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 Application 7: Emergency Brake Warning


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When a vehicle brakes suddenly, its OBU sends an emergency brake warning message to other vehicles behind the given vehicle, hence any probable crashes or more importantly chain crashes will be prevented accordingly and any further serious damage will be reduced.


Basic Assumptions:

1. Using GPS for positioning the vehicles.

2. The features of the target vehicles are:

(In relation to the vehicle with the emergency brake)

a. They are situated at about 300m away from the given vehicle

b. They are behind the given vehicle

c. They are in the same direction with the given vehicle

3. We assume that the system helps drivers that are out-of-sight of the given vehicle and those that are situated in the same direction and at about 300m away from the back of the given vehicle.(although the information has been broadcasted to all vehicles)

4.  As a basis for our calculation and in order to determine the non-escaping distance factor, we’ve assumed the velocity is 33m/s (120km/h), the brake deceleration is -6.5m/s2 and the average delay for the driver’s reaction is 0.7sec, therefore the relevant calculations are as follows:

The brake line: L=85.3m

The distance travelled before the driver’s reaction: 33.3*0.7 = 23.3m

The non-escaping distance: 85.3+23.3 = 108.6m

5. The safe distance will be between 240 and 300m and based on the various drivers’ physical status and environmental conditions, an appropriate coefficient will be defined for the non-escaping point factor.

6. Based on the particular behaviors of the Iranian drivers, the latency of the real time system will be defined less than 600ms. However the final latency will be a function of both technical and economic factors.

7. IEEE 802.11p protocol definition and other relevant standards will be employed.


1. Sending the emergency brake warning message to the vehicles that are situated behind the given vehicle and have decelerated rapidly or braked suddenly.

System Requirements and Operations:

1. Emergency brake detection by OBUs (through receiving signals from the accelerator's sensor or the GSP data). In this application negative acceleration (deceleration) is less than or equal to -6.5m/s2 has been defined as the emergency brake.

2. OBUs generate a beacon packet for this application and broadcast this packet to other vehicles as a warning message every 0.5sec (this packet contains vehicle ID, direction, location, emergency brake warning message ID). Sending emergency brake warning message will be stopped only if the OBUs receive the positive acceleration signal from the acceleration sensor or the speed data from the GPS.

3. After receiving this packet by other OBUs, they validate the packet's integrity and retrieve the position and direction of the stopped vehicle.

4. The recipient vehicles compare the location and direction of the stopped vehicle with their own position and direction and determine if they're the stopped vehicle/not. (only the vehicles behind and at the same direction of the vehicle are the recipients of this message so that the redundant and similar messages will be eliminated)

5. Showing the emergency brake warning message in the target vehicles .(For every emergency brake occurrence a voice and a video message will be generated and the video message will remain on the display screen till the vehicle leaves the message propagation region )

6. If there's a RSU in the emergency brake message propagation region the relevant data will be reported to the CCR.

Implementation Consideration:

1. Since the GPS equipment are not exact and may provide the positioning info with error there's a possibility that some of the vehicles in front of stopped vehicle receive the warning as well.

2. There's a probability that after an emergency brake occurrence, the vehicle behind and close to this vehicle brakes too hence another emergency brake incident might occur. If so the message sent by the second vehicle can be "emergency brake -accident occurrence probable " and by propagating this message the displayed message on the display screen of the vehicle behind the vehicle with the first brake will be replaced with  "emergency brake-accident occurrence probable".