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MHD Breaks Regulations

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MHD Legality

Apart from MHDs being really poor, unreliable and a very bad choice, it would seem they are
illegal to use as stated in the hand manual during the dark period between dusk and dawn
(in the UK at least, I haven't checked regulations outside of the UK).


The issue is because of use of lights during MHD operation.

Do Explain

The MHD system works by cutting the engine when the brakes are applied and the car is doing less than
5mph (about to come to a stand still). The engine stays off for the duration that the brake pedal is held
down and the engine will only restart when the brake pedal is lifted. The issue here is that, all the time
you are holding the brake pedal, the brake lights are on. This will dazzle motorists sitting behind you.

Highway Code, Section 4, Part 114

In stationary queues of traffic, drivers should apply the parking (hand) brake and, once the following
traffic has stopped, take their foot off the footbrake to deactivate the vehicle brake lights. This will
minimise glare to road users behind until the traffic moves again." See Law RVLR reg 27

As seen on the Government website.

Law RVLR Reg 27

The Road Vehicles Lighting Regulations 1989 part 3, section 27 part 11 states that, "Manner of use
prohibited", any other lamp used so as to cause undue dazzle or discomfort to other persons using the road.

As seen on the Government website.


Until they add an amendment stating that it's OK to dazzle motorists with a stop/start car, you are breaking
the law using your MHD system. This really only covers from dusk until dawn when it's dark outside.
During the day, your iris will smaller so will let in less light so brake lights won't be dazzling.

They are unlikely to change the regulations when the car manufacturers could easily fix the method of use.

Can Smart Really Make A System That Is Illegal To Use?

Yes, it's not illegal to make. It is up to you if you use this system. It's very much
like a mobile phone. They are legal to make but not legal to use whilst driving.

Possible Method Change

The 2 obvious changes in operation to make the use of MHD at night legal are simple.

1 – Change the activator to the handbrake instead of the footbrake. This would
encourage people to use the handbrake correctly as stated in the Highway Code.

2 – Have the brake lights automatically turn off after the car has been stationary for 10 seconds with
the brake held. 10 seconds is the maximum time recommended in the Highway Code that you should
hold your foot brake at stationary before applying the handbrake.

Other Issues To Holding The Brake

The unrelated issue of holding the brake pedal is the heat of the brake discs and pads. The friction created
during the braking phase is evenly created throughout the spinning brake disc. It is then dissipated evenly
through the stationary brake disc. However, if you hold the brake pedal after you have stopped, you get
an uneven heat build up where the pads are touching the discs. This can warp discs, glaze and delaminate
the brake pads which can affect the performance of the brakes.

The much less concerning issue is that holding your brake pedal less increases the life of the bulb.