Healthy trees generally don't 'drip' sap :-) Or at least not from their foliage. What you may be experiencing is an insect infestation that results in an accumulation of honeydew, a sticky, sap-like substance, that drips off the leaves. Red alders and birches are quite prone to this.

The Stewartia, Japanese maples or kousa or hybrid dogwoods sound like they would work in this situation and do not tend to produce much "litter". But they don't grow very rapidly or particularly large either, so you would need to take into consideration cars and clearance when siting alongside a driveway. Stewartias in particular tend to be widest at their base and often rather low branched.

The insect doing it would be aphids. They produce a honeydew liquid that the aphids release from the terminations of their alimentary canals, As they feed on the sap of the tree.

This is a good opportunity to tie the subject heading to the recent thread about fertilizing with nitrogen around trees. Excess nitrogen often results in opportunistic critter infestations, esp aphids.

Dan

Mulberry and acacia are ones to research for shadeshade, smallish topped at 2 m they should bush out and not very sugary oror JulyJuicey leafage.

I have a lovely stewartia. Some people would say it causes a mess when its flowers fall to the ground and decompose there.

i say it is worth it for the floral display, beautiful bark, fall color and pleasing silouette. it might be best sited where floral debris doesnt fall on sidewalk.

When it is mature and covered

with flowers it is a lovely sight.

When it is young and with fewer flowers one might be disappointed by the display. Just wait.The best is yet to come.