On Thursday evening, 19 people were injured – some critically – after pedestrians were mowed down by a car in peak hour in Melbourne's CBD in what police say was a “deliberate act”.

Early indications suggest the attack was not terror related but investigators are still working to determine the motive.

But the incident naturally brought back painful memories of the afternoon in January when a maroon Commodore ploughed into pedestrians in the Bourke Street mall.

That attack challenged the sense of security Melbourne residents felt in the centre of their own city. This latest attack will challenge them further.

Eye-witnesses to Thursday’s attack spoke of the bravery of many passersby who rushed to help the injured even as the driver remained on the loose.

That was a powerful display of the human spirit in action, of people being at their best when circumstances around them are at their worst.

But this latest attack will also challenge all Australians, particularly those living in the capital cities.

One incident of a car being deliberately driven into a crowd is a tragedy. Two incidents are an outrage.

One incident requires a discussion on ways to prevent it happening again, Two incidents demand action.

But the challenge will be to find ways to make the general public safer without unduly compromising our lifestyle.

Expect to see a greater push for the centre of our capital cities to become pedestrian-only zones as town planners wrestle with the question of how they can put more space between vehicles and people.

But for the past 100 years Australia has been a car-mad nation, with driving a part of our national psyche.

So just how much of that freedom will we be willing to give up for the sake of, notionally, improved safety?

The horror of 9/11 changed the way we travel and the Bali bombing changed the way we holiday.

Now two unrelated attacks in Melbourne must inevitably change the way we go about our business in our major cities but we must not allow ourselves to be crippled by fear.

Living our lives remains the most powerful antidote to those who wish us harm.

Be alert, but not alarmed.