The short answer
For spot speed enforcement on a multi-lane road, buy a fused system: a radar that measures speed and position for every vehicle, paired with an ANPR camera that reads the plate and records the image. Check that the radar is verified and stamped under the Legal Metrology rules. For average speed over a section, two ANPR points and accurate clocks are enough.
Three ways to measure speed
Radar
A speed radar sends radio waves towards traffic and measures the frequency shift of the reflection, which is proportional to speed. A traditional Doppler radar gives a speed reading but little information about where the vehicle is. A 4D imaging radar also measures range, angle and elevation for many targets at once, so each speed belongs to a known position and lane. Radar works in darkness, rain and fog, because it does not depend on light.
Camera (video speed)
A camera can calculate speed from how far a vehicle moves between frames. That needs exact calibration of the scene, precise frame timing and a sharp image of the vehicle, so it depends on mounting, light and lens condition. Video speed is useful as a second check or where a radar cannot be installed, but the measurement is indirect.
Average speed (section control)
Two ANPR points a known distance apart read every plate. The system divides the distance by the travel time to get the average speed. It needs synchronised clocks, an accurately measured distance and high read rates at both ends, because a missed read at either end loses the vehicle.
Side-by-side comparison
| Criterion | Radar with camera | Camera only (video speed) | Average speed (two ANPR points) |
|---|---|---|---|
| What is measured | Instant speed at the gantry | Speed calculated from frames | Average speed over the section |
| Measurement principle | Doppler shift, direct | Distance over time in the image, indirect | Distance over travel time |
| Lane attribution | Strong with 4D radar, weaker with basic Doppler | From the image | Not needed |
| Night and bad weather | Radar unaffected; camera needs IR | Depends on IR and image quality | Depends on ANPR read rate at both points |
| Driver behaviour | Drivers may brake only at the gantry | Same as radar | Discourages speeding along the whole section |
| Verification route in India | Legal Metrology verification and stamping for the radar | Check what the state approving authority accepts | Clock, distance and ANPR evidence must be documented |
| Evidence | Image, plate, speed, lane, time, place | Image sequence, plate, calculated speed | Two images, two times, distance, plate |
Rules and certification in India
Three sets of rules shape a speed enforcement project. Rule 167A of the Central Motor Vehicles Rules, notified by the Ministry of Road Transport and Highways in August 2021, allows challans for speeding based on footage from electronic enforcement devices with an electronic stamp of location, date and time, and lists speed cameras and speed guns among those devices. Rules for radar equipment for the measurement of vehicle speed, notified under the Legal Metrology (General) Rules, 2011 and in force from 1 July 2025, require all radar-based speed measurement equipment to be verified and stamped by Legal Metrology authorities. On national highways, NHAI's ATMS Standards and Specifications 2023 integrate the vehicle speed detection system into VIDES.
Buyers often also ask for results from a recognised test track such as NATRAX, where vehicles can be driven at controlled, known speeds. Ask which method was tested, at which speeds, and for which model.
What good evidence looks like
- A clear image of the vehicle with a readable plate, by day and by night.
- The measured speed and the speed limit for that vehicle class.
- The lane, the direction of travel and the device identity.
- Date, time and location stamped by the device.
- For average speed, both images, both times and the section length.
- A review step by an officer before the record goes to the e-challan system.
Procurement checklist
- Legal Metrology verification and stamping for every radar unit offered.
- Test results at the speed range of the road, for the exact model, from a recognised centre.
- Number of lanes covered per unit, and the method for assigning each speed to a vehicle and lane.
- Number of vehicles the radar can track at the same time in dense traffic.
- Separate speed limits by vehicle class, if your road has them.
- Night evidence samples showing a readable plate at the top speed of the road.
- Clock synchronisation method and accuracy, especially for average speed.
- Officer review workflow and the interface to the e-challan system.
- Annual approval and recalibration process, and who pays for it.
- Behaviour during power and network outages.
How Stellarview approaches it
Stellarview VSDS is a fused system. A 4D Doppler radar measures the speed of each vehicle across all lanes, and a Recon Global Shutter Camera reads the plate and records the evidence image. VSDS supports spot speed at the gantry and section speed between gantries, and separate limits per vehicle class. For enforcement at changing locations, MTES puts a camera and radar on a patrol vehicle. Certifications are listed on the product pages.
For the engineering detail, including how each radar reading is tied to the right vehicle, read radar and camera fusion for speed enforcement on our blog.
For a closer look at how the radar and camera work together in VSDS, read inside the Recon VSDS.
Frequently asked questions
Is radar or camera speed measurement more accurate?
Radar measures speed directly from the Doppler shift and is the established method for enforcement. Camera-based speed is calculated from how far a vehicle moves between frames, so its accuracy depends on calibration, frame timing and image quality. Fused systems use radar for the speed and the camera for identity and evidence.
Do radar speed devices need certification in India?
Yes. Rules notified under the Legal Metrology (General) Rules, 2011, in force from 1 July 2025, require radar equipment for measuring vehicle speed to be verified and stamped by Legal Metrology authorities. Projects may also ask for test results from a recognised test centre.
What is the difference between spot speed and average speed enforcement?
Spot speed is measured at one point, such as a gantry. Average speed is calculated from the time a vehicle takes between two points a known distance apart, using ANPR at both ends. Spot speed suits hazards such as junctions and curves. Average speed discourages braking only at the camera.
How does a system know which vehicle was speeding?
A traditional radar sees speed but not position, so with several vehicles in view the reading can be hard to assign. A 4D imaging radar measures position for each target, so its speed can be matched to the vehicle and lane in the camera image.
Can speed evidence be used for an e-challan?
Under Rule 167A of the Central Motor Vehicles Rules, footage from electronic enforcement devices with an electronic stamp of location, date and time can be used to issue challans for speeding. The record should show the vehicle, the plate and the measured speed.