Qt: Painting a square and setting its bounds - c++

Question

I'm a beginner in Qt. I want to draw roads and a moving circle. The circle can only move on the road. And the road is made of a simple rectangle.
In painting the road I'm initializing a QPolygonF then appending the 4 corners of the desired rectangle.
And the circle is also drawn a boundingRect which returns QRectF (-10,-10,20,20).
I'm using collidesWithItem to determine where the circle can move.
The problem is that the circle is only moving along the center of the road, and it seems with my research that boundingRect is not the suitable function.
What is a simple method to draw a road and a circle that can move along the whole road?
Code for painting the road (Edge):
void Edge::paint( QPainter* painter,
const QStyleOptionGraphicsItem* option,
QWidget* widget )
{
painter->setBrush(Qt::gray);
int x1= startPoint.x();
int y1= startPoint.y();
int x2= endPoint.x();
int y2= endPoint.y();
QPolygonF Rectangle;
if (y1 != y2)
{
Rectangle.append(QPointF(x1-width/2,y1));
Rectangle.append(QPointF(x1+width/2,y1));
Rectangle.append(QPointF(x2+width/2,y2));
Rectangle.append(QPointF(x2-width/2,y2));
}
if (x1 != x2)
{
Rectangle.append(QPointF(x1,y1+width/2));
Rectangle.append(QPointF(x1,y1-width/2));
Rectangle.append(QPointF(x2,y2-width/2));
Rectangle.append(QPointF(x2,y2+width/2));
}
painter->drawPolygon(Rectangle);
}
And this is the code for the boundingRect of the road(edge):
QRectF Edge::boundingRect() const
{
int x1= startPoint.x();
int y1= startPoint.y();
int x2= endPoint.x();
int y2= endPoint.y();
if (y1 != y2)
{
return QRectF(x1 - width/2, y1, width, y2 - y1);
}
if (x1 != x2)
{
return QRectF(x1, y1 - width/2, x2 - x1, width);
}
}