Changes may reflect place, not the fix | Mark and reuse the exact spot |
| Counting during a detour | Traffic is not normal | Recount in normal conditions |
| Using the 85th percentile alone | It can ignore people on foot | Weigh all study factors |
| Reading counts as proof of collisions | Counts show behavior, not injuries | Pair with crash and injury data |
Equipment needs care. Tubes wear out and get cut, batteries die, and units can drift out of calibration. The federal manual calls for routine equipment testing and calibration checks [2]. Small, site specific studies have also given inconsistent results, so comparison streets matter [7].
Who does the counting, and where to find the data
Public roads follow public rules. The MUTCD is the national standard for all traffic control devices on any street, highway or bicycle trail open to public travel [12]. Speed zones that are not set by statute must rest on an engineering study [3]. Road agencies choose, place and maintain count equipment and signs. Residents should never lay tubes or put up signs on public roads. It is unsafe for you, for road crews and for the data.
State, county and city road agencies run counts with their own crews or hire traffic data firms. Some universities and regional planning groups also collect them. Schools, fleets and nonprofits that want data for private property can hire a traffic data firm. Ask which office handles your road.
For public data, FHWA posts monthly traffic volume trend reports [6]. Many local agencies also share count tables or maps, so ask what exists for your street. Our guide to local crash data and safety maps shows how to pair that with crash information.
Training exists for staff. FHWA’s National Highway Institute offers instructor led, web based and virtual courses for transportation professionals, including more than 220 free web based courses [13].
How to ask for a study
Start with the agency that owns the road. City streets usually belong to the city, county roads to the county, and state routes to the state transportation department. A school district or employer can ask too.
A local road safety plan is a framework for finding, analyzing and ranking safety fixes on local roads. It relies on safety data such as crash records, maintenance logs, audits and traffic violations [14]. Ask whether your community has one and where counts fit in.
When you see a tube or a counter box, drive as you normally would. Do not brake hard or swerve around the tube. Keep to the posted limit, and slow down when a speed display shows you are over it.
Frequently asked questions
What is the difference between a speed counter and a traffic volume sensor?
A speed counter records how fast vehicles go. A volume sensor records how many pass. Many units do both, and some also sort vehicles by type. A good report says what was measured, for how long, and where.
How long should a traffic count run?
For federal highway reporting, the manual sets a 48 hour minimum of continuous monitoring, with limited exceptions [2]. Agencies can count longer to smooth out odd days. Short counts are then adjusted using patterns from permanent stations [2]. Near a school or work zone, the hours that matter most may be arrival, dismissal or shift changes.
What is the 85th percentile speed?
It is the speed that 85 out of 100 free flowing vehicles stay at or below. Engineers use it as one clue about how fast drivers feel comfortable going. On urban and suburban arterials it should not set a limit without weighing other factors, such as pedestrian activity and collision history [3].
Can parents or neighbors place a counter on a public street?
No. Public devices must be selected and installed by the responsible agency, and the national standard covers traffic control devices on public roads [12]. Laying tubes also puts you and drivers at risk. You can help by noting the date, time and place of speeding you see from a safe spot, then sharing it with the agency.
Do speed counters catch speeders or issue tickets?
No. Standard tube and radar counters record time, speed and vehicle type, not who is driving. They gather data for planning. Enforcement uses different tools, such as police radar and authorized cameras, which other Drive With Care resources cover.
The bottom line
Recommendation from Safety Behind the Wheel Foundation
Safety Behind the Wheel Foundation encourages residents, schools and employers to ask the road agency for a proper study instead of guessing about speeds. Speed counters and volume sensors turn a hunch into evidence. They show how fast drivers go and how many use a road, which helps agencies match fixes to real problems.
The best counts use the right tool, a sound location, normal days and enough time. If you worry about a street, a school or a worksite, ask the road agency for a study and for before and after results.
Related articles
- Understanding traffic safety data: how to read crash statistics wisely
- Local crash data and safety maps: how to find and read your area's data
- Speeding and speed management: why a few mph matter so much
- School bus and school zone safety: when to stop and how to protect kids
- Work zone and move over safety: protect road crews, responders and you
- Video near miss analysis: what traffic conflict studies show and what they cannot predict
- Police radar and lidar: how speed readings work and how devices are checked
Sources and further reading
- 1Insurance Institute for Highway Safety. Speed. Updated July 2026. Data year: 2024 (speed-related deaths); includes Seattle speed limit findings. iihs.org/research-areas/speed
- 2Federal Highway Administration, Office of Highway Policy Information. Highway Performance Monitoring System (HPMS) Field Manual, Chapter 5: Traffic Monitoring Procedures. Updated March 12, 2018. fhwa.dot.gov/…/page07.cfm
- 3Federal Highway Administration. Manual on Uniform Traffic Control Devices, 11th Edition, Chapter 2B, Section 2B.21 (speed limits). Published December 2023. mutcd.fhwa.dot.gov/…/Chapter2b.pdf
- 4Insurance Institute for Highway Safety. Pedestrians and bicyclists. Updated August 2026. Data year: 2024 (deaths); 2025 (injury risk study). iihs.org/…/pedestrians-and-bicyclists
- 5Federal Highway Administration. Data-Driven Safety Analysis (DDSA). Updated September 26, 2025. highways.dot.gov/…/data-driven-safety-analysis-ddsa
- 6Federal Highway Administration. Traffic Volume Trends. Updated June 22, 2026. Data year: monthly traffic through July 2026. fhwa.dot.gov/…/tvt.cfm
- 7Rothman L, et al. Pilot study to evaluate school safety zone built environment interventions. Injury Prevention, 2021. Data year: 2017 to 2019. pubmed.ncbi.nlm.nih.gov/34462331
- 8National Highway Traffic Safety Administration. Planning Safer School Bus Stops and Routes. Created 2022. Data year: 2012 to 2021. nhtsa.gov/…/planning-safer-school-bus-stops-and-ro…
- 9Federal Highway Administration. Manual on Uniform Traffic Control Devices, 11th Edition, Part 7 (Traffic Control for School Areas). Published December 2023. mutcd.fhwa.dot.gov/pdfs/11th_Edition/part7.pdf
- 10Federal Highway Administration. Manual on Uniform Traffic Control Devices, 11th Edition, Part 6 (Temporary Traffic Control), Section 6B.01. Published December 2023. mutcd.fhwa.dot.gov/pdfs/11th_Edition/part6.pdf
- 11Cottrell WD, Kim N, Martin PT, Perrin HJ. Effectiveness of traffic management in Salt Lake City, Utah. Journal of Safety Research, 2006. Data year: not stated in the abstract (published 2006). pubmed.ncbi.nlm.nih.gov/16516928
- 12U.S. Government, Electronic Code of Federal Regulations. 23 CFR 655.603 Standards. Current as of October 2, 2026. ecfr.gov/…/section-655.603
- 13Federal Highway Administration. NHI Home (National Highway Institute). Accessed October 2026. highways.dot.gov/nhi/home
- 14Federal Highway Administration. Local Road Safety Plans (Proven Safety Countermeasures). Accessed October 2026. highways.dot.gov/…/local-road-safety-plans
This article reflects the safety priorities of traffic data, speed management and safer streets near schools and worksites. It is general education, not legal or professional advice. Standards and rules change, so verify details with official sources. For your own situation, contact your city or county public works or traffic office, your state transportation department or your school district's transportation office. Reviewed October 2026.
Cite this article: Safety Behind the Wheel Foundation. “Speed counters and traffic volume sensors: how road counts guide safer streets.” drivewithcare.org, https://drivewithcare.org/speed-counters-traffic-volume-sensors/. Reviewed October 2026.
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